NATIONAL BUREAU OF STANDARDS REPORT

3057

Performance of Drr’freez Driers in a Freon-12

Refrigerating System

bv•/ Minoru Fujii

Albert W. Diniak

Ernest E. Hughes

C. W. Phii.lips

P. R. Achenbach

Report to

The Office of The Quartermaster General

Department of the Army

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards is suggested in the following listing of

the divisions and sections engaged in technical work. In general, each section is engaged in special*

ized research, development, and engineering in the field indicated by its title. A brief description

of the activities, and of the resultant reports and publications, appears on the inside of the back

cover of this report.

Electricity. Resistance Measurements. Inductance and Capacitance. Electrical Instruments. Magnetic Measurements. Applied Electricity. Electrochemistry.

Optics and Metrology. Photometry and Colorimetry. Optical Instruments. Photographic

Technology. Length. Gage.

Heat and Power. Temperature Measurements. Thermodynamics. Cryogenics. Engines and

Lubrication. Engine Fuels. Cryogenic Engineering.

Atomic and Radiation Physics. Spectroscopy. Radiometry. Mass Spectrometry. Solid State

Physics. Electron Physics. Atomic Physics. Neutron Measurements. Infrared Spectroscopy.

Nuclear Physics. Radioactivity. X-Rays. Betatron. Nucleonic Instrumentation. Radiological Equipment. Atomic Energy Commission Instruments Branch.

Chemistry. Organic Coatings. Surface Chemistry. Organic Chemistry. Analytical Chemistry.

Inorganic Chemistry. Electrodeposition. Gas Chemistry. Physic^ Chemistry. Thermochemistry. Spectrochemistry. Pure Substances.

Mechanics. Sound. Mechanical Instruments. Aerodynamics. Engineering Mechanics. Hydraulics. Mass. Capacity, Density, and Fluid Meters. Organic and Fibrous Materials. Rubber. Textiles. Paper. Leather. Testing and Specifications. Polymer Structure. Organic Plastics. Dental Research.

MetaUurgy. Thermal Metallurgy. Chemical Metallurgy. Mechanical Metallurgy. Corrosion.

Mineral Products. Porcelain and Pottery. Glass. Refractories. Enameled Metals. Concreting Materials. Constitution and Microstructure. Chemistry of Mineral Products.

Building Technology. Structural Engineering. Fire Protection. Heating and Air Conditioning. Floor, Roof, and WaU Coverings. Codes and Specifications.

Applied Mathematics. Numerical Analysis. Computation. Statistical Engineering. Machine

Development.

Electronics. Engineering Electronics. Electron Tubes. Electronic Computers. Electronic

Instrumentation.

Radio Propagation. Upper Atmosphere Research. Ionospheric Research. Regular Propagation Services. Frequency Utilization Research. Tropospheric Propagation Research. High

Frequency Standards. Microwave Standards.

Ordnance Development. These three divisions are engaged in a broad program of research

Electromechanical Ordnance, and development in advanced ordnance. Activities include

Ordnance Electronics. basic and applied research, engineering, pilot production, field testing, and evaluation of a wide variety of ordnance materiel. Special sk^s and facilities of other

NBS divisions also contribute to this program. The activity is sponsored by the Department of

Defense.

Missile Development. Missile research and development: engineering, dynamics, intelligence,

instrumentation, evaluation. Combustion in jet engines. These activities are sponsored by the

Department of Defense. • Office of Basic Instrumentation • Office of Weights and Measures.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

IOO3 -2 O-4 S32 January 15, 1954 3057

Performance of Drifreez Driers in a Freon-12

Refrip;erating System

by

Rinoru Fujii

Albert W, Diniak

Ernest E. Hughes

C. W. Phillips

P. R. Achenbach

Heating and Air Conditioning Section

Gas Chemistry Section

to

The Office of The Q'u^rtermaster General

Department of the Army

<NB^

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, repr

unless permission iso 25.D,C, Such perm

cally prepared If tha

. 1 r- 11- 1 1I pert, Is prohibited Approved for public release by the

. idards, Washington Director of the National Institute of

>rt has been speclfi- Standards and Technology (NIST) for it. own use, on October 9, 201 5.

PERFCRKANCE OF DRIFREEZ DRIERS IN A FREON-12

REFRIGERATING SYS TEN

by

Minoru Fujii, Albert W. Diniak, Ernest E. Huc^hes,

C. W. Pbillips, and P. R. Achenbach

ABSTRACT

An :i nvestif^ation of the performance of Drifreez

driers for removiny moisture from a Freon-12 refrimeratinp; system and the possible attendant

hazards from, pulverization of the calcium carbide

desiccant and the circulation of acetylene in the

’^efrimerant circuit was m.ade for thf^ Office of

The Quartermaster General. The dr''ing characteristics of the Drifreez drier v/ere compared

with those of a silica gel drier intended for the

same purpose and for systems of similar size.

The teat results showed that the Drifreez drier

xvas superior to the silica gel drier in the

amount of x^rater that could be removed with equal

values of end dryness up to 40 ppm or conversely

the Drifreez drier x^jas superior in that it could

prodxice a loxfer end dr3 mess than a silica gel

drier for equal xvater remioval in the range from

5 to 20 grams of water. The Drifreez drier was

shoxAm to have a lov/er drx'-ing rate than the silica

gel drier. The hazard tests shov/ed that the

highly-explosive miaterial, copper acetylide, x^ras formed in the refrigerating system hj the reaction

of acetylene v/ith the cooper tubing and that a

mild explosion coxxld be initiated b^r cutting the

tubing with a hacksaw after removing the refrigerant. Another hazard was shovm to be possible

if a Drifreez drier or anxr other drier using

calcium carbide as the desiccant was emploxxed to

remove moisture from a closed s^'^stcm in which air

alone xvas being circulated. Vibration tests

showed that the calcium carbide in the Drifreez

drier pulverized to some extent but the particles

leaving the drier were too small to clog strainers

and xTOuld probably not score compressor cxz-linder

walls

.

I
I. INTRODUCTION

In response to a request from the Office of The

Cjuartermaster General, dated May 27, 1952, an investisration of the performiance of certain Drifreez driers

manufactured by Berna Corporation, Richmond Hill, N.Y. , was undertaken as a basis for determ^ining the desirability of using these or other driers employing calcium. carbide as the desiccant in Quartermaster refrigeration units. The investigation was divided into

three main Darts, as follows: (A) Tests to determine

the effectiveness of the Drifreez driers in removing

water in a Freon-12 refrigerating system^ with respect

to speed of drying, total capacity for water absorption,

and end dryness of the system, (B*) tests to determine

whether or not any hazards would result from the

presence of acetylene gas in the S 3rstem, produced from

the reaction of calcium carbide and water, and (C)

tests to determiine whether the calcium carbide granules

would pulverize under vibration and contribute foreign

matter in a solid state to the remainder of the refrigerant system. The description of the apparatus, test

procedure, and results for the three parts of the investigation are presented separately under the following

headings

Part A Drying Tests

Part B Hazard Tests

Part C Vibration Tests

An overall evaluation of the Drifreez drier is m.ade

at the end of the report.

II
PART A, DRYING TR3TS

I

,

introduction

The effectiveness of the Drifreez drier containing

calcium carbide for removing moisture from circulation

in a refrigerating system using dichlorodifluoromethane

(Freon-12) was studied on request of The Office of the

Ouartermaster General, The performance of the specimen

driers was studied with respect to end dryness, speed

of drying, and total moisture removing capacity with

some consideration being given to size, weight, and cost

in comparison with other driers now being used. Tests

of a silica gel drier were also made for comparison.

Some of the more desirable properties of a

refrigerant drier are as follows:

1. Ability to produce a low final moisture content.

2. Non-deliquescent

,

3. Non-frangible.

4. No corrosive or erosive end products.

5. Chemicall}’’ inert with respect to refrigerant

and oil.

6. The rate of moisture rem*oval may be of importance

for some applications.

Hitherto moisture determinations in refrigerating

units depended on sampling a large quantity of refrigerant

from the system and measuring the average moisture content

of the entire sample. The inherent disadvantages of such

methods can be eliminated by the application of an

electrolytic method developed by E. R. Weaver at the

National Bureau of Standards. This method of moisture

measurement em.ploying an electrolytic film permitted

making observations in rapid succession at the inlet

and outlet of the drier so that the drying process could

be followed as it progressed.
I

- 2 -

II, Description of Specimens

Development of the Drifreez driers by the manufacturer

was in progress during the course of the tests. Five

different models were submitted at different times by the

manufacturer. The several models differed prim.arily in

the method of embedding the calcium carbide granules and

in the number and kind of filter pads and screens used

at the txvo ends of the drier. The vibration tests were

made on the first model and drying tests were made with

specimens of the second, third and fourth models.

The specimens were identified as folloxvs:

DRIFREEZ CARTRIDGE

Berna Corporation

Richmond Hill Ig, K. Y.

1/3 to 1 horsepower

The nameplate carried the following information:

"This cartridge removes all m.oisture occurring

in refrigeration units using Methyl Chloride

or Freon under all normal operating conditions, Do not install on Sulfur Dioxide or

Ammonia S3rstems, Installation is directly

into the liquid line before the expansion

valve.

The dimensions of the cartridges were as follows:

The desiccant was described by the manufacturer as

hard crystalline acetylenogens containing trace elements.

This can be more simply described as impure calcium

carbide. It was granular in form ranging from 0,4 In. to o mesh, approximately. About 43 grams of desiccant were

used in each cartridge. Item l^Fig, 1, is an exterior

view of one of the Drifreez driers.

Length from flare to flare

Diameter, outside

Weight, approximately

3 /^' in. flare connections

6 cu. in, capacity

6-3/4 in.

1-5/16 in.

232 grams
1

Fig. 1

“ 3 -

^ The second model submitted is shoivn disassembled

as -^-tem 1 in Fig. 2. Calcium carbide granules, weighing approximately 43 grams but with slight variation in amount among several cartridges, \uere spread on a fine steel wool pad. This pad was rolled up for insertion into the cartridge as shoT-vn in Fig. 2. A steel wool pad was used at each end of the steel v7ool roll. On the outlet end of the drier tvio fine wire screens with a thin wool filter sandwiched betxveen were used whereas a single fine wire screen was used on the inlet end. The screens v/ere approximately 65 x mesh and appeared to be made of bronze xvire. The brass end- soft-soldered to the cylinder. A perforated baffle was placed between the wire screen and the brass end-piece on the outlet end.

The third ^model submitted is shoxvn disassembled

as Item 2 in Fig. 2. In this model the calcium carbide granules were rolled up in a steel wool pad backed up outlet edges of these pads were folded oyer the calcium carbide as shown in Fig. 2 before rolling them up. A single layer of screen was used at each end of the roll and there were no wool

either end. The end-piece and baffle features v;ere identical to those on the first model tested.

The fourth model submitted had a cylindrical body of glass for observation purposes and xvas considered to be a demonstration unit. It is shown as item 2 in Fig. 1 . The dimensions of ^ this cartridge were about the same as those of the previous models except that the brass

diameter and 3/4 in thick. The glass cylinder xitted against shoulders machined in tne ilanges^and v/as sealed with a rubber gasket at either end. Six studs were used to draw the flanges

the gaskets and the cylinder. The center portion of the cylinder was filled with granulated calcium carbide, and fine steel x^ool pads filled the remaining space at either end. A fine mesh screen x^^as placed dov/nstreara of the steel wool pad xvithin the cartpdge on the outlet end. The entire specimen weighed n34 grams at the end of the test. The original weight of the calcium carbide was reported by the manu- lacturer to be 43 grams whereas the weight of the mixture of calcium carbide and calcium hydroxide was 58 grams at the end of the test. The original weight of the calcium carbide could not readily be computed
- 4

-

from the final weir;ht of the raixture because of the

impurities present.

The silica gel drier used for the test was identified as follov.-s:

Fueller Brass Company

Maximum horsepox»7er - 1

The dimensions of the cartridge were as follows:

Length from flare to flare

Diaraeter, outside

Weight

3/c in. flare connections

cu, in. capacity

o j.n.

1-15,'^16 in.

572 grams

o

The desiccant was granular silica gel areighing lie

grams. The desiccant filled the entire c^^linder except

for a flat screen at the inlet end and a cone-shaped

screen at the outlet end. The brass end-pieces were

screwed to the crlinder and soft-soldered.

III. Description of Apparatus

1. Refrigeration Unit - (See Figs. 3 and 4)

the system was charged \i:

beginnins” of the test.

' l/3-horsenower air-cooled condensing unit (R) with

an evaporator (K) wade of 3/^" copper tubing was enplo3'’ed,

„th 4.5 pounds of Freon-12 at the

A beat load was imposed on the

evaporator by passing electric current through nichrome

wire wound around the evaporator coil vjhich was insulated

.

electrically from the heating element v/ith asbestos.

Current flow was adjusted by means of rheostat (M) to

control the load. The thermostatic expansion valve was

exchanged to an automatic expansion valve (G) during the

course; of testing to provide steadier operating temperatures and to avoid freeze-up at the valve due to

hunting. An oil separator (N) was used to minimize

the f] o’V of oil through the evaporator portion of the

s 3^stem. A by-pass arrangement (C) was installed between the discharge drier dovmstream of the oil

separator and the suction line for the purpose of

introducing' water into the s^rstem. The specimen drier

( --) was installed in the liouid line in accordance v^rith
FIG

G.3



the manufacturer's instructions. The overall vievj of

the refrigeration unit and moisture measureinent apparatus is sho’en in Fig. 3.

2 , Moisture Measurement

Tlie measurement of moistiire in the refrigerant was

made using a method developed by E. R. Weaver of the

I’'ational Bureau of otandards in which a h3rgroscopic

film of sulphuric-phosphoric acid mixture was applied

to the surface of a detector exposed to tbs refrigerant

gas and water vapor mixture in the refrigerating S3r3tem,

The film reached equilibrium with the water vapor in the

refrigerant gas rapidly forming a solution, tli' electro ccal conductivity" of vrhich was a measure of the wat-r

content of the refrigerant vapor. Gompres.sed air of

hnown moisture content T-as used as a comparison gas to

calibrate the film after each reading. By'- adjusting the

pi’essure of ':.he compressed air until the same conductivit

was obtained, the two gases were made to have the same

concentration of 'firater. The unknoi'n water content was

then calculated from the tro pressures and the hnown

content of the compressed air from the following formula:

Wx =

^/Vhere ’fx ”

¥c Fx =

Pc =

¥c Pc

Px

unknoi-m water content in refrigerant

at zero gage preSvSure.

Irnov/n water content of comparison air

at zero gage pressure.

absolute pressure at v;hich a conductivity

reading v/as obtained on the unkno-'m refrigerant gas.

absolute pressi'.re of the comparison air

at which the same conduct j.vity/ reading

was obtained as for the refrigerant gas.

Goricections were made for the deviation in behavior

of all the gases involved, including water vapor, from

the ideal pas laws. The comprevssed air used for comparison was saturated at between 1000 and 2000 pounds

per square inch gage pressure and Vv’as permitted to

expand to get a comparison reading. For very dry

conditions in a refrigerating sy'"stem, hovi-ever, a

secondaryr comparison air v/as sometimes used. This

secondary," supplyy of air is a ey/li^cder of fairly?- dryjair whose moisture content had been previously determined byj comparison with the saturated air. For these

tests the compressed air required '^or making all moisture
- 6 -

determinations was obtained from a sinp;le cylinder by

means of a parallel circuit with a saturator in one

branch. Because of the pressure relationships involved,

the electrical method could be used to make moisture

measurements in refrigerant in the gaseous state only.

The sampling 33^3 tern is shovrn diagramrnatically in

Big, 5 and certain com.ponents are identified by letter

in Figs, 3, 4 and 6 also. Two sampling points, one on

each side of the drier (A), were established on the

liquid line. Two small bore tubes, each dipping into

the main stream of liquid refrigerant at the sampling

points, carried a portion of the liquid refrigerant to

heaters (D, E) v/here it was completely vaporized. The

heaters (D, E) consisted of the electric heating elements

regulated by rheostats to supply the required amount of

heat for complete vaporization of the liquid refrigerant.

To attain an efficient vaporizing process, the refrigerant tubing was wrapped around the heaters in a spiral

and was thermally insulated from the surrounding air.

The vaporized refrigerant then flowed to specially-built,

pressure-tight detector blocks (B, C) containing the

detectors (K), (See Fig, 6) where the moisture determinations were made. By manipulating these control valves

in the detector block, either refrigerant vapor or the

compressed air used for comparison could be caused to

flow past the h3rgroscopic film on the detector (K).

After passing through the detector blocks (B, C) the

refrigerant was returned to the suction line for recirculation, The flow through the sampling circuit was

controlled by valves (F) so that very little refrigerant

was permitted to bypass the evaporator (H), Compressed

air used for comparison v;as exhausted to atmosphere.

The piping circuits used for connecting the compressed

air supply to the detector blocks are shown diagrammatically in Fig, 6, The gas connections were so arranged

that the same cylinder of compressed air (I) supplied

the ^Mry” comparison air directly and the 'Vet” comparison gas through saturator (Jj, As shown in Fig, 6,

either of the two comparison gases could be directed

to the two detector blocks (B, C).

The detectors (K) were connected to a Weston galvanometer (Q) (See Fig, 3) which served as a null point

indicator. An Esterline-Angus time recorder (L) was

used to record the net operating time of the refrigeration

unit, A potentiometer (P) was used to measure the temperatures of detector blocks (B, C) and of the gas samples.
SAMPLING

SYSTEM

O

GAS

CONNECTIONS

- 7

-

IV. Test Procedure

A total of seven series of tests were made to

determine drying performance of three models of the Drifreez drier and one model of a silica gel drier, A

fifth model of the Drifreez drier v/as not tested because

the investigation was terminated.

Each drying test was begun with a nev; drier in a

well-dried s 3?’stem, Wlien initial wetness of the system

was high, other spare driers were used to reduce the

moisture content in the system. Knom quantities of

water were added in the suction line b]^ means of the

bypass between the discharge line and the suction line,

utilizing the pressure difference across the compressor

to inject the water. The quantity of water added at

one time varied from 0.1 gram to 20 grams. With a refrigerant charge of 4.5 lb of Freon 12, 0.1 gram of

water uniformly mixed in the system would produce a

concentration of about 50 ppm by weight if the effect

of the oil in the system is neglected. At first,

smaller amounts of water were added and larger amounts

were added later. The moisture content at the inlet

and at the outlet of the specimen dryer was measured

alternately as fast as the apparatus permitted.

The compressor served as a good means of mixing

water in the refrigerant. Care was taken not to damage

it by adding water too rapidly. The s3/stem was operated

with an evaporator temperature above 32 °F during most

of the investigation to prevent freezing of water in

the evaporator and at the expansion valve orifice.

V, Test Results

1. Drying Tests

Preliminary tests were made of two specimens of the

second model of Drifreez cartridge. The results of these

tests were not suitable for plotting in graphical form

because of difficulties v/ith the moisture measuring

apparatus and freezing of the v;ater at the expansion

valve. These preliminary tests did indicate that a low

end dr3mess could be obtained with the Drifreez driers.

Fig. giving results of a different specimen of

Drifreez drier also indicates that low end dryness was

attained.
Tests v/ere made of three specimens of the third model

of the Drifreez driers submitted by the m.anufacturer . In

this model the calcium carbide granules were rolled up

in overlaid pads of steel wool and wool felt as shown in

item 2 of Fig. 2, A total of o*2 grams of water was introduced into the system during tests of the first

specimen in 9 injections ranging in magnitude from 0,1

gram to 5 grams. The moisture content in parts per

million at the inlet to the drier is plotted in Fig, 7

whereas that at the drier outlet is plotted in Fig, ^ for the 225 hours of the test. Moisture contents were

observed just before and for several hours after 7 of

the 9 injections. When a small amount of water (0.1

gram) was added, the system was dried to approximately

the initial condition in 3 to 4 hours.

When 5.0 grams of v\rater were added at one time over

90 hours v/ere required to lower the moisture content of

the system from the maximum value of 131 ppm to 10 ppm

by weight. The moisture content remained reasonably

steady at a value near 125 ppm for about 3.5 hours

after injecting the 5 grams of water. This suggests

that the amount of water added was more than could be

dissolved in the entire refrigerant charge so there was

probably free water floating on the liquid refrigerant

in the receiver. The moisture content at the drier

outlet was almost as high as at the drier inlet during

this 90 hour period indicating that the moisture was

absorbed very slowl^^ by the drier. The close agreement

in moisture content between the drier outlet and drier

inlet V7as undoubtedly caused in part the bypassing

of moisture around the calcium carbide granules. Because

a relatively small amount of calcium carbide was rolled

up in steel wool and felt in the Drifreez cartridge,

there would be many paths through the drier for x^rhich

the moisture-laden refrigerant xTOuld not come in contact

with the calcium carbide. Since the solubility of water

in Freon-12 at 32 °F is of the order of 25 ppm by weight,

(A.S.R.E. Data Book, 7th Edition), free water was probably

released in the evaporator of the refrigerating system

for 50 or more hours after the injection of the 5 grams

of water because the moisture content at the outlet of

the drier exceeded this value. The moisture released in

the evaporator v/ould gradually be returned to the compressor by the refrigerant vapor as the system became

dr^’-er

,
1H9I3M A9 NOinniW d3d SidVd

i31Nt d3AdQ iV 2t N03bd Nl d3iV/V\

100

125

150

175

200

225

1H9I3M A9 NOinniW b3d SlbWd

131100 d3Aaa IV 91 N03d3 Nl d31VM

100

125

150

175

200

225
m

c

,v:

I

I

’7 X •'

"in

^
During the tests of the second specimen of the third

model of the Drifreez drier 20 grams of ivater were added

to the system at one time to determine the capacity of

the drier for absorbing water. With an initial moisture

content of 16 ppm before injection of the v/ater, the

moisture content was reduced back down to 40 ppm after

7 days operation. An additional two days of operation

did not reduce the moisture content below 40 ppm which

is not dry enough to prevent freezing-out of free water

at the expansion valve for suction pressures Delow

freezing.

Calcium carbide reacts with water to form calcium

hydroxide and acetylene in accordance with the follox'^ing

equatioh

:

From the atomic weights of the several elements it can

be shown that 43 grams of calcium carbide would react

with about 24 grams of water to produce about 50 grams

of calcium hydroxide and 17 grams of acetylene. The

test of this second specimen indicated that 20 grams

of water was somev/hat in excess of the capacity of this

drier when applied to a system operating at evaporator

temperatures below 32 ®F, whereas the theoretical equation

indicated a capacity of about 24 grams of water. The

difference may be due to impurities in the calcium carbide.

A similar test v/as made of a third specimen of the

third model of the Drifreez drier with 5 grams of water

being added at three different times. The moisture

content of the system was reduced to a loxv value with

a spare drier before adding the first increment of 5

grams. Four days later the moisture content w^as IS ppm

at which time the second increment was added. In another

four days of continuous operation the drier had reduced

the moisture content to 22 ppm whereupon the third 5

gram, injection was made. After the third injection of

5 grams 11 days were required to reduce the moisture

content to 17 ppm.

The results obtained with the latter two specimens

indicated that the third model Drifreez cartridges had

a useful moisture absorbing capacity between 15 and 20

grams of water when used in a Freon-12 system that is

to be operated at evaporator temperatures a little below freezing.
10

One test was made with the foiirth model of Drifreez

drier - the demonstration model shown as item 2 of Fig. 1

with a cylindrical glass body. The test was started v/ith

an initial moisture content of 17 ppm and 10 grams of

water were injected at one time. The drying curve is

shown in Fig, 11 for the ensuing 60 hours during which

the moisture content was reduced from a peak value of

150 ppm to a final value of about 30 ppm. There is a

likelihood that the moisture content at the drier inlet

remained near 150 ppm for three hours or more until all

of the excess water in the receiver or in the compressor

could be dissolved by the refrigerant passing through

the system. Observations were taken for only about one

hour immediately after injection of the water. Fig. 11

indicates that the moisture content varied somewhat for

repetitive readings at close time intervals. This could

be caused by random variations in the moisture content

at the receiver outlet since the liquid refrigerant and

water in the receiver would not necessarily remain wellmixed, The moisture content at the drier outlet averaged

about 5 ppm lower than that at the drier inlet, A comparison of Fig, 11 with Fig, 7 and ^ shov;s that the

fourth model Drifreez cartridge reduced the moisture

content more rapidly than the third model even though

more water was injected at one time in the fourth model.

This difference is probably accounted for by the better

contact between the desiccant and the moisture-laden

refrigerant in the fourth model in which the calcium

carbide granules filled the entire cross section of the

cartridge in the center portion.

After the 60 hours operation illustrated in Fig, 11

an additional 5 grams of water were injected into the

system. After about 5 days operation a pressure drop

of 110 psig developed between the drier inlet and outlet

indicating a stoppage. Inspection revealed that the

stoppage was caused by the calcium hydroxide deposited

in a pasty mass on the downstream side of the calcium

carbide granules.

One test was made with a silica gel drier, A total

of 25.15 grams of water was added to the system in 13

injections ranging in amount from 0,1 gram to 10 grams.

Moisture observations were made at the inlet and outlet

of the drier before and after 10 of the 13 injections

of water. The results are plotted in Figs. 9 and 10.

for the inlet and outlet of the drier, respectively.
001

i H 9 1 3 M N 0 I n 1 I W

Z 1 d3d S I a V d

i 3 “I N 1 d 3 A » Q 1 V N 0 3 d 3 N I d 3 i V M

25

50

75

100

125

150

175

200

225
1

'4
100

125

150

175

200
TT» p w ’ w

t*

s
ABSORPTION BY CALCIUM CARBIDE DRYER

150

o

Qj 130

90

HOURS
f

I"

w4
Fig, 10 shows that the system was dried to approximately the original dryness in 1 to 2 hours after injection of 0.1 gram of v/ater. Hov/ever, the moisture content

at the outlet of the drier increased appreciably for a

short time when this small amount of. water was added at

one time showing that the desiccant did not remove all

the moisture during its first passage through the drier.

The end system wetness at the drier outlet gradually

increased as the amount of water adsorbed increased.

This result is expected with an adsorption-type drier.

After 5.05 gram.s of water had already been adsorbed,

the addition of 5 more grams of water at one time caused

the moisture content at the drier outlet to rise to 55 ppm

for a brief tirfie. Fir. 10 indicates that after 26 hours

operation the moisture content was reduced to an average

value of 34 Ppm. During this 26-hour period, the expansion valve x\^ould be expected to freeze up at evaporator

temperatures lower than 32 ° F.

Pxablished data by Veltman and Waring4^ on the iwaterholding capacity of silica gel in equilibrium with

liquid Freon-12 show that the residual v/ater in Freon-12

v/ould be about 33 Ppm after adsorbing an amount of water

equivalent in percentage to that adsorbed by the test

specimen at this stage of the test. These results

indicate that the capacit^-^ of this model silica gel

drier was less than 10 grams of water v/hen used in a

Freon-12 system, operating at an evaporator temperature

a little belov/ freezing.

A comparison of Figs. 9 and 10 reveals that the

peak value of moisture content at the silica gel drier

outlet was significantly lower than the peak value at

the drier inlet following every addition of water to

the system. This relation indicates more rapid removal

of water by the silica gel drier than v/a

;

observed with

the Drifreez drier, and sug-^'ests that silica gel i/on.ld

be superior to calcium carbide for applications in

X'/hicb relatnvel;'" large quantities of v.?ater were sxiddenly added to the stream of circulating refrigerant.

^ Desirable Desiccant Properties for Refrigerant Water

Removal, by P. 4. Veltman and C, E, Waring, Refrigerating Engineering, December 1947.
o Kiscellaneous Observations

In addition to the moisture absorption characteristics of the Drifreez driers certain other features

were observed.

A Drifreez cartridge was completely filled vjith

granular calcium carbide without steel wool filters or

pads included and water wras passed through the cartridge

so that it could react with the calcium carbide. The

calcium hydroxide resulting from the reaction became so

densely compacted in the cartridge that the water flow

through the cartridge could not be maintained. When the

cartridge was cut open it was found that the carbide had

not been completely consumed and the calcium hydroxide

had to be removed with a hammer and chisel. This result

indicates that the products from the reaction of calcium

carbide and water occupy more space than the original

carbide granules and reveals the importance of providing

some room for expansion in the drier cartridges as they

are manufactured.

A Drifreez cartridge was subjected to an internal

hydrostatic pressure of 1700 psig for one hour without

rupturing the cartridge.

The effect of temperature on the capacity of the

Drifreez driers was not investigated because the

desiccant was of the chemical absorption type and,

therefore, not likely to be significantly affected by

the usual range of refrigerant liquid line temperature.

The calcium hydroxide formed by the reaction of

calcium carbide v/ith water has a secondary dehydrating

effect, but this effect is not likely to be of important

magnitude in the presence of calcium carbide that has

not reacted with water.

The filtering ability of the several Drifreez driers

tested v/as examined hy inspection of the expansion valve

strainer and the system receiver at the conclusion of

testing. The strainer at the inlet of the expansion

valve contained very little foreign material resembling

Ca(0K)2. The receiver was cut in tv7o and the interior

inspected. There was no dirt nor deliquescent material

found in the receiver. The liquid refrigerant in the

receiver contained very little impurity.
- 13

Fo tests were made of the Drifreez drier vrith re

frigerants other than Freon-12.

3 . Comparison of Electrolytic and Gravimetric

Methods of Moisture Determination

Comparison was made of the electrolytic method of

moisture measurement with the phosphoric pentoxide

(P2 O

5

) gravimetric method. The electrolytic method,

developed by Mr, E, R, Weaver of the National Bureau

of Standards, was used for the Drifreez tests because

it permitted rapid readings of more than one point

or continuous readings of one point whereas the P 20 ^ gravimetric method required comparatively large

samples for batch observation and provided an a^'^’erage

indication of the moisture content of the gas extracted

from the system over an extended period.

Several m.ethods of comparing the two t3rpes of

measurement were tried.

Method I involved a simultaneous removal of two

separate vapor samples from a cylinder containing

commercial liquid dichlorodifluoromethaae (Freon-12).

One sample was passed through a phosphorus pentoxide

absorption train (P2 O 5 method) whereas the other sample

was passed through a detector block (electrolytic method)

by means of which electrical determinations were made

at frequent intervals during the run. (The phosphorous

pentoxide absorption train is explained later,) The

valves obtained were plotted against the volume of

vapor that had passed through the absorption train, A

typical result is shovm in Graph 1, Fig. 12 , and in

Table 1. The area under the curve obtained is proportional to the weight of water in the entire sample

of refrigerant. The variation in moisture content revealed by the electrolytic measurement was believed to

be caused by the evaporation of liquid refrigerant in

the supply cylinder necessary to replace the vapor

drawn as a sample. The evaporation of liquid refrigerant both chills the liquid at the surface and changes

the moisture content of the vapor in contact with the

liquid. Table 1 shovtjs good agreement between the

average moisture content valves observed by the two

measuring methods.
- 14 -

TABLE 1

Comparison of Gravimetric and Electrolyt^ic Methods

of Moisture Determination

Moisture Content of

Freon, ppm by We ight

Method Electrolylsic Gravimetric

(or Test Procedure) Method- Method

I 4 3

ft 7

n 26 23

II 13 14

ff 24 26

ff 26 27

III 7, 7 9

tf 7, 7 9

ff 11,11 12

Method II involved placing the detector-block directly

in the line from the cylinder to the absorption train so

both apparatuses would be making determinations on identically the same sample of gas. The sketch of the apparatus

employed is shown in Fig, I 3 . (A) is the absorption train

which was flushed thoroughly with dry air before the

analysis. The absorption bulbs were allowed to reach temperature and pressure equilibrium and then weighed, A

measured volume of Freon-12 gas was then passed through

the train after which the bulbs were again flushed with

dry air to remove all traces of refrigerant. The bulbs

were again left until temperature pressure equilibrium

was attained and then weighed. The gain in weight was

the amount of water absorbed by ^2 ^

5

* Referring to

Fig. 13i, (B) shows the apparatus used to provide dry

air at point "a” in the absorption train and (C) is air

enlarged sketch of one of the absorption bulbs.

Resu] ts obtained from Method II are shown in Graph

2, Fig, li2, and in Table 1. The resulting curve was
MOISTURE

content

in

FF^EON-IZ,

PARTS

PER

MILLJON

BY

WEIGHT 25

20

15

10

5

0

30

25

20

15

10 FLOW, CUBIC FEET

FIG. 12
r
PHOSPHORUS

PENTOKIDE

ABSORPTION

METHOD

15

not as regular as uras expected and the slow flow of Freon

gas past the detector prevented taking rapid and numerous

electrical determinations. Method II still involved the

disadvantage of evaporating liquid refrigerant in the

supply tank.

Method III made use of a 1-1/2 cubic foot commercial

refrigerant cylinder which was charged with moist Freon12 vapor to a pressure just below the saturation pressure

of Freon-12 at room temperature (70 psig). With this

source of moist'’’ro-laden gas and the same apparatus as

shown in Fig. 13 , moisture determinations were made

electrically beiore and after each gravimetric determination, and the results were compared. The size of

the cylinder made it possible to run duplicate and

triplicate analyses before having to recharge the

cylinder. Results are shovm in Table 1,

Tests were not made for all the ranges of moisture

level. However, the results of these comparison tests

indicate that the agreement between the two methods is

close enough to validate the conclusions reached in

the tests of the Drifreez drier.

VI. Discussion

The test results showed that the Drifreez drier

removed moisture from liquid Freon-12 in the test

system to an end drimess of less than 5 ppm. This

permits operation of a Freon-12 refrigerating sT^stem

v/ithout release of free v;ater at the expansion valve

for refrigerant temperatures dov/n to approximately minus

15°F, as indicated by the saturation curve for Freon12 in the A.3.R.E. Data Book, 7th Edition, page 329.

To compare the Drifreez drier with another drier,

a silica gel. drier of knovm trade acceptance was selected

Test results showed that this drier, when first installed

dried the test S3"stGm to below 5 ppm. The selection of

size of the silica gel drier for comoarison was based

on the respective manufacturers^ rated maximum horsepower capacity and on the size of flare connection. The

length of the silica gel drier was about 1-1/4'* longer

and the diameter was 5/^'** larger than that of the Drifreez drier. Those differences made the total internal

volume of the silica gel drier approximately twice that

of the Drifreez drier. The Drifreez drier used a steel

wool cushion for the desiccant and a filter which

occupied a relativel3

r large space in the cylinder of
- 16 -

the drier whereas the entire space in the cylinder was

occupied by drying agent in the silica gel drier. Thus,

the drying ability of approximately 43 grams of calcium

carbide, which removes water chemically, was compared

with approximately 12 cu. in. or ll6 grams of silica

gel which removes water by adsorption. Based on price

quotations obtained December IS, 1953 from an established refrigeration -wholesaler in Washington, D. C.,

normally stocking the two driers involved in these

tests, in lots of 25 or more, the wholesale price of

the Drifreez drier was |3.5^ each, whereas that of the

silica gel drier was |3*60 each.

Except for the drying rates observed at conditions

of peak wetness, the instantaneous drying rate of both

driers was small enough that the relative drying rates

could best be compared by observing the number of hours

required for each drier to reduce the system wetness

after comparable amounts of moisfure have been removed

by each drier.

Beginning with a residual moisture content of

3 ppm after absorbing 3.1 grams of water during the

first 127 hours of the test, the first specimen of the

third model Drifreez drier reduced the system dryness

to about 74 ppm in approximately 26 hours after

another 5 grams of water were added as shown in Fig, S.

In the same length of time, the silica gel drier reduced the system moisture content to about 34 ppm after

5 grams of water v;ere added when it had already adsorbed

5,05 grams of water during the first 141 hours of the

test and the residual moisture content was l6 ppm after - that time as shown in Fig, 10. This comparison indicates more rapid moisture removal by the silica gel

drier during the period cited. However,, the Drifreez

drier continued to absorb moisture and, in 96 hours

after adding the 5 grams of water, had reduced the

moisture content of the system to approximately 13 ppm

at the drier outlet. The relatively rapid drying of

the silica gel dryer is further shown in Fig, 10 by

the drop in system wetness to about 37 ppni in 2-1/2

to 3 hours after the 5.0 grams were added at 141

hours.
Fi,2;. 10 shows that after the test had been in prog

ress for 74 hours, but prior to adding 2,0 grams to the

system with the silica gel drier, the syst-em wetness

was approximately 16 ppm, and 6? hours after adding the

2.0 grams, making a total of 5.05 grams, the system

wetness vras again l6 ppm as indicated at an elapsed

tim.e of 141 hours in both Figs. 9 and 10. Data by Veit

man and Waring ^ show that l6 ppm was an equilibrium

value of water in liquid Freon-12 for the percentage

of water adsorbed by the silica gel at this stage of

the test. By comparison, the Drifreez drier vras able

to reduce the system wetness to an average of 13 ppm

after absorbing ^.1 gramas of water as shovm on Fig. S.

The test results showed that the third specimen

of the third model Drifreez drier, after absorbing

15 grams v.’-as able to reduce the system wetness to

17 ppm whereas tests of the second specimen of the

third model Drifreez drier showed that when about

20 grams of water was absorbed b^^ the drier the system wetness was reduced to apnroximately 40 ppm.

Veltman and Waring ^ show that the silica gel drier

could be expected to adsorb only 5 grams of water

without exceeding an end dryness of 17 ppmx, and

about 12 gramis of water without exceeding an end

dryness of 40 ppm.

The silica gel drier was able to reduce the

system wetness to approxim.ately 60 ppm after adsorbing 25.15 grams compared to 40 ppm for a Drifreeze

drier after absorbing 20 grams. Since freezing problems occur at or below 20 to 30 ppm in Freon-12, the

ability of the driers to reduce system wetness below

these levels was of primar^r interest in this investigation. However, the abilit^^ to dry to a s^’^stem

wetness in the range above 20 to 30 ppni might be

useful in service or maintenance work as an initial

step in drying a very wet system. It XTOuld be expected

that the Drifreez drier would lose its ability to remove water from the refrigerant when all of the calcium

carbide had been consum.ed whereas the silica gel could

continue to adsorb some water but with an ever-increasing end wetness of the system.
VII. Conclusion

The dr^^'in^ tests of’ various r.odeIs of the test

specimen Drifreez (calcium carbide) drier, compared with

a 12 cu in. silica s;el drier, lead to the following

conclusions:

1. For a given end dr^mess belovj 30 ppm, the

Drifreez drier removed and held more water than the

silica gel drier. Moisture levels belov; 30 ppm are of

particular interest in Freon->12 s^/stems operating at

32° F or lower because freezing at the expansion valve

is probable at higher moisture levels.

2. The instantaneous drying rate of the silica gel

drier was higher than that of the Drifreez drier.

3. For equal m.oisture removal betx^een 5 grams and

20 grams in each of the tv70 types of driers, the Drifreez

drier reduced the system wetness to a lox-^rer end point

than the silica gel drier.

4. The canacit^^ of the third model Drifreez drier,

containing 43 grams of commercial grade calcium carbide,

for absorbing X''rater was between 15 and 20 grams of water

when used in Freon-12 systems operating at evaporator

temperatxrres no lower than 20° F.
19

PART B. HAZARD TESTS

I. Introduction

As requested bv the Office of The 'Quartermaster

General, tests were made to determdne whether or not

hazard resulted from the use of Drifreez driers (with

calcium carbide as the desiccant) in a refrigerating

system employing dichlorodifluoromethane (Freon-12)

as the refrigerant.

Moisture removal by calcium carbide driers is

achieved b3

r chemical reaction (absorption) which results in the formation of acet^dene and calcium hydroxide. The main objective of this investigation was to

evaluate the degree of hazard due to the presence of

acet^rlene in the refrigerating system and to determine

whether or not a secondary product, copper acet3’’lide,

was form.ed in the S 3^stera from, the reaction of the

acetylene with the copper in the piping circuit.

First, the products form.ed bv chemiical reaction

as a result of using Drifreez driers for the removal

of m.oisture in Freon-12 refrigerat ion unit were determined. The prim.ar3

’’ products of the reaction of

calcium carbide and water were found to be acet3/'lene

and calcium hn^-droxide. As will be shoiAm later, there

was strong evidence that the acet3

^ 1 ene reacted wi.th

the copper in the S 3 ''stem to produce copper acet3'-lide.

It was assumed for tr.e purpose of this investigation that acetylene had no effect on lubricating

oil, refri.gerant , motor windings, or internal parts

of the systeiT. other than the copper tubing. The next

step was to stud3

;- possible hazards that might arise

from these products and the degree of danger under

certain conditions.

II. Study o f the Pro ducts of Reaction

1 . Acetylene

Acetylene formed hy the reaction of calcium carbide and water, circulated in the system, xvith the

Freon-12. Any excess of acet3 'lene over that v/hich

could be dissolved in Freon-12 would be expected to be
tra^-ned ir the recf^iiyar and condenser. ^ one-third-ton

air-cooled condensing- unit X'/ith a specially built evaporater was used for the tests. Gas samrles x-zere extracted

for analysis and the acet^rlene content in the Freon-12 was

determined by the mass spectrometer from^ time to tim^e both

dur-inm the drying tests and durinm other special tests

m.ade for the purpose of evaluating acet^'-lene production

xn the refrigerating system.

One gas sample xvas drax^m from the vapor space in the

top of the receiver after about ^9 grams of water had

been injected into the system.. At this time a total of

seven Drifreez driers had been used in the system..

( 35.15 gram.s of xvater adsorbed by three silica gel driers

were not included in the F9 grams mentioned above.) The

analysis of the gas showed the acetylene content in the

sample to be 12.95 percent hy volume.

Three yas samiples were drax^m from the discharge line

betx/\reen the oil separator and the condenser when the

refrigeration unit x^^as in operation. After about 26

grams of xvater had been absorbed by five Drifreez driers,

the acetylene content of the gas xvas 0.^2 percent, by

volume. The second gas sample indicated 2.04 percent

acetylene, by volume, after ^-9 grams of xvater had been

absorbed by seven Drifreez driers. The third sample -indicated 7.6 percent acetylene, by volume, after 121

grams of xvater had been absorbed bx^ nine Drifreez driers.

Table 2 shoxvs, in chronological order, the amount

of water added, txrpe and purpose of driers used, leakages in the system., and acetylene content. The table

xvas prepared from the drying test data. It can be seen

that silica gel driers and Drifreez driers were used

not only for observations on the drier performance,

but also for dri^-ing out the system either after a test

specimien had ceased to dehydrate the system, or preliminary to beginning a new test. At the beginning of

the test, 4.5 pounds of Freon-12 were charged into the

system. Additional refrigerant had to be added to the

system later because of leaks xvhich developed during

the test, and the amount added xvas not measured. Other

niechanical difficulties interrupted the tests and made

xt necessary to replace the specimen driers twice during the course of the tests. Because several of the

tests xvere not made to determine the total absorbing

capacity of the driers, precise relationships between

tv

e

acetylene content, amount of water injected, and

amoxxnt of desiccant consumed xvere not established.
No literature was found on solubiliti^ of acetylene

in Freon-12 and no tests were made to determine it.

Table 2 shows that 52 grams of water were added to

the system after the last leak occurred in the system on

November 7 necessitating the addition of some Freon. The

eauation for the reaction of calcium carbide and water

ohows that the absorptir^n of 36 grams of water produces

aroroxiraately 26 grams of acet3?"lene. Thus, the absorption of 52 grams of water after Novem.ber 7 would produce

arnroximately 37.6 grams of acetylene or O.OS3 lb. If

the system were completel3

?' empty on November 7 when the

leak was discovered and a full charge of lb of

Freon-12 were added at that time, the system would contain a little over l.F percent, by weight, of acetylene

on February 3 after 52 grams of water had been absorbed.

A concentration of l.i7 uercent, by weight, of acet^/lene

in Freon-12 would correspond to a concentration of about

6.5 nercent acet^rlene by volume at atmospheric pressure

with both materials in gaseous state.

The measured concentrat' on of acet^^lene b^r volum.e

in the discharge line was 7.6 percent on February/ 3

after 52 grams of xvater had been absorbed subseauent to

the last addition of Freon-12 to the sA'-stem. The composition of the m;ixture of Freon-12 and acetArlene vapors

in the discharge line should be about the same as that

of the liquid in the receiver v/hen the refrigerating

unit is in steady operation. The difference between the

observed volum.etric concentration of 7.6 percent acetA'’-

lene in the discharge line on FebruarAr 3 and the value

of 6.5 percent computed fromi the anticipated am.ount of

acetylene formed in the system is not considered significant, but may indicate that the systemi v/as not com.- pletely empty on November 7 A-rhen the last leak occurred.

The samples of the Freon-12-acet3rlene mixture analA''zed

on November 14 and 17 shoA-^jed a concentration of 2.04 percent acetA'-lene in the discharge line and 12.95 percent

in the vapor space in the receiver. This comparison

suggests that all 0/ the acetA’’lene produced was not

readil^r absorbed in the liquid Freon-12 in the receiver.

As mentioned previously/- in this report, no measurement

of the solubilit^" of acetylene in liquid Freon-12 was

made

.
Table 2. Tabulation of Water Addition and Driers

Used on 1 /3-ton Refric:erating Unit

t ^ T ? ?

' ,, ^Tvoe of new' _

’ location , Date ' Water , drier Purpose , of re- t Acetylene,

, added , installed * drier ,frigef-antT by volume

, r » T leak r

r Gram.s t 1 T Percent

Sept T •

T f t t 16 t — t DriDreez t Test T — T — 17 1 .0 t t T — f — 19 t 1.0 f f f — t — 24 T — T Dri freez ? Test t Valve t — 24 T 0.1 t f T

'—

T — 26 T 0.1 f t T — y — Oct

t T r » T 1

t 0.1 T t t — y 6

f — t Drifreez f Test T — y — 14 T — t Do T Do t — y — 15 t 0.1 T r t — y — 15 T 0.1 T f T — y — T t t 1 y 15 t 0.6 t t t y 16 f 0.1 f r t — y ...

16 t 0.1 f ! f — y — 17 t 0.1 f f f t — 17 T 2.0 t t T — y — f !

r T y 20 t 5.0 t f t — y 24 t 0.1 f t t —- y — 24 T 5.1 f T t Line y . — . 27 t ^.1 t T T — y — 27 f — f Drifreez ! To dry f —_ y —

2^

! t T system t r

t — r Silica Gel T Test f — y — 2g T 0.1 T T 1 ^ y ^ _ 2S f o.s t ! f — y 29 f 0.1 T t t — y — 29 T 0.1 t t t — T — f T t f y 29 t 1.0 t y t — y 30 T 0.1 t t f — y 31 f 0.1 f t t —_ y 0.^^2

31 T 0.75 T ! T — y ( sampled from

31 f 2.0 f r t

’ discharge line

)

Nov. T T t t y 3

t 5.0 t r T — — — f 4

f 5.0 t f t — y ^ _

5

T 0.1 f t T y — . —

5

T 10.0 f t f — — y 6

'

’ Silica Gel’ To dr3

'‘ ’

’ '

' system '
Table 2 (Continued)

r t

^ I Water

I added

f

’T^'-'pe of new^

' drier ’

' 'aistalled ’ Purpose

of drier

f f

’ Location ’

’ of re- ’

^ fri^erant '

' leak ’

Acetylene

,

by volume

Lov. ^ Grams ’ 6 ’ 20.0 t Drifreez ’ Test

7

t t 7

! t 7 7

T T Silica Gel’ To dry

f 7 7 system

7

T T Drifreez ’ Test

7 ’ 20.0 ’ '

f T T

T

T

t

?

t

T

t

T

t

t

f Detector ' block ’ _ _ — ’

T

T

t

?

Percent

14 7

7

7

7

7

7 17 7

7

7

7 19 ’ 5.0 ’ Drifreez

24 ’ 5.0 7 2^ ’ 5.0 7 Dec

.

r 7 9 ’ 10.

U

’ Drifreez

12 ’ 5.0 7 19 ’ 2.0 7 Feb. 7 7 3

7 7

t T

t t

f t

t t

T ?

f t

t t

? T

' Test ’

T T

T ?

t t

’ Test ’

f f

T t

t t

T T

t f

t T

f t

’ 2.04

’ ( sampled from

* discharge line

)

’ 12.95

’ (receiver)

t

t

T

’ 7.6

’ ( sampled from

’ discharge line

)
» 2 /, -

2 , Calcium H^^droxide

Since the products formed hy the reaction of calcium carbide and water are knov/n to be calcium hydroxide and acet^rlene, no analyses were made to identify

calcium hydroxide as one of these products.

Calcium hydroxide formed by the reaction of calcium carbide and water should be retained in the drier

to avoid clog^^ins: of the system or damage to the com.- pressor

Observation made on the glass cylinder Drifreez

drier (the demonstration model) indicated that the steel

wool filter in the drier had retained large am.ounts of

the calcium h^rdroxide in a pasty form. When the thermostatic expansion valve was replaced by an automatic

expansion valve after eight Drifreez driers were used,

the strainer at the inlet of the former was inspected.

A very small amount of the paste-like substance which

appeared to be calcium hydroxide was found. The amount

was considered to be negligible.

The receiver was cut in half at the conclusion of

the tests and its interior was inspected. Calcium hydroxide was not found.

3 . Copper Acetylide

The tubing used in most Freon refrigeration systems is made of copper. It was not known whether or

not the acetylene produced by a Drifreez drier would

react with copper to form copper acetylide in the

presence of Freon-12 in a refrigeration system.

The copper tubes used in the refrigeration unit

employed for the drying tests were cut open at the end

of the drying tests. The inside surface of the tubing

throughout the system was found to be coated with a

black material. This coating was thicker and darker

in color im.mediately downstream from the drier.

The black material was identified by means of the

following analyses:

(a) When pieces of tubing with the black coating

were heated over a bunsen burner, small puffs of smoke
were emitted from the open ends. Upon close investigation, after the tubinv had been permitted to cool, it

was found that the black coating had disapneared. Subsequent reheating of the same tubin'^ did not cause any

further puffing.

(b) i/ifhile one of the pieces of tubing taken from

the refrigeration system was being cut v/ith a hacksaxv,

a mild explosion occurred which produced a dull yellow

flash and some greyish-black fumes. A sm^all spot of

soot remiained on the sav/blade. It is unlikely that

acetylene gas caused the explosion because the mass

spectrometer analysis of the '^ases liberated upon heating the coated copper tubing showed only minute quantities of acetylene.

It has been established'^^ that dry cuprous acetylide is exceedingly explosive and is sensitive to shock,

friction, and heat.

(c) Mien pieces of the copper tubing v/ere heated in

vacua, much carbon dioxide, some carbon monoxide, and

hvdrogen were evolved as determined b^r a miass spectrometer.

This indicated tl at the black material was probably an

organic substance. A minute amou.nt of acetvlene was also

indicated, but this may have been desorbed from the surface .

(d) Pieces of the coated copper tubing were treated

with dilute hydrochloric acid in vacua and the gaseous

product was examined in a mass spectrometer. The record

from the analysis showed large ion currents at masses

24, 25 , and 26 in the correct abundance ratio for

acetylene. The ion current at mass 26 was 2200 divisions

or over two-thirds full scale and acetiO. ene was by far

the most abundant component observed. In contrast, only

a sm.all amount of acetylene was found when the coated

copper tubing was heated without the acid treatment .

It is well knowR'^ that the addition of dilute acids

to copper acetylide releases acetylene according to the

equation

'^Handbook of Dangerous Materials by N. Irving Sax.

The Chemistri^ of Organic Compounds by James B. Conant
(e) Some of the strips of the coated copper tubine!:

were treated with dilute hydrochloric acid and the gas

evolved was bubbled through an amrnoniacal solution of

cuprous chloride v/hich was kept in a reduced state by hydroxyl amine hydrochloride. A red precipitate was observed.

It is knovm ~ that v^hen acet3?’lene is shaken with

amrnoniacal cuprous or silver solutions, precipitates of

m.etallic acetylide are found as indicated by the following equation:

C2H2 + Chi2Cl2 + 2NH^0H -> CU2C2 + 2NH4CI + 2H2O

Thus, the gas liberated upon treatment of the coated

Conner tubing v/ith dilute h^/drochloric acid was confirmed

by an independent chemicaj method as being acetylene.

The conclusion dravm from the above observations and

experiments ivas that the black material covering the inner surface of the copper tubing contained copper acetylide which was formed by reaction of the copper with

acetylene liberated by the several Drifreez dehydrators.

Referring to Fim. 1, both tubes numbered "I"’ were

taken from the liquid line upstream of the drier and

those numbered ”2” were taken from the liquid line downstream of the drier and were cut longitudinally. A

piece of new unused copper tube is numbered ”3" and is

shovm for comparison purpose. The hacksaw blade used

to cut the tubes is numbered "4”. The arrow pointing

at the black soot on the hacksaw indicates the location

at which explosion occurred v^hile cuttino; one of the

tubes numbered ”1", The arrow pointing at the lower

picture of tube numbered ”1” indicates the part of tube

that was being cut when the explosion occurred.

III. Results of Hazard Tests

Among the three products (calcium h3rdroxide,

acetylene, and copper acet3;’lide) formed b37- chemical

reaction of calcium carbide, water, and copper in a refrigeration svstem, calcium hydroxide was not found to

have escaped from the Drifreez drier in sufficient

quantity to cause clogging of the lines or dam.age to

the compressor, nor was it considered to be hazardous

chemically in the refrigeration S 3^stera. However, further investigations were made to determine the


conditions that mifrht be hazardous and to stud3

r the

possible extent of damage by acetylene and copper acet^^-

lide. Possible hazards could be explosion of copper

acetylide, dissociation of acetylene, and explosion of

air-acetylene mixtures.

1 . Dissociation of Acet-^^lene in Presence of Freon-1 2

Tests were made to find the amount of Freon--12

necessar3

^ to prevent dissociation of acet3’’lene under 300

pounds per square inch gaye pressure. According to t-'^e past experience of the acet^flene manufacturing^ industr^^,

the causes and conditions for the spontaneous dissociation of aceti'-lene were not precisel^^ knovm. A comprehensive studv of the conditions of heat pressure, friction,

or impact that would initiate dissociation of acet^'^lene

would be hazardous so it was decided to detonate a sample

with a spark plug, if possible. Although it happens infrequently, a spark ma^T- be produced in the refrigerant

vapor in a hermetically-sealed motor-compressor unit by

failure of the windings of the motor.

The apparatus emploT/ed is shox\m in Fiy. 2. All of

the tubes of the system shown in the figure were filled

with water b^,'- means of vacuum pump C. Acet^’-lene was

produced in the generator A by dropping water from funnel B. The acetylene gas thus produced and Freon-12

from tank J were charged into the 100 ml burette F to

obtain the desired miixture. B3/ manipulation of valves,

the acetyl ene-Freon mixture was charged into bomb H.

This xvas accomplished b3

^ means of vacuum created in the

S3^stem b3

'- the vacuum pump C. The gas mixture was compressed usini3- air from tank 1 through reservoir E to

bomb H by manipulatinp; suitable valves. The mixture in

bomb H was ignited b3

'' a m.arine spark plug K installed

on top of the bomb C. The spark plug was fastened by

a ball sleeve fitting so that if an explosion toolc place

the spark plug would serve as a safet37

" valve.

Under 300 pounds per square inch gage pressure, the

gas m.ixture did not explode when its composition was C

percent Freon-12 and 92 percent acet’-’lene (by volume)

measured at atmospheric pressure. Under the same pressure, it exploded most of the time when the composition

was 7 percent Freon-12 and 93 percent acet3’'lene (’03?- volume) measured at atmospheric pressu.re. Inconsistenc3

'- in explosiveness at the latter concentration was believed to be due to differences in the time lag between
charging the gases and igniting the mixture under pressure

during the several tests. Appreciable change in composition of the mxixture v/as expected after the pressure was

applied because of the difference in solubilities of

acetylene and Freon-12 in v/ater and the change in the relative partial pressures. In order to find the true composition under pressure, a gas mixture ox f percent Freon-12

and 92 percent acetylene (b^^ volume) was charged in the

bomb and pressure was applied by exactly the same proceedure. At the time when it v/ould normally be ignited,

the mixture was extracted from the bomb into a glass

tube sampler for analysis. The mass spectrometer showed

that the real composition under pressure v^/as 9.2 percent

Freon-12 and 90.^ percent acetylene (by volum.e) .

2 . Explosiveness of Ideal Fixture of Acetylene

and Air in Presence of Freon-12

The amount of Freon-12 necessary to prevent explosion

of ideal mixtures of acet3xlene and air under one and two

atmospheres of pressure was determined.

The apparatus emplo^/ed was simxpler than the one used

for dissociation of acetylene under pressure. The desired composition of a mdxture of Freon-12, air, and

acet^rlene was obtained in a 100 ml burette. It was then

transferred to a spherical explosion pipette b^?- means of

a levelling bottle. One or two atmospheres of pressure

was applied to the gas mixture bv the levelling bottle

and it was ignited by a spark produced between electrodes

in the pipette. A rubber cap placed on the charging

inlet of the pipette served as a safet}/- valve when

explosion occurred. Mercury instead of water was used

in the pnpette levelling bottle and the lines.

It was found that under one atmosphere pressure, a

concentration above 12 percent Freon-12 (by volume)

prevented explosion of ideal mixtures of air and acetylene (7 to 13 percent acet^dene by volume). Ideal mixtures of air and acet^dene exploded in mixture with 12

percent Freon-12 (b]/- volume).

Under two atmospheres of pressure, mixtures containing more than I6 percent Freon-12, b^x volume, did not

explode with ideal mixtures of air and acet^/'lene.
FIG. 2



3 • Explosion of Copper Acet'^^lide

Attempts v/ere made to explode the deposit found on

the inside surface of the copper tubin,p taken from the r frigeration system. Heating, scratching, and tapping di

not cause it to explode.

Hox'/ever, a mild localized explosion v/as observed

while sawing one of the pieces as described earlier in

this report.

Copper acetylide oxidizes rather rapidly when expos

to the a-’r which may accomit for the inabilit3

'- to deliberately produce an explosion by heating, tapping, or

scratching the deposit.

4 . Explosion of Compressor and Copper Tubin?^

A hermet icall^T- sealed l/5-horsepox^rer compressor was

subjected to an explosion of an ideal mixture of air and

acet\''lene to determine its ability’’ to X'/ithstand such an

explosion or to examine the extent of its damage.

Fig. 3 shows the anparatus eniploved. Acetylene was

'generated in the cartridge C, filled with calcium carbid

bxr feeding water from cx/linder E. A separator D was use

to separate water and calcium hxrdroxide from acetylene.

After acetylene was charged into the compressor in the

explosion test chamber A, air was charged from compressedair tank, F, which can be seen behind the gage board G.

The initial pressure to which the corapressor was charged

with acetxrlene and the final pressure to which it was

charged b].'' adding air were selected so that the mixture

would fall in the ideal mixture range (from 7 to 13

percent acetxxlene, by volume). No Freon-12 icas present

in the compressor during these tests. The compressor,

charged with the mixture, "'’as heated b3

* heat lamps to a

tem.perature higher than the ambient temperature, •-/hich xvas about 40'’ F. The mixture v/as igr.ited by an ordinary

automobile spark plug.

Under l60 pounds per square inch gage pressure, the

mixture of air and acetxrlene (7.2 percent by volume)

exploded on ignition. The explosion caused the compressor to fail at the welded joint of the steel housing.

Fio-. 4 shoxfs the interior of the test chamber after the

explosion. Fig. 5 shows the close-up of the damaged

corapressor. A and B are the upper and lower half of the

O.

CD

(DO.

CD
- 30 -

housin.e^, respectively. C is the motor and compressor.

D is the spark plug, shovm in Fig. S only.

Prior to the ahoYe test during v;hich explosion of

the compressor took place, explosion or rupture of l/4-in.

copper tubing occurred while acetylene was being generated

The line immediatel3r after the generator was clogged up

by calcium hydroxide which was pushed into the line by

rapid generation of acetylene. This created high pres-,

sure in the line and probably/' caused the explosion.

Pieces of copper tubing which failed are shoivn in Fig. 6.

The arrows indicate the direction of gas flow. It x^as

not kno’m x^hether the explosion x^^as caused by dissociation of acetylene or bx^ detonation of copper acetylide.

The localized areas of rupture indicate ver^'- sudden rises

in pressure because the tube X'/as not noticeably stretched

even a short distance from the ruptured areas.

IV. Discus s ion and Conclusions

The hazard tests developed the following information

regarding the effects of using Drifreez driers (calciuim

carbide) to absorb water in a Freon-12 refrigerating

system:

(1) The normal products of the reaction of calcium

carbide and x^ater; namelA^, acetylene and calcium hydroxide, were produced in the sx'-stem.

(2) Nearl^^ all of the calcium h^^droxide ims retained

in the drier cartridge. That which escaped from the

drier did not present an3

^ problem of clogging in the

strainers or refrigerant lines.

(3) As much as 13 percent, bx^ volume, of acetylene

xvas found in the vapor space of the receiver after 09

grams of water had been absorbed in seven Drifreez driers.

(4) At 300 psig pressure, 9 percent or more of

Freon-12, b^'' volxime, prevented the ignition of acet^/'lene

gas by means of an electrical spark.

( 5 ) At atmospheric pressure concentrations of

Freon-12 above 12 percent, b^r x^^olume, prevented explosions of ideal mixtures of air and acetylene when a

spark discharge was used for ignition. At tx-tro atmospheres

pressure, concentrations of Freon-12 above I 6 percent

Mere required to prevent similar explosions.





Ik

Uit
- 31

(6) The explosion of a mixture of 7 percent acetylene and 93 percent air reptured a typical hermetic compressor-motor housing. Copper tubing was also ruptured

either by the dissociation of acetylene gas or copper

acet3rlide.

(7) Copner acetvlide vra.s formed as a coating on the

inside of the copper tubes in a Freon-12 refrigerating

system in which about 121 grams of water had been absorbed by 9 Drifreez driers. The presence of copper

acet^^lide was established by:

(a) The sudden disappearance of the black

coating accompanied b^^ mild puffs or

explosions upon being heated.

(b) A small explosion during the process

of cutting the coated copper tubing

with a hacksaw at the conclusion of

the test.

(c) The production of acet^^-lene in appreciable quantities when the coated tubing was treated in vacuum with dilute

h3’’drochloric acid as compared with only

miinute quantities of aceti^lene released

on heating pieces of the copper tubing

in vacuum without acid treatment.

A principal hazard arisinf;" from the use of calcium

carbide as a drier is related to the highly explosive

nature of copper acet^^lide who’ ch was shovm to have been

deposited in the refrigerating s^^stem. 1/ifhile it was

not shown that the copper acetylide would explode in

a normally-operating Freon-12 refrigerating system, and

no information has come to our attention to indicate

that an^?- of the thousands of s^^stems in use that contain Drifreez driers as reported b^,^ the m.anufacturer has

experienced an explosion, it was shoim that cutting of

a copper refrigerant line internally coated v/ith copper

acetvlide did produce a mild explosion. Such an explosion might occur during the repair of an^’- simdlar

system in the field to which heat, friction, or sharp

blows were applied to copper tubing from which the refrigerant had been remioved.

The absorption of 121 grams of water in a l/3-hp

refrigerating system, as in the case of these tests,

probably represents an exaggerated example of drying

refrigerating units although such an occurrence in

practice is not impossible.
> •-

With one possible exception, it is considered very

unlikely that a combination of circumstances would ever

exist in the use of a refrigerating system that would

produce an explosive mixture of air and acetylene;

Freon-12 and acetylene; or air, Freon-12, and acet3rlene

inside the system even though spark discharges or local

heating might occasionally occur in a hermetic motorcompressor housing that could act as an ignition source.

In view of the relativel3

’’ low precentages of Freon-12

required to inhibit explosions of acetylene or acetylene-air mixtures, it is probable that difficulties

with high discharge pressure would occur in a refrigerating system long before enough acet3'-lene was present

to explode in mixture with Freon-12 or before enough

air could be drawn into a system to approach explosive

mixtures with acet3’'lene and Freon-12.

A calcium carbide drier should never be used to dry

air b3

'- itself in a refrigerating S3rstem because, without

the Freon-12 being present, an explosive mixture might

readil3r be produced. In new installations or in service

work on repaired systems, dr3^ing is often done by circulating air throughout the S3’"stem with driers in the

circuit prior to charging with refrigerant. The use of

calcium carbide driers in such a manner, because of the

wide range of explosive mixtures of acet3/lene and air,

could create a serious hazard, either from possible

internal explosion or from external ignition of the gas

being purged from the S3'-stem.

It is recognized that several conditions would have

to be favorable for explosion hazards, resulting from

use of calcium carbide in a refrigerating S 3/stem using

Freon-12, to actuall37- culminate in an explosion. A

conclusion drawn from these tests is that it is possible

for these conditions to be produced or to occur simultaneously in a normal refrigerating system.

The Drifreez driers examined in these tests were

constructed in such a manner that expansion and heating

of the calcium carbide particles was adequately distributed so as to prevent abnormal pressures or hot

spots in the drier. The use of calcium carbide as a

filler for drier cartridges without regard for the

expansion and extreme heating that occurs as water is

absorbed would present a serious hazard and should be

avoided
33 ~

PART C, VIBRATION TEST

I . .introduc tion

The Office of The Quartermaster General requested

that vibration tests be made to determine v/hether or not

the Drifreez cartridges were able to withstand the kind

of vibration encoir.ntered in field refrigeration units

driven by four-cycle gasoline engines without excessive

breakage and pulverization of the desiccant.

Representative vibration of dryers installed in

field refrigeration units operated by four“C3rcle gasoline

engines was difficult to determine because of the many

variables involved. Probable factors affecting the

-frequency and amplitude of the vibration which in turn

would affect the acceleration x^ere considered to include

the follox'j'ing:

1. Speed of rotation of 4 cycle-gasoline engine.

2. Mounting of refrigeration unit.

3. Mounting of other components of refrigeration

unit

4. Location, position (vertical or horizontal)

and mounting of dryer.

5. Size, shape, and length of tube connecting

the various components,

II . Description of Test Specimen

The specimen Drifreez dryer xvas- identified as follows:

Drifreez Cartridge

Berna Corporation

3

/gtf flare connections

6 cubic inch

An exterior viex'/ of the specimen on the vibration

table is sho'wn in Fig. 1, New cartridges X'/hich were sealed

to prevent deterioration of the dehydrating material xvere obtained from the manufacturer. The specimen used for

this test was of the txrpe originally submitted for this

investigation. The desiccant used in the brass cartridge

was rav calcium carbide particles ranging in size from

0,4 in, to f mesho
- 34 -

Approximately 43 grams of the desiccant were dispersed

on a layer of fine steel wool (See Fig. 2). The size of

particles and total weight of the desiccant varied among

the units and it was not uniformly distributed on the steel

wool. The steel wool containing the desiccant was rolled

(See Fig. 3) and inserted in a bra'=s cartridge which had

64 X mesh brass wire screens soft-soldered into both

ends. At the outlet end of the cartridge a steel wool

pad vras sandwiched between the screen and steel wool

roll. The ends were soft-soldered to the cylinder.

Baffles were inserted between the caps and screens at

the two ends to support the screens.

The physical dimensions of the drier cartridge were

as follows:

Length 6-3/4’'^ (total)

Diameter....... 1-5/16” (O.D.)

Weight, about -254 grams (total with caps)

Volume, about - 6 cu. in. (screen to screen)

III. Test Method and Procedure

Since it was desirable to make an accelerated test

and because the vibration in actual use with a gasoline

engine is subject to so many variables^ an arbitrary

selection of the most severe vibration that could be

obtained with an available vibrator was used. This was

a simple harmonic motion with a frequency of 55 cycles per

second and a total amplitude of 0.075 in which corresponded

to a maximum acceleration of 373.5 ft/sec2 or 11.6 g. A

2 -cylinder, 4-C 3?-cle gasoline engine running at 3300 rpm

would have 55 power strokes per second and could produce

a vibration similar to that selected. The vibrator used

was identified as All-American Vibration Fatigue Testing

Machine, Model 25 HA, manufactured by the All-American

Tool and Manufacturing Company, The vibration machine is

shoivn in Fig. 1. A vibration test at resonant frequency

was not made because such a frequency would usually be

avoided, if possible, in an actual installation.

Two driers were installed horizontally in parallel

liquid lines connected to the receiver outlet of a 3/4

hp refrigerating unit. One of the driers v/as subject

to vibration by the vibrator and the other V7as subject

to the normal vibration of the test 3/4 H.P. refrigerating

unit only, which was negligible. Freon 12 was used as

the refrigerant and the flow in the parallel liquid lines

was alternated ever3

r 30 seconds from one drier to the

other by means of solenoid valves shoivn in the background


Fig. 2

s^

CT)

ti H

P-4

VIBRATION

- 35 “

of Fig. 4 and a timing switch not shovm. Alternating the

liquid flow between driers caused a sudden application of

the liquid pressure to the interior of the driers or

pulsing action similar to that occurring in a refrigerating unit when it operated intermittently. The

refrigerating unit was operated continuously v/ith a

suction pressure of about 40 psig during the vibration

tests.

In order to detect and compare the pulverization of

desiccant resulting froi]i vibration, a filter containing

two layers of fine n^j-lon cloth was placed downstream of

each drier. These filters were made from the same piece

of nylon cloth. The filters are shown in the foreground

of Fig, 4. The nylon cloth, cut open, is shovm in Fig. 5.

After assembling the refrigerating unit, the system

was thoroughly dried out using a vacuum pump. Although

several breaks in tube connections occurred during the

test as a result of the vibration, the system probably

remained fairly dry because it was operated at a positive

pressure throughout the refrigerant circuit.

The drier to be vibrated was securely fastened at

the middle of the cartridge to the table of the vibrator.

{See Fig. 1). The simple harmonic motion was horizontal

and the center line of the drier was perpendicular to

the table movement.

The total amplitude was set at 0,075” at the start

of the test and there v/as no need of further adjustment.

A General Radio Strobotac was used to set the frequency

at 55 cycles per second by illuminating the shaft of the

vibrating mechanism under the table. The specimen drier

was subjected to vibration for a total of 100 hours.

IV. Test Results

Fig. 2 shows the desiccant on the steel wool as it

appeared at the conclusion of the test. The specimen

on the left was not subject to vibration and the one on

the right was subject to vibration for 100 hours. No

difference was observed between the portions of desiccant

in the two specimen driers.

Fig. 5 shows the two nylon cloth filters cut open;

the one on the right was used on the liquid line downstream of the drier attached to the vibrator table,
and the one on the left was installed on the other leg of

the parallel circuit doiAmstream of the drier not subject

to vibration. Following the test the nylon filter installed after the vibrated drier was found to have been

discolored slightly more than the other. Microscopic

examination showed that the discoloration was

caused primarily by oil and fine parti^cles of the

desiccant. The powdered desiccant was so fine and in

such small quantity, it was impossible to identify the

powder by naked eye. Also it v/as impractical to separate

the particles from the nylon cloth for any quantitative

or qualitative analysis because of the glutenous effect

of the oil deposit on the nylon.

Under microscopic observation, the size of holes

in the nylon cloth of the filters was measured to be

about 3.3 microns (0.00012 in.) in length and 1.7

microns (0.00007 in.) in width. The size of the

particles found on the filter downstream from the

vibrated drier ranged from B to 32 microns ( 0.00032 to

0.00125 in.) in diameter. On the other filter, size of

the particles xvas about 2 microns (0.0000^ in.) in

diameter.

Fig, 3 shows the two steel wool filters in which the

calcium carbide was wrapped as they appeared at the end

of the test. No difference between the two was observed.

No change in pressure drop across either drier

developed during the 100-hr vibration test. This indicates that any particles larger than 32 microns that

might have been separated from the original calcium

carbide granules and retained within the Drifreez cartridge had no measurable effect on its overall pressure

drop,

V. Discussion and Conclusion

The severity of the vibration vi^ith an acceleration

of 11,6 g, can be gauged to some extent by the 16 failures

of the system during the 100-hour test that were attributable to vibration. The failures could be summarized

as follows; one broken thermocouple, one loosened Allen

screw on hub of vibration shaft, two worn out V-belts

between electric motor and adjustable-speed couplings,

12 refrigerant leaks at flare joints and soldered joints.
FIG. 4-

FIG
I
The usual tubing loops used for absorbing vibration in a

refrigeration-unit failed to prevent breakages during th

test. Plastic tubing was finally substituted for copper

tubing to connect the dryer on the vibrator table into

the refrigerant circuit. The plastic tubing was success

ful in preventing joint failures.

The test results showed that the pulverization of

the desiccant was increased by the severe vibration of

the vibrating table as compared to that observed for

the normal vibration of the small refrigerating unit

used for the test,. However, the particles of material

collected dox-mstream from the driers ranged in diameter

from 0.0000^ in. to 0,00125 in, and would probably pass

through any screen or strainer likel3r to be used in the

refrigerating circuit. It is also probable that particles in this size range would pass between the pistons

and cxT-linder walls of a compressor without significant

cutting action. If the vibration caused particles

larger than 0,00125 in. to be dislodged from the

calcium carbide granules they were retained by the

filter in the outlet of the driers. The pulverization

of the calcium carbide was undoubtedly decreased by

the fact that the pieces xfere distributed rather

sparsely on a steel x-^rool pad which acted as a cushion.

It was not possible from the data taken to express

the effect of the 100-hour accelerated vibration test

in terms of equivalent days, months, or years of normal

vibration in a refrigerating system with respect to the

pulverization of the desiccant.

Vibration tests of the drier in a wet refrigerating

system Mere not made. When the desiccant, calcium carbide, reacts with ’water, it forms calcium hydroxide and

acetylene. No tests were made to determine the ability

of the filter in the drier to retain the fine poxfder of

calcium h3/droxide under vibration. However, in mixture

with liquid Freon the fine powder of calcium hydroxide

was observed to become very past3/ and thus not likely

to flow through the filters. The evidence that calcium

hydroxide in liquid Freon became pasty and was not

likely to flox"7 through the filters was obtained b3^ observation of the glass C3^1inder drier used during the

drying tests described in Part A, The glass cylinder

was partially filled with calcium carbide wathout a

wrapping of steel wool, and a steel wool pad was
-

”5J 0o _~

inserted at either end of the glass cylinder. As the

desiccant reacted with moisture in the system, calcium

hydroxide accumulated upstream of the filter pad at the

outlet of the container. A distinct boundary between

the steel wool and calcium hydroxide was observed and

the pressure drop across the drier increased to 110

pounds per square inch gage indicating that the drier

was clogged up.

It was concluded from these tests that granular

calcium carbide, embedded in steel wool, would pulverize to some extent during normal operation in a dry

refrigerating system because of vibration, but the

particles that left the drier would be too small to

clog strainers and would probably not score compressor

cylinder walls.
- 39

VT. Evaluation of the Drifreez Drier

The investigation of the Drifreez drier indicated

the follov/ing conclusions with respect to its dr3'-ing

characteristics, the hazards associated with its use,

and its pulverization characteristics.

1.

For values of end dr^mess in the important range

below 30 ppm, the model of the Drifreez drier used for

the investigation removed and held more water than a

silica gel drier of about twice the internal volume and

selling currently for equal price. On the other hand,

the instantaneous drA'-ing rate of the silica gel drier

was hieher than that of the Drifreez drier. After

having removed equal amounts of moisture in the range

between 5 and 20 grams, the Drifreez drier produced a

lower end dr^/Tiess than the silica gel drier of equal

cost

.

2.

Tv;o sources of hazard were revealed from the

introduction of acetylene gas into a refrigerating

system as a result of the reaction between calcium

carbide and water. These hazards are (a) the production of copper acetylide, b^?- reaction with copper

tubing, which is sensitive to explosion b^T- heat,

friction, or impact, and (b) the production of an

explosive mixture of air and aceti^'lene if a calcium

carbide drier were used to dry a s^/stem while air

only was being circulated in the s^’-stera. Although

it is believed that the simultaneous development of

all the circumstances necessar3

'' for an explosion

would not occur very often, either hazard is possible

in normal operation or servicing of a refrigerating

system.

3

.

Vibration pulverized the granular calcium

carbide to some extent even v/hen embedded in steel

wool as it was in the Drifreez drier, but the particles

that left the drier were too small to clog strainers

normally/ used for liquid refrigerant and would probably not score compressor cylinder walls.
V
THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The functions of the National Bureau of Standards are set forth in the Act of Congress, March

3, 1901, as amended hy Congress in Public Law 619, 1950. These include the development and

maintenance of the national standards of measurement and the provision of means and methods

for making measurements consistent with these standards; the determination of physical constants and properties of materials; the development of methods and instruments for testing materials,

devices, and structures; advisory services to Government Agencies on scientific and technical

problems; invention and development of devices to serve special needs of the Government; and the

development of standard practices, codes, and specifications. The work includes basic and apphed

research, development, engineering, instrumentation, testing, evaluation, calibration services, and

various consultation and information services. A major portion of the Bureau’s work is performed

for other Government Agencies, particularly the Department of Defense and the Atomic Energy

Commission. The scope of activities is suggested by the listing of divisions and sections on the

inside of the front cover.

Reports and Publications

The results of the Bureau’s work take the form of either actual equipment and devices or published papers and reports. Reports are issued to the sponsoring agency of a particular project or program. Published papers appear either in the Bureau’s own series of publications or in the

journals of professional and scientific societies. The Bureau itself publishes three monthly periodicals, available from the Government Printing Office: The Journal of Research, which presents

complete papers reporting technical investigations; the Technical News Bulletin, which presents

summary and preliminary reports on work in progress; and Basic Radio Propagation Predictions,

which provides data for determining the best frequencies to use for radio communications throughout

the world. There are also five series of nonperiodical publications: The Applied Mathematics

Series, Circulars, Handbooks, Building Materials and Structures Reports, and Miscellaneous

Publications.

Information on the Bureau’s publications can be found in NBS Circular 460, Publications of

the National Bureau of Standards ($1.00). Information on calibration services and fees can be

found in NBS Circular 483, Testing by the National Bureau of Standards (25 cents). Both are

available from the Government Printing Office. Inquiries regarding the Bureau’s reports and

publications should be addressed to the Office of Scientific Publications, National Bureau of Standards, Washington 25, D. C.