Thermoelectric gas dryer

A method and apparatus for removing moisture from a sample stream of gas while preventing dilution of the concentration of components of successive increments of the sample stream of gas.

BACKGROUND OF THE INVENTION 
The present invention pertains to a method and apparatus for drying a 
sample stream of gas and more particularly to a new and useful method and 
apparatus for removing the condensate from a gas stream while minimally 
interfering with the flow of the sample stream. 
Presently, gas analyzers have instantaneous measurement of component 
concentration capabilities. However, in some applications, such as the 
measurement of component concentration in automobile exhaust gas, the 
instantaneous analysis capability is defeated by its moisture content. 
Although the analyzer can respond instantaneously to concentration 
changes, the moisture in the sample stream of exhaust gas must be removed 
in order to permit an accurate analysis of the component concentration. 
Prior art gas dryers have a significant deficiency in this area. Referring 
to FIG. 1, it can be seen that the gas concentration at 5 is significantly 
different from the concentration at 6 corresponding to the inlet and 
outlet of a volume 9 used for removing moisture from a sample stream of 
gas. As illustrated in FIG. 1, prior art condensate removal systems have 
an intake 7 and exhaust 8 with a large volume 9 therebetween. This large 
volume causes a mixing or integrating between the sample arriving at 
intake 7 and the sample contained in the volume used in cooling the sample 
gas. As a result, the concentration at exhaust 8 will not immediately 
represent the concentration at intake 7 but will initially represent the 
concentration in volume 9, then represent the mixture of the concentration 
in volume 9 plus the concentration at intake 7, and finally represent the 
concentration at intake 7. This process can best be seen through a 
comparison of curve 2A and 2B of FIG. 2. Curve 2A represents the 
concentration of a gas to be measured at intake 7 with respect to time. 
Curve 2B represents the concentration of the same gas at exhaust 8 with 
respect to time for the same time period. Since both concentrations are 
measured on the same time scale, it can easily be seen that a time delay X 
occurs between the reduction of concentration at intake 7 and a 
significant decrease in the measured concentration at exhaust 8. Should a 
second pulsed increase in concentration occur before the concentration at 
exhaust 8 decreases to its minimum, curve 2B will begin to increase and an 
accurate measurement of the concentration minimum is lost. Loss of 
accurate measurement will also occur when the concentration increases to 
an extremely high maximum for a short duration (spiking) and continue at a 
low value thereafter. 
The problem of missing the maximums and minimums of a component gas 
concentration in a sample gas stream is particularly critical in the 
automobile exhaust testing area. The carburetor balance of an automobile 
engine is indicated by the various concentrations of carbon dioxide and 
oxygen in the exhaust stream. When a particular cylinder is misfiring, a 
radical change in the oxygen concentration in the exhaust will occur. 
While the component concentration measurement device is capable of sensing 
this radical change in concentration when it is used by itself, the 
combination of the measurement device with a prior art gas dryer greatly 
reduces sensitivity as illustrated in FIG. 2. 
SUMMARY OF THE INVENTION 
The present invention eliminates the problems of prior art moisture removal 
systems through the use of a design which combines a solid state cooling 
device, a peltier block, in conjunction with gravity moisture separation 
housing having a wick to produce "plugged flow" and to draw the moisture 
out of the housing. A solid state cooling block is used with a temperature 
dependent current control device to reduce the temperature of the gas 
sample, thus instantaneously compensating for temperature changes in the 
sample gas stream and promoting condensation of any moisture which may be 
contained within the sample. The condensate is separated from the gas 
sample through gravity and settles to the bottom of the housing. The 
housing has three intersecting paths. One of the three paths receives the 
flow of a sample gas stream. A second path exhausts the sample stream. The 
third path has a hygroscopic wick disposed therein to draw moisture from 
the intersection point of the first two paths, partially through gravity, 
partially through the pressure of the flow of the sample stream and 
partially through the hygroscopic nature of the wick material. This 
hygroscopic wick will prevent the exhaust of the sample gas through the 
third path so that "plugged flow" is achieved. By providing fast, 
efficient cooling through the use of a solid state cooling device, the 
length and cross sectional area of passages for sample gas flow may be 
reduced. Through plugged flow, the sample gas travels quickly from inlet 
to outlet. Thus, no intermixing of successive increments of the sample gas 
stream take place and increased accuracy in instantaneous component 
concentration gas analysis is possible.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 3 illustrates a housing 10 mounted on the surface of heat sink 14 by 
screws 16 and 18. Within housing 10 are three intersecting paths 
comprising an inlet 20 for fluid communication with a sample source (See 
FIG. 5), an outlet 22 for fluid communication with a sample analysis 
device (FIG. 5), and a drain 24 for removal of moisture from the housing 
10. Inlet 20 is preferably located at an approximate 45.degree. angle with 
outlet 22 and extends through a significant portion of housing 10 although 
any angle is permissible provided that both inlet 20 and outlet 22 are 
above a horizontal line traversing the intersection point. Inlet 20 is 
preferably of lesser diameter than outlet 22 to avoid a pressure buildup 
which would force a portion of the sample stream of gas to exhaust through 
drain 24. Inlet 20 and outlet 22 are sized for a gas flow rate within the 
range of 400 to 600 cubic centimeters per minute although one skilled in 
the art may readily alter the sizes of the preferred embodiment to accept 
different sample stream flow rates. In the preferred embodiment, inlet 20 
is 1/16" to 3/16" in diameter and outlet 22 is 1/8" to 1/4" in diameter. 
Drain 24 may be of any suitable size although the preferred embodiment 
illustrates the drain as having the same diameter as counterbore 25 for 
ease of manufacture. Housing 10 may be manufactured of any heat conducting 
material although the preferred embodiment uses stainless steel to prevent 
corrosion from any corrosive effects of the sample gas stream which is 
passing through outlet 22. Disposed within drain 24 is a hygroscopic 
wick-like substance 26 to draw any condensed moisture out of outlet 22. In 
the preferred embodiment it was discovered that the top of wick 26 
preferably is located in such a manner that it extends toward opening 27 
to engage any condensate droplets that may settle across the opening to 
drain 24. Wick 26 comprises a hygroscopic fibrous material wound around a 
wire core in the preferred embodiment. However, wick 26 may be of any 
hygroscopic material having water absorbing tendency and may be as simple 
as a pipe cleaner or a rolled piece of paper towel. Also within housing 10 
is a well 28 to accept a thermistor 30 for controlling the current flow in 
solid state cooling device 32 (see FIG. 4). 
Referring now to FIG. 4, a side view of the present invention illustrates 
solid state cooling device 32 as being mounted on heat sink 14. Solid 
state cooling device 32 is preferably a peltier block of the type 
manufactured by Materials Electronic Products Corporation and may be 
obtained through commercial sources. Cooling device 32 is constructed to 
have a hot surface 34 and a cold surface 35. When current travels through 
cooling device 32, hot surface 34 increases in temperature and cold 
surface 35 decreases in temperature. Thus, the surfaces of cooling device 
32 respond to changes in current flow instantaneously. 
Thermistor 30 detects the temperature of housing 10 which is a function of 
the temperature of the sample stream of gas and varies its resistance 
according to the temperature of housing 10. Thermistor 30 is connected to 
variable current source 36. Current source 36 is connected to thermistor 
30 through connector 37a and 37b and to solid state cooling device 32 
through connectors 38a and 38b. Thermistor 30 detects the temperature of 
housing 10 and feeds this information to current source 36, which varies 
the current flow to solid state cooling device 32. When the temperature of 
housing 10 exceeds a predetermined maximum, an increased current flow is 
passed through solid state cooling device 32, causing an instantaneous 
temperature change in hot surface 34 and cold surface 35 of cooling device 
32. A greater current through cooling device 32 will increase the 
temperature differential between hot surface 34 and cold surface 36 of 
cooling device 32 as indicated in the McGraw Hill Encyclopedia of Science 
and Technology, published by McGraw-Hill Book Company, Inc., 1971, in its 
description under "thermoelectricity" at page 602 of Volume 13. Since the 
volume necessary for cooling the temperature of the sample gas to its dew 
point is kept to a minimum through the use of cooling device 32, no 
expansion area is necessary. Prior art required an expansion area to 
augment cooling of the sample gas. The present invention does not require 
an expansion area. Thus a sample gas stream will flow through the present 
invention without having to fill an increase in volume at the point of 
cooling. Therefore, no mixing between successive increments of gas flow 
takes place and the gradual decrease in measured concentration of a 
component gas (as illustrated by curve 2B of FIG. 2) is eliminated. 
Heat sink 14 is illustrated as having cooling fins to conduct the heat 
generated by the hot surface of solid state cooling device 32. Heat sink 
14 may be of any standard design provided that it is sized to conduct the 
heat generated by the hot surface of electronic cooling device 32. Housing 
10 is illustrated as being in close contact with cooling device 32 to 
promote temperature reduction in outlet 22 through conduction of heat from 
housing 10 to cold surface 35 of device 32. Although the drawings 
illustrate housing 10 as being fastened to heat sink 14 through the use of 
screws, any method may be used. A thermal grease is preferably used to 
coat the surface of solid state cooling device 32 to promote conduction 
between housing 10 and cold surface 35 of solid state cooling device 32 
and between hot surface 34 of solid state cooling device 32 and heat sink 
14. 
In actual operation a sample gas stream enters inlet 20 and passes through 
to outlet 22. In the path comprising inlet 20 and outlet 22, the 
temperature of the gas stream is reduced to promote condensation of 
moisture. Moisture, when condensed, will travel down the walls of inlet 20 
and outlet 22 to drain 24 and come in contact with wick means 26. Wick 
means 26 must be positioned so that it will engage and droplets of 
condensate which might, through surface tension, settle at the opening 27 
of drain 24. Preferably wick means 26 is positioned so that its top end is 
near the lower portion of the inlet channel 20. The condensate will then, 
through the combined forces of gravity, and the hygroscopic properties of 
wick means 26, pass through drain 24 to outside ambience. The gas sample, 
entering housing 10 at a slight pressure, will be forced out through 
outlet 22. Wick means 26, being saturated with mositure, will greatly 
retard gas flow to the point of blocking any fluid communication of the 
sample gas between drain 24 and outside ambience while the gas stream is 
being passed through inlet 20 and outlet 22 thus achieving a "plugged 
flow" condition. A "plugged flow" condition is one wherein a sample stream 
of gas flows through a conduit without dilution, either from outside the 
conduit or within the conduit. The use of wick means 26 in combination 
with solid state cooling device 32 aid in producing a gas flow having 
concentrations resembling that of curve 2A of FIG. 2 while prior art 
devices produce concentration curves resembling 2B of FIG. 2 when sample 
gas concentrations vary according to curve 2A. 
FIG. 5 illustrates a block diagram of a typical system wherein the present 
invention is used. Sample gas source 40 is an automobile exhaust for this 
sample; however, source 40 may be any source which produces a sample 
stream of which a component gas concentration is to be measured. Source 40 
is connected through inlet conduit 41 to a block representation 42 of the 
present invention. Block representation, or dryer, 42 is connected through 
exhaust conduit 43 to an analyzer 44 which is capable of measuring 
instantaneous concentration changes. Analyzer 44 is an oxygen analyzer in 
this example. However, analyzer 44 may be a carbon dioxide or carbon 
monoxide analyzer or any analyzer having instantaneous measurement 
capability. 
As illustrated in FIG. 1 and FIG. 2, a component gas concentration can 
change radically from one moment to the next particularly when source 40 
is an automobile exhaust. FIG. 1 shows a prior art device having an inlet 
7 with a gas concentration 5 and an outlet 8 with a gas concentration 6. 
As illustrated in FIG. 2, the gas concentration changes according to curve 
2A which illustrates an instantaneous change in gas concentration. Curve 
2B illustrates the gas concentration seen at exhaust 8 over a period of 
time. As can be seen in a comparison between curve 2A and curve 2B, an 
indication of the change in gas concentration on the measuring device has 
a time delay depicted as X. Using the prior art dryer of FIG. 1, dilution 
of successive increments of gas will occur in the volume necessary to cool 
the gas. Using the present invention gas dryer the sample gas flows from 
source 40 through inlet conduit 41 to dryer 42. Since dryer 42 has no 
expansion chamber and is quickly cooled, the component concentration of 
the sample stream from source 40 will not be diluted by prior increments 
of the sample stream and a measured concentration curve approximating that 
of 2A instead of that of 2B will result. Thus, the component concentration 
of the increment of sample stream entering the dryer through inlet conduit 
41 will be the same when that increment exits dryer 42 to exhaust conduit 
43 and flows to analyzer 44. 
While the present invention is shown by way of the preferred embodiment, it 
is to be understood that the preferred embodiment is by way of 
illustration only and in no way limits the present invention which is to 
be construed only in light of the following claims.