Confidential Global Cooling Inc. Page 1 June 2013

FLAMMABILITY	RISK	ASSESSMENT	OF	R-170	(ETHANE)	FOR	USE	IN	VERY-LOW	

TEMPERATURE	FREEZER	THERMOSIPHON

SUBMITTED	BY:	GLOBAL	COOOLING	INC.,	6000	POSTON	RD.,	ATHENS,	OH	45701

JUNE	2013
Confidential Global Cooling Inc. Page 2 June 2013

Table	of	Contents

1 GLOSSARY	OF	TERMS	AND	ACRONYMS............................................................................................3

2 SUMMARY ..........................................................................................................................................6

3 BACKGROUND ...................................................................................................................................7

3.1 Introduction .........................................................................................................................................7

3.2 VLT	Freezer	Study	Model .................................................................................................................9

3.3 Use	Profile	of	VLT	Freezers .......................................................................................................... 11

3.4 Data	References................................................................................................................................ 12

4 DETAILED	DESCRIPTION	OF	THE	PRODUCT ................................................................................ 12

4.1 The	Stirling	Engine.......................................................................................................................... 12

4.2 The	Thermosiphon.......................................................................................................................... 13

4.3 Heat	Rejection................................................................................................................................... 14

4.4 Electronic	Components.................................................................................................................. 14

5 GAS	LEAKAGE	EVENTS................................................................................................................... 17

5.1 Manufacturing	Site .......................................................................................................................... 17

5.2 Servicing	Site..................................................................................................................................... 17

5.3 Non-Conforming	Leaks .................................................................................................................. 18

6 RISK	ASSESSMENT ......................................................................................................................... 19

6.1 Introduction ...................................................................................................................................... 19

6.2 Probability	of	Leaks ........................................................................................................................ 23

6.3 Failure	Rate	Estimates ................................................................................................................... 26

6.4 Fault	Tree	Analysis.......................................................................................................................... 28

7 RISK	MITIGATION.......................................................................................................................... 29

8 DISPOSAL	ISSUES ........................................................................................................................... 30

9 BIBLIOGRAPHY............................................................................................................................... 30
Confidential Global Cooling Inc. Page 3 June 2013

1 GLOSSARY	OF	TERMS	AND	ACRONYMS

Absolute	 risk:	 the	 quantitative	 expression	 of	 risk	 due	 to	 a	 specific	 cause	 in	 terms	 of	 both	

impact	and	probability.

AC:	Alternating	current.

Accident	 sequence:	 a	 specific	 unplanned	 event	 or	 sequence	 of	 events	 with	 a	 directly	

attributable	undesired	consequence.

Acute	hazard:	the	potential for	injury	or	damage	resulting	from	an	instantaneous	or	relatively	

short	duration	exposure	to	the	effects	of	an	undesirable	event.

ADL:	Arthur	D.	Little,	Inc.

AICC:	adiabatic,	isochoric,	complete	combustion.

ASHRAE:	American	Society	of	Heating,	Refrigeration	and	Air-conditioning	Engineers

Baffle:	a	surface	used	for	deflecting	fluids,	usually	in	the	form	of	a	plate	or	wall.

Cabinet	 heater:	 an	 element	 embedded	 behind	 the	 thermal	 breaker	 (or	 mullion)	 for	 the	

purpose	of	providing	heat	to	the	freezer	gasket	in	order	to	prevent	condensation	or	sweating.

CFM:	Cubic	foot	per	minute,	a	rating	typically	used	for	fans.

Consequence:	the	direct	result	of	a	hazardous	event.

DC:	Direct	current.

Deflagration:	 Propagation	 of	 a	 combustion	 zone	 at	 a	 velocity	 that	 is less	 than	 the	 speed	 of	

sound	in	the	unreacted	medium.

Episodic	event:	an	event	of	limited	duration,	usually	associated	with	an	accident.

Equivalence	Ratio:	ratio	of	the	fuel-to-oxidizer	ratio	to	the	stoichiometric	fuel-to-oxidizer	ratio.

Event:	a	particular	occurrence,	sometimes	referring	 to	one	or	more	specific	steps	in	a	 failure	

sequence	leading	to	an	accident.

Fault	 tree:	 a	logic	 diagram	illustrating	 a	 ‘top-down’	 chain	 of	 events	 showing	 a	 ‘top-event’	 of	

interest	and	all	of	the	significant	event	sequences	and	combinations	of	failures	that	can	lead	to	

it.

Fill-tube:	a	small	diameter	tube	where	a	gas	charge	is	entered	into	the	thermosiphon.	The	filltube	is	pinched	off	to	make	a	gas-tight	seal	and	to	retain	the	charge.

Flash	point:	the	temperature	of	flammable	material	at	which	there	is	sufficient	vaporization	to	

support	combustion	of	the	material.

Fluid:	gas,	vapor	or	liquid.

Hazard:	 a	 chemical	 or	 physical	 condition	 that	 has	 the	 potential	 for	 causing	 damage,	 injury,	

fatality	or	other	type	of	harm	or	loss.
Confidential Global Cooling Inc. Page 4 June 2013

Hazard	 analysis:	 the	 identification	 and	 classification	 of	 hazardous	 events,	 which	 may	

sometimes	be	accompanied	by	the	determination	of	the	probabilities	of	their	occurrence	and	/	

or	their	consequences.

Hazard	identification:	a	process	of	design	and	operations	review	with	a	purpose	of	recognizing	

and	noting	significant	hazards	to	people	and	property.

Hazard	models:	mathematical	relationships	that	describe	the	physics	and	chemistry	of	hazard	

phenomena.

Hazard	 zone	 (or	impact	 area):	 the	 zone	 or	 region	 where	 hazard	impact	 has	 the	 potential	 to	

occur.

Impact:	 the	 ultimate	 potential	 result	 of	 a	 hazardous	 event,	 expressed,	 depending	 on	 the	

particular	 application,	 in	 terms	 of	 injuries,	 fatalities,	 environmental	 or	 property	 damage,	

business	interruption,	etc.

Impact	criteria:	 values	 of	appropriate	physical	parameters	 that	are	used	in	conjunction	with	

hazard	models	to	define	hazard	zones	or	impact	areas.

KG:	the	deflagration	index,	measured	in	bar-m/s.	A	constant	that	defines	the	maximum	rate	of	

pressure	rise	with	time	(dp dt)max of	a	deflagration	in	a	volume	V,	according	to	the	equation:

KG = (d p dt)max ⋅ V3

Likelihood:	a	measure	of	the	expected	frequency	of	an	event	occurring.

MIE:	minimum	ignition	energy.	A	measure	of	the	least	energy	required	to	ignite	a	combustible	

mixture.	Hydrocarbon	refrigerants	all	have	a	MIE	of	around	0.25mJ (Kunz	1998).

NFPA:	National	Fire	Protection	Association.

Relative	risk:	the	expression	of	risk	in	terms	of	how	it	compares	to	another	risk.

Refrigerant:	the	fluid	used	for	heat	transfer	in	a	refrigerating	system.

Risk:	the	potential	for	realizing	undesired	impacts	as	a	result	of	an	event,	facility	operation,	or	

other	activity.

Risk	 assessment:	 the	 methodical	 estimation	 and	 evaluation	 of	 quantitative	 or	 qualitative	

measures	of	risk.

Risk	 evaluation:	 the	 assessment	 of	 risk,	 coupled	 with	 an	 appraisal	 of	 the	 significance	 of	 the	

results.

Risk	management:	 the	 systematic	 control	 of	 hazards	 via	evaluations,	 decisions	and	activities	

relating	to	risk	acceptability,	risk	mitigation	and	the	effectiveness	of	risk	assessment.

Risk	 mitigation:	 the	 implementation	 of	 appropriate	 measures	 for	 reducing,	 eliminating	 or	

transferring	risk.
Confidential Global Cooling Inc. Page 5 June 2013

Risk	reduction:	the	introduction	of	modifications	to	an	existing	design, facility	or	activity	that	

tend	to	reduce	either	the	probability	or	potential	impact	of	a	hazardous	event.

RTD:	Resistance	temperature	device	usually	used	for	measuring	temperature.

Societal	risk:	a	measure	of	risk	where	the	impact	relates	to	society	in	general	rather	than	to	an	

individual.

Stirling	engine:	a	 closed-cycle	 heat	engine	 that	is	 capable	 of	 being	 used	as	a	 heat	 pump	 or	a	

power	producer.	In	the	application	discussed	in	this	report,	the	Stirling	engine	is	always	used	

as	a	heat	pump.	It	pumps	heat	from	the	cabinet	to	the	immediate	environment	surrounding	the	

cabinet.	The	Stirling	engines	used	in	VLT	freezers	are	hermetic	and	are	charged	with	helium.

TEWI:	Total	Equivalent	Warming	 Impact.	A	calculation	 that	determines	 the	equivalent	global	

warming	over	a	fixed	period,	usually	10	years,	of	the	refrigerant	and	use	energy	of	a	freezer.

Thermal	breaker:	A	flat	surface	or	flange	separating	the	outside	surface	of	the	cabinet	from	the	

interior	surface.	It	is	typically	manufactured	from	a	low	thermally	conducting	material.

Thermosiphon:	 a	 device	 for	 transferring	 thermal	 energy	 by	 the	 use	 of	 buoyancy	 or	

gravitational	 effects.	 In	 this	 case,	 all	 reference	 is	 made	 to	 a	 two-phase	 thermosiphon	

employing	 a	 refrigerant	where	 the	 heat	is	 rejected	 by	 condensation	 and	heat	is	 accepted	 by	

evaporation	or	boiling.

UL:	the	Underwriters	Laboratories	Inc.

Uncertainty:	a	reflection	of	the	accuracy	and	/	or	precision	of	an	estimate.

VLT:	Very	low	temperatures	usually	below	-70°C.

VLT	 freezer: a	 freezer	 for	 storage	 of items	 at	 VLTs,	 usually	 employed	 in	 biosciences,	

pharmaceutical	 and	 bio-storage	 industries. Also	 know	 as	 an ultra-low	 temperature	 or	 ULT	

freezer.
Confidential Global Cooling Inc. Page 6 June 2013

2 SUMMARY

In	 order	 to	 move	 away	 from	 ozone	 depleting	 chemicals,	 very	 low	 temperature	 freezer	

manufacturers	 have	 been	 employing	 refrigerants	 such	 as	 508B.	 Unfortunately,	 these	

refrigerants	 have	 been	 identified	 with	 extremely	 high	 global	 warming	 potentials.	 An	

alternative	 to	 508B	 is	 R-170	 (ethane),	 which	 is	 known	 to	 have	 minimal	 impact	 on	 global	

warming.	However,	R-170	is	flammable	and	therefore	its	potential	use	in	this	application	must	

be evaluated	in	terms	of	safety	during	freezer	manufacture,	shipping	and	use.

We	 have	 found	 that	 the	main	 risk	is	 a	 fire	 or	 explosion	inside	 the	machine	 compartment	 at	

initial	powering	of	the	VLT	freezer.	We	believe	that	this	event	could	occur	because	shipping	or	

transportation	 stresses	 are	 the	 most	 severe	 and	 that	 they	 could	 damage	 the	 thermosiphon	

tubing	resulting	in	a catastrophic	leak	of	R-170.	The	risk	of	such	a	scenario	requires	the	leak	to	

occur	in	the	last	30	minutes	of	transportation,	for	both	fans	to	not	work	at	the	switch	on	event	

and	for	an	ignition	source	to	be	present.	We	calculate	that	the	frequency	of	such	a	combination	

of	events	to	be	2.4	x	10-11 /yr.	

This	risk	assessment	is	based	on a	number	of	sources	including data	presented	by	Arthur	D.	

Little	in	their	report	‘Risk	Assessment	of	Flammable	Refrigerants	for	Use	in	Home	Appliances’1

and	other	data	collected	by	Unilever	in	their	risk	assessment	for	the	use	of	R-290 in	ice	cream	

cabinets.	 In	addition,	Global	Cooling	has	also	retained	UL	 for	evaluating	 the	safety	of Stirling	

engine	cooled	VLT	freezers	employing	thermosiphons	charged	with	R-170.

As	part	of	the	risk	assessment,	we	characterized	R-170	by	comparing	it	to	other	hydrocarbon	

refrigerants	 already	 approved	 for	 household	 refrigerators	 and	 commercial	 applications.	 We	

are	asking	the	EPA	to	approve	the	use	of	up to	100g	of	R-170	for	use	in	VLT	freezers.

 1 Arthur	 D.	 Little,	 Risk	 Assessment	 of	 Flammable	 Refrigerants	 for	 Use	 in	 Home	 Appliances,	 September	 10,

1991.
Confidential Global Cooling Inc. Page 7 June 2013

3 BACKGROUND

3.1 Introduction

Global	Cooling	Inc.	manufactures Stirling	engine	cooled	VLT	freezers. The	Stirling	refrigeration	

system	 (consisting	 of	 the	 Stirling	 engine	 and	 thermosiphon) installed	 in a VLT	 freezers	 is	

shown in	 Fig.	 1.	 The	 Stirling	 engine	 employs	 helium	 gas	 as	 its	 working	 medium.	 Thermal	

transport	from	the	interior	of	the	freezer	is	accomplished	by	the thermosiphon	that	contains	a	

refrigerant,	currently	R-508B.

Fig.	1 Main	Components	of VLT	Freezer with	Stirling	Refrigeration	System

Recently,	a	number	of	enterprises	and	concerns	have	lobbied	for	natural	refrigerants	owing	to	

their	better	fit	with	sustainability	objectives (ASHRAE	2011).	Global	Cooling	Inc.	has	therefore	

conducted	 a	 sustainability	 evaluation on	 a	 27.5 cu.	 ft. freezer	 where	 the	 thermosiphon	

refrigerant	 has	 been	 replaced	 with	 the	 hydrocarbon	 R-170	 commonly	 known	 as	 ethane

(Berchowitz	 and	 Kwon	 2012).	 	 The	 tests	 were	 conducted	 with	 100g	 of	 R-170	 versus	 the	

original	charge	of	200g	of	R-508B. Aside	from	the	lower	refrigerant	charge and	zero	ODP2,	the	

global	warming	potential	of R-170	is	5.5	(GWP100)	versus	13,396	(GWP100)3 for	R-508B.

Two baseline	cascade	systems	were included	in	the	study,	one	with	blowing	agent	HFC-245fa	

and	 the	 other	with	 cyclopentane. The	 cascade	 system	is the	 current	 benchmark	 VLT	 freezer	

system and	the	use	of	cyclopentane	represents	units	sold	in	the	US	with	lowest	TEWI.	Owing	

mainly	to	low	use	energy,	the	Stirling	has	a	substantially lower	TEWI	grade	than	any	cascade	

system. Fig.	2	shows	a	comparison	of	the	Stirling	VLT freezer against	the	baseline	cascade	VLT	

 2 Ozone	Depletion	Potential

3 This	is	a	measure	of	global	warming	potential	over	a	period	of	100	years	compared	to	CO2.	CO2 =	1	(GWP100)
Confidential Global Cooling Inc. Page 8 June 2013

freezers.	 A	 typical	 household	 refrigerator	 is	 included	 for	 reference.	 Each	 displacement	 of	 a	

cascade	VLT	 freezer	by	a	similar	sized	Stirling	VLT	has	the	potential	of	saving	the	equivalent	

TEWI	of	nine household	freezers.	In	addition to	the	TEWI	advantage,	we found	no	substantial	

difference	in	performance	when	using R-170 against	R-508B and	therefore decided	that	a	full	

evaluation	of	the	potential	use and	safety	concerns of	R-170	was	warranted.

Fig.	2	Estimated	TEWI	for	Cascade,	Cascade	with	Cyclopentane	Blowing	Agent	and	Stirling

Since	R-170	is	flammable, the	consequences	of	this	had	to	be	fully	investigated	so	that	proper	

handling,	 transport	and	 use	 procedures	 could	 be	identified. The	 flammability	issue	 has	 been	

the	major	area	of	concern	and	our	risk	assessment	is	largely	about	this	item.	Other	aspects	of	

safety	 are,	 of	 course,	 also	 studied	 but	 they	 do	 not	 represent	 any	 significant	 issues	 not	

ordinarily	handled	by	procedures	already	adopted	for	the	safe	use	R-508B. Table	1	compares

some	 pertinent	 properties	 of	 R-508B, R-170, R-290	 (propane) and	 R-441A.	 R-170	 falls	 into	

safety	 group	 A3 (ANSI/ASHRAE	 2010) compared	 to	 A1	 for	 R-508B.	 R-290	 and	 R-441A,	

refrigerants	approved	by	EPA/SNAP,	are	both	in	safety	group	A3	and	are	therefore	subject	to	

maximum	 charge	 quantities. R-170	is	 very	 similar	 from	a	 safety	 standpoint	 to	R-290	and	R441A	and	VLT	freezers	are	also	similar	to	the	applications already approved by	SNAP for	use	

for	 these	 refrigerants.	 Therefore	 data	 collected	 and	 studied	 under	 risk	 evaluation	 for	 R-290	

and	R-441A	will	be	pertinent	to	this	study. Where	use	differences	occur	they	will	be	identified	

and	appropriate	considerations	will	be	introduced.

0.00 

10.00 

20.00 

30.00 

40.00 

50.00 

60.00 

70.00 

CASCADE CASCADE 

(CYCLOPENTANE) 

STIRLING HOUSEHOLD 

FRIDGE (US) 

TEWI [tonnes CO2] 
Confidential Global Cooling Inc. Page 9 June 2013

Table	1	Physical	Properties	of	R-508B, R-170, R-290 and	R-441A

Refrigerant R-508B R-170 R-290 R-441A

Common	or	Trade	Name

Suva	95	

(DuPont) Ethane Propane HCR	188	C1

Chemical	Formulae CHF3/CF3CF3 C2H6 C3H8

Blend	Ethane,	

Propane,	Isobutene	

and	n-Butane

Molecular	Weight 95.39 30.07 44.1 48.23

Boiling	Point	(1	atm)	[°C] -88.27 -88.7 -42 -42

Freezing	Point	[°C] Not	known -183.3 -188 -188

Critical	Temperature	[°C] 14.0 32.2 96.7 117.3

Critical	Pressure (abs) [MPa] 3.926 4.8839 4.25 4.3

Flammable	Limits	in	Air [%]

(Lower	to	Upper) None 3	to	12.4% 2	to	11%

(Kaufeld	2012) 2	to	11%

Minimum	Ignition	Energy	[mJ] Not	applicable 0.285 0.25 Estimate	similar	to	

Propane*

Auto-ignition	temperature	[°C] None 515 470 450

Heat	of	Combustion	[MJ/kg]

(Higher	Heating	Value) N/A 51.9 50.35 Probably	similar	to	

Propane

Ozone	Depletion	Potential	(ODP) 0 0 0 0

Global	Warming	Potential	(GWP)

(IPCC	2007) 13,396 5.5 3.3 0

4

EPA/SNAP Approval VLT	Freezers Requesting	

VLT	Freezers

New	retail	

stand-alone	

refrigeration

New	household

refrigeration

Safety	Group (ANSI/ASHRAE	

2010) A1 A3 A3 A3

Maximum	charge quantity	[g] None Requesting	

100 150 57

*	Minimum	ignition	energies	tend	to	be	similar for	hydrocarbons	(Kunz	1998)

3.2 VLT	Freezer	Study	Model

The	standard	freezer	chosen	for	this	study	is	a -86°C	780	liter	(27.5 cu.	ft.)	upright	(as	opposed	

to	 chest)	model	 referred	 to	 as	 the	 SU780U (Fig.	 3).	 The	 SU780U	is	 an	 example	 of	 the	most	

common	 size	 of	VLT	 freezer typically	 sold	 to	 bio-repositories,	 the	 pharmaceutical	industries

and	 universities.	In	 distinction	 to	 the	 universally	 applied	 cascade	 refrigeration	 systems,	 this	

unit	utilizes	 a	 Stirling	 refrigeration	 system	 with	 a	 thermosiphon.	 For	 this	 study,	 the	 current	

thermosiphon	 charge	 of	 200g	 of	 R-508B	 has	 been	 replaced	 with	 100g	 of	 R-170	 (Ethane).	

Except	for	appropriate	labeling	and	marking,	the	R-170	unit	is	in	all	other	respects	identical	to	

the	R-508B	unit.

 4 According	to	manufacturer,	A.S.	Trust	and	Holdings.
Confidential Global Cooling Inc. Page 10 June 2013

											

Fig.	3 Model	SU780U VLT	Freezer

As	 shown	 in	 Fig.	 1,	 the	 machine	 compartment	 of	 Stirling	 engine	 units	 is	 at	 the	 top	 of	 the	

cabinet.	Removing	the	top	cover	of	the	freezer	provides access	to	all	the	essential	mechanical	

components.	 Fig.	 4 shows	 the	 machine	 compartment with	 the	 top	 cover	 removed. The	

thermosiphon	(2)	is	shown	with	its	insulation	removed.	 It	is	attached	to	the	cold	head	of	the	

Stirling (1) and	enters	the	cabinet through	a	collar to	surround	the	inner	liner. There	are	four	

braze	 connections	 that	 are	 made	 in	 the	 mechanical	 compartment	 when	 attaching	 the	

thermosiphon	 to	 the	Stirling	engine. The	 thermosiphon	 fill	 tube	cannot	be	clearly	seen	but	it	

enters	 the	 thermosiphon	 condenser	 at	 the	 cold	 head	 of	 the	 Stirling	 engine.	 The	 electronic	

boards	 are	 shown	 at	 (3),	 (4)	 and	 (5).	 The	 inverter	 board	 is	 (4)	 and	 is	 the	 most	 relevant	 to	

safety	issues,	as	will	be	discussed	later.	Other	items	of	interest	is	the	door	switch	(14),	battery	

(12)	and	the	two	fans	at	(10)	and	(11).
Confidential Global Cooling Inc. Page 11 June 2013

Fig.	4 Major	Components	in	the	Machine	Compartment

3.3 Use	Profile	of	VLT	Freezers

VLT	 freezers	are	generally	 clustered	 from	 perhaps	as	 few	as	 five	or	 six	 units	 side	 by	 side	 to	

many	hundreds	within	a	large	building	in	 the	 form	 sometimes	 referred	 to	as	a	 freezer	 farm.

This	 study	 will	 consider	 risks	 associated	 with	 end-use.	 Transportation	 risk will	 not	 be	

evaluated	since	it is	already	handled	under	49	CFR	173.174	(refrigerating	machines).	UN	3358	

Class	 2.1	 REFRIGERATING	 MACHINES	 containing	 flammable,	 non-toxic,	 liquefied	 gas.	 IATA	

special	provision	A103	also	applies.	

Cold Head

Collar

Braze 

Connections

Thermosiphon 

condenser surrounds 

the cold head
Confidential Global Cooling Inc. Page 12 June 2013

3.4 Data	References

No	 safety	 data	 exists	 for	 the	 subject	 situation	 of	 a	 VLT	 freezer	 with	 Stirling	 refrigerating	

system.	 We	 therefore	 had	 to	 assemble	 close	 equivalent	 data	 that	 was	 sourced	 from	 the	

following:

a. Risk	Assessment	of	Flammable	Refrigerants	for	Use	in	Home	Appliances	by	Arthur	D.	Little,	

Inc. (Arthur	D.	Little	1991)

b. Use	of	R290	in	Ice	Cream	Cabinets	by	Unilever.

c. Tests	done	under	Global	Cooling	Inc.’s	commission	by	UL	according	to	test	standard	UL471.	

This	standard	has	been	revised	in	November	2012	to	include	laboratory	refrigerators	and	

freezers.

d. ASHRAE	15-2010,	Safety	Standard for	Refrigeration	Systems (ANSI/ASHRAE	2010).

e. Combustion	studies	specifically	addressing	R-170	(ethane).

4 DETAILED	DESCRIPTION	OF	THE	PRODUCT

4.1 The	Stirling	Engine	

The	Stirling	engine	employs	Helium	gas	as	its	working	medium	in	a	closed	cycle.	There	is	no	

phase	 change	 of	 the	 working	 medium.	 The	 cycle	 compresses	 and	 expands	 the	 gas	 in	

segregated	areas	in	order	to	provide	a	cold	heat	acceptor (also	referred	to	as	a	cold	head) and	

a	warm	heat	rejecter.	The	configuration	used	in	this	product	is	a	free-piston	arrangement.	The	

free-piston	 arrangement	 uses	 no	 mechanical	 linkages;	 the	 moving	 parts	 are	 moved	 by	 a	

combination	 of	gas	 pressure	 forces,	 springs	and	a	linear	motor.	No	 oil	is	 used,	lubrication	is	

provided	 by	 gas	 bearings	 and	 consequently	 very	 long	 life	 and	 high	 reliabilities	 are	 assured.	

Since	 there	are	no	mechanical	linkages,	 the	amplitude	of	 the	piston	can	be	controlled	by	 the	

applied	voltage.	This	allows	the	heat	lifting	capacity	of	the	unit	to	be	controlled.	Controlling	the	

capacity	 in	 this	 manner	 is	 in	 strong	 distinction	 to	 conventional	 compressors,	 which	 adjust	

their	 cooling	 effect	 by	 on/off	 modulation.	 Figure	 5 shows	 the	 basic	 arrangement	 of	 a	 freepiston	Stirling	engine	with	 some	 of	 the	important	 parts	 noted. The	 particular	model	 used	in	

the	 SU780U	 freezer	 is	 the	 M600,	 also	 manufactured	 by	 Global	 Cooling	 Inc.	 The	 M600 is	

pressurized	 to	 30bar	 (abs)	 with	 helium	 gas. Maximum	 input is	 1.2kW and	 minimum/	

maximum	 voltage	is	108/264VAC (Global	 Cooling	 Inc.	 2013).	At	 steady	 state	 conditions,	i.e.,	

when	the	target	temperature	is	reached,	the	controller	backs	off	the	power to	around	300W	by	

reducing	the	input	voltage.
Confidential Global Cooling Inc. Page 13 June 2013

Fig.	5 Free-Piston	Stirling	Engine	Details (Schematic)

4.2 The	Thermosiphon

The	thermosiphon	is	a	3/8	inch	copper	tube	that	wraps	around	the	inner	liner	of	the	 freezer	

and	connects	the	thermosiphon	condenser	on	the	Stirling	engine.	It	is	hermetically	sealed	by	a	

pinch-off	tube	and	contains	only refrigerant,	in	this	case	R-170.	There	are	no	internal	moving	

parts	 in	 contact	 with	 the	 refrigerant.	 The	 general	 arrangement	 of	 the	 thermosiphon	 can	 be	

seen	in	Fig.	1. The	thermosiphon	operates	at	the	saturation	pressure	of	the	refrigerant	for	that	

operating	temperature.	For	example,	if	the	acceptor	of	the	Stirling	is	at	-90°C,	then	refrigerant	

condensation	 takes	 place	 at	 the	thermosiphon	 condenser	 at	 0.938bar.	 This	 condensate	 runs	

down	 the	 thermosiphon	 by	 gravity	 and	 evaporates	 in	 the	 tubing	 section,	 which	 is	 the	

equivalent	of	the	evaporator	of	a	conventional	compressor	system.	Since	the	tube	is	essentially	

unrestricted,	the	evaporation	pressure	and	the	condensation	pressures	are	the	same	(different	

only	 due	 to	 the	 vapor	 pressure	 drop and	 gravity which	 is	 very	 small)	 and	 therefore	 the	

temperature	 of	 both	 the	 evaporation	 and	 condensation	 sections	 are	 almost	 identical.	 This	

condition,	 called	 isothermal	 heat	 transfer,	 is	 extremely	 efficient.	 Characteristics	 therefore	 of	

the	thermosiphon	are:

a. Isothermal	heat	transfer

b. Operates	at	any	temperature	below	the	critical	point,	in	this	case	32.2°C

Electrical Power Input

Thermosiphon 

Condenser

Heat Rejecter 

(warm head)

Thermosiphon 

Connector

Displacer

Piston

Linear 

Motor

Passive 

Balancer

Acceptor 

(cold head)
Confidential Global Cooling Inc. Page 14 June 2013

c. No	internal	moving	parts.

d. Very	low	refrigerant	charge	for	a	given	heat	transfer rate. In	this	case	100g	or	less.

e. No	oil	that	may	clog	capillary	tubes,	a	known	difficulty	with	conventional	compressors	in	

the	VLT	application.

f. No	driers	or	other	components	that	may	complicate	servicing.

4.3 Heat	Rejection

Blowing	 air	 past	 radial	 fins	 at	 the	 heat	 rejecter	 removes	 the	 heat	 from	 the	 system.	 Two

variable	 speed fans	are used in	 series	 so	as	 to	 provide	 high	 rejection	 of	 heat	 during	intense	

cooling	 periods	 such	 as	initial	 cool	 down	 and	 door	 openings. The	 two-fan	 arrangement	 also	

serves	 to	improve	 reliability	of	 the	system	since	 the	system	will	operate	with	one	 fan	but	at	

reduced	capacity.	The	Stirling	engine	is	much	more	efficient	than	its	compressor	counterparts	

and	therefore the	amount	of	heat	rejected	is	roughly	half	that	of	similarly sized	conventional	

systems.	Room	air	is	drawn	in	through	a	grill	above	the	door,	passes	through	a	tunnel	and	over	

the	rejecter	 fins	on	the	Stirling,	through	the	machine	compartment and	rejected	at the	upper	

right	side	of	the	freezer.	The	air	picks	up	heat	from	the	electronic	controller	as	well.	The	flow	

of	air	is	one	pass	and	the	distance	from	the	inlet	to	the	outlet	is	about	700mm.	The	maximum	

volumetric	flow	is	214CFM.

4.4 Electronic	Components

The	SU780U	is	able	to	take	universal	input	voltages.	The	line	voltage	is	converted	to	DC	voltage

by	the	power	electronics and	this	is	provided	to	an	inverter	that	provides	alternating	current	

to	the	Stirling	engine	at	a	fixed	frequency	but	a	variable	voltage	depending	on	cooling	demand.	

Electrical	 components	 must	 be	 evaluated	 for	 possible	 ignition	 sources.	 Compared	 to	 typical	

VLT	freezer	cascade	systems,	the	Stirling	refrigeration	system	has	a	number	of	key	differences	

regarding	 the	 electrical	 system.	 The	 following	items	 are typically	noted	 as	 potential ignition	

sources (Arthur	D.	Little	1991):

a. Thermostat	or	temperature	control	switch.	Typical	on	/	off	systems	have	a	make	/	break	

switch	 to	 turn	on	 the	compressor	when	 the	 temperature	is	 too	high	and	 turn	it	off	when	

the	 temperature	is	 too	low.	The	 thermostat	is	located	in	 the	cold	space	and	switches	line	

voltage.	 The	 Stirling	 system	 uses	 a	 temperature	 transducer	 called	 an	 RTD, which	 is	

energized	by	a	continuously	connected low voltage	source of	5VDC. The	RTD	is	completely	

encapsulated	by	a	metal	sleeve.

b. Defrost	control,	heater	and	thermostat.	These	items	are	not	used	in	VLT	freezers.

c. Energy	saver	switch.	This	item	is	not	used	in	the	SU780U.
Confidential Global Cooling Inc. Page 15 June 2013

d. Interior	light.	This	item	is	not used	in	VLT	freezers.

e. Automatic	icemaker.	This	item	is	not	used	in	VLT	freezers.

f. Cabinet	 or	 gasket	 heaters.	 Resistance	 heaters	 are	 used	 in	 many	 VLT	 freezer	 cabinets	 to	

warm	critical	areas	so	as	to	prevent	sweating	on	the	exterior	surfaces	of	the	cabinet	or	on	

the	 gasket.	 	In	 the	 SU780U,	 these heaters	 are	 placed	 behind	 the	 thermal	 breaker	 on	 the	

front	surface	of	the	cabinet.	The	thermal	breaker	on	the	door	is	not	heated.	For	R-170	units,	

a	solid-state	switch	driven	by	a	controllable	duty	cycle	modulates	the	gasket	heaters.	

The	 following	 items	 are	 specific	 to	 the	 Stirling	 system	 and	 are	 listed	 as	 potential	 ignition	

sources:

a. Thermal	limit	switch	(called	out	as	a	 thermal	 fuse	in	 the	schematic	diagram,	Fig.	6).	This	

item	opens	 the	circuit	 to	 the	Stirling	engine	when	 the	outside	surface	 temperature	of	 the	

Stirling	 engine	 exceeds	 110°C.	 This	 is	 only	 likely	 to	 happen	 in	 a	 locked	 reciprocator	

condition	(equivalent	to	a	locked	rotor	condition on	conventional	systems).

b. Door	alarm	switch.	This	item	signals	an	alarm	to	sound	if	the	door	has	been	open	beyond	a	

user	specified	time.	The	switched	voltage	is	3.3VDC.

c. Dry	 contact	 relay.	This	is	energized when	an	alarm	 condition	exists	according	 to	 several	

user	settable	conditions.	The	maximum	allowed	switched	voltage	is	24VDC.

d. Power	switch.	This	is	where	the	SU780U	is	switched	on.	It	is	located	at	the	top	rear	of	the	

freezer	next	to	the	line	cord.	The	line	voltage	is	rated	from	108	to	264VAC,	50	or	60Hz.

e. Battery	 switch.	 This	 is	 to	 energize	 a 6V backup	 battery	 to	 maintain	 temperature	

monitoring	and	storage	of	critical	information	during	a	power	failure. It	is	located	near	the	

power	switch.

f. Line	fuse.	Two	line	fuses	are	used	to	the	power	board	and	two	to	the	Stirling	engine.	These	

are	sealed	devices	 that	permanently	open	circuit	when	current	conditions	 for	 those	lines	

are	exceeded.	They	are	located	in	the	machine	compartment.

g. Inverter	board.	The	Stirling	engine	takes	an	AC	input and	adjusting	this	AC	voltage	controls	

the	capacity	of	the	Stirling.	The	inverter	board	takes	DC	power	from	the	power	board	and	

converts	it	 to	a	 controlled	AC	 voltage	 by	 use	 of	an	H-bridge	 circuit.	 The	H-bridge	 circuit

components	are	capable	of	energetic	failure.	We	calculate	that	the	energy	liberated	in	such	

a	failure	to	be	75J.	Well	above	the	MIE	of	R-170.

The	electrical	schematic	for	the	SU780U	is	shown	in	Fig.	6.
Confidential Global Cooling Inc. Page 16 June 2013

Fig.	6 Electrical	Schematic	Diagram	for	the	SU780U Freezer

PCBͲGUIͲ4.1: User Interface

PCBͲM6ͲINV: Inverter Board

PCBͲM6ͲPWR: Power Board

Line Filter

ͲSCHAFFNER FN2070Ͳ12Ͳ06

ͲUL E3087691

AC Power Switch DPDT

ͲCherry WRG32F5FBBNN

ͲUL E23301



AC Power Input Receptacle

ͲQualtek 703WͲ00/04

ͲUL AXUT2.E139592

NonͲLine Isolated

HͲBridge

NonͲLine Isolated Power Supply

5Vdc @ 2A

ͲCUI VSKͲS5Ͳ15UA

ͲUL E222736

12Vdc @ 2.1A

ͲCUI VSKͲS25Ͳ12U

ͲUL E222736

24Vdc @ 1.66A

ͲTDKͲLambda KMS40Ͳ24

ͲUL E135494

NonͲLine Isolated Power Supply

380Vdc @ 5A

ͲCOSEL DPF1000

ͲUL E132067

15Vdc @ 333mA

ͲCUI VSKͲS5Ͳ15UA

ͲUL E222736

Line Filter120/240VAC @ 16A

ͲSchurter FMBAͲ0172Ͳ1670

ͲUL EN133200

Line Fuse

ͲLittelfuse 0314012.HXP

ͲUL E10480

Line Fuse

ͲLittelfuse 0314012.HXP

ͲUL E10480

TVS

ͲLittelfuse V275LA20AP

ͲUL E320116, E56529,

E135010

Earth Ground

M600 Stirling Cooler

PCBͲM6ͲACCEL:

ͲGlobal Cooling Inc.Accelerometer

RTD (Cold Head Temperature)

ͲJMS Southeast

17436C

Thermistor (Warm Site Temperature)

ͲCantherm CWF3AA103G3380

M600 Coil

ͲGlobal Cooling Inc

UL ....

M600Fan

ͲNidec X17L24BHM5Ͳ06

Optical Isolator

Avago

HCPLͲ073A

ͲUL FPQU2.E55361

RS232 Communication

Ͳ Texas Instruments

MAX232DR

Ͳ UL...

RS232 Communication

Ͳ Texas Instruments

MAX232DR

Ͳ UL...

Current Sense Transformer

ͲCR Magnetics

CR8348Ͳ2000

ͲUL XODW2.E235509

Voltage Sense Transformer

Ͳ Tamura Power Transformer

3FDͲ212

ͲUL XOKV2.E95844

Microprocessor

Microchip Technology PIC18F6722ͲI/PT

(10.0 MHz Operating Frequency)

5V 12V

Power Supply

Status Signals

Electronics Fan

ͲNidec X17L24BHM5Ͳ06

Control

Feedback

Signals

Gasket Heater

ͲSpringfield Wire 34152

Ͳ55 W (2333 Ohms)

Gasket Heater Relay

ͲTE Connectivity RZ01Ͳ1A4ͲD005

ͲUL E214025

380 Common

+ 380Vdc

LCD Display Driver

ͲSolomon Systems

SSD1963QL9

Ethernet Driver (Not Used)

ͲMicrochip Technology

ENC624J600ͲI/PT

(4.0 MHz Operating Frequency)

Real Time Clock With Lithium

Battery

Dry Contacts Relay

ͲTE Connectivity RZ01Ͳ1A4ͲD005

ͲUL E214025

Battery Charge Controller

ͲTexas Instruments

UC2906DW

ͲUL

Microprocessor

ͲMicrochip Technology

PIC32MX360F512L

(8.0 MHz Operating Frequency)

Auxiliary Relay (Not Used)

ͲTE Connectivity RZ01Ͳ1A4ͲD005

ͲUL E214025

LCD

ͲSharp LQ070Y3DG3B

Buzzer

ͲPUI Audio XͲ4033ͲTFͲLW80ͲR

RTD (Box Air Temperature)

ͲJMS Southeast

17436D

Door Switch

ͲOmron Electronics

D3DͲ221

ͲUL WOYR2.E41515

Thermal Fuse

ͲSunshine

KSD301ͲP

ͲUL XAPX2.E213536

RTD

ͲJMS Southeast

17436D

Battery

ͲPanasonic – BSG

LCͲR067R29

ͲUL BAZR2.MH13723

Line Fuse

ͲLittelfuse

0216008.HXP

ͲUL E10480

Line Fuse

ͲLittelfuse

0216008.HXP

ͲUL E10480

Line Fuse

ͲLittelfuse

0326008.MXP

ͲUL E10480
Confidential Global Cooling Inc. Page 17 June 2013

5 GAS	LEAKAGE	EVENTS

5.1 Manufacturing	Site

There	 are	 two	 items	 that	 require	 leak	 testing.	 The	 first	 is	 the	 Stirling	 engine,	 which	 having	

being	charged	with	helium	is	placed	in	a	vacuum	chamber	with	a	helium	leak	detector.	There	is	

a	 leakage	 threshold	 that	 conforms	 to	 at	 least	 a	 10-year	 useful	 life.	 If	 the	 unit	 meets	 that	

criterion,	 then	 it	 is	 passed	 for	 integration	 with	 the	 cabinet.	 The	 second	 item	 is	 the	

thermosiphon, which	 is	 charged	 with	 R-170.	 Leakage	 from	 this	 item	 is	 considered	 a	 safety	

issue	due	to	the	flammability	of	R-170.	The	thermosiphon	evaporator	section	is	sourced	from	a	

supplier.	As	purchased,	it	has three brazed	joints	 that	are	certified	leak-free	by	 the supplier.

The	 M600	 Stirling	 engine	 is	 attached	 to	 the	 thermosiphon	 by	 brazing	 two	 elbow	 tubes	

between	the	thermosiphon	evaporator	and	the	thermosiphon	condenser.	The	total	number	of	

shop-brazed	 joints	 is	 four.	 The	 fill	 tube	 is	 attached	 to	 the	 thermosiphon condenser	 by	 the	

vendor of	the	Stirling	head	assembly and	is	certified	to	be	leak-free.	A	silver-soldered	joint	is	

made	 on	 the	 fill	 tube	 to	 extend	 it	 so	 that	 it	 can	 be	 conveniently	 pinched	 off	 after	 the	

thermosiphon	has	been	charged.	The	charging	process	consists	of	attaching	a	vacuum	pump	to	

a	manifold	that	attaches	to	the	fill-tube.	A	vacuum	is	pulled	on	the	thermosiphon	to	less	than	

60	micro-Torr and	held	for	15	minutes.	Any	unacceptable	leak	will	not	allow	a	vacuum	of	this	

level	to	be	maintained.	This	step	confirms	the	integrity	of	the	thermosiphon.	An	R-170	cylinder	

is	 also	 attached	 to	 the	 vacuum	 manifold	 by	 way	 of	 a	 valve	 and	 capillary	 line.	 Once	 the	

evacuation	process	has	been	completed,	a	valve	to	the	vacuum	process	is	closed	and	R-170	gas	

is	allowed	 to	enter	 the	 thermosiphon	by	opening	a	valve	in	 the	capillary	line.	The	amount	of	

gas	charge	is	determined	on	a	weight basis	by	an	electronic	scale. Once	the	correct	charge	has	

been	placed,	the	valve	is	closed	and	the	fill-tube	is	pinched-off	with	a	special	tool	to	form	a	gas	

tight	seal. Removal	of	the	remaining	piece	of	the	fill-tube	from	the	manifold	results	in	a	leakage	

of	about	0.02g.	

5.2 Servicing	Site

If	 for	any	 reason	 servicing	is	 required	where	 the	Stirling	engine	must	be	 removed,	it	will	be	

necessary	 to	 remove	 the	 refrigerant.	 The	 device	 recommended	 for	 this	 purpose	 is	 the	

Caresaver	Universal	 Refrigerant	 Recovery	Unit.	 The	 unit	 has	 an	 onboard	 storage	 capacity	 of	

250g,	 which	is	 sufficient	 for	 two	 SU780U units charged	 with	 R-170.	 In	 order	 to	 remove	 the

refrigerant,	 the	 thermosiphon	 fill	 tube	 will	 be	 penetrated	 with	 a	 refrigerant	 servicing	 tool.	

Connections	to	this	tool	will	be	by	Schrader	valve,	which	opens	only	when	a	properly	equipped	

refrigerant	hose	is	fully	seated	onto	the	valve.	No	significant	refrigerant	leakage	is	anticipated	
Confidential Global Cooling Inc. Page 18 June 2013

during	 the	 removal	 of	 the	 refrigerant.	 However,	 when	 the	 refrigerant	 is	 returned	 to	 an	

appropriate	 cylinder,	 leakage	 is	 anticipated	 when	 the	 refrigerant	 hose	 is	 detached	 from	 the	

cylinder.	The	amount	of	refrigerant	leaked	is what	resides	within	the	Schrader	valve	when	it	is	

disconnected.	 We	 estimate	 that	 to	 be	 0.02g.	 We	 anticipate	 a	 similar	 leak	 when	 the	

thermosiphon	is	refilled	after	repair	and	the	refrigerant	hose	is	disconnected.	We	anticipate	no	

leakage	after	repair	since the	tube	is	sealed	by	a	pinch	off	tool	that	has	exceptional	reliability	

for	this	purpose.

5.3 Non-Conforming	Leaks

Leaks	 that are	 not	 anticipated	 during	 the	 normal	 course	 of	 business	 are	 referred	 to	 as	 nonconforming	leaks.	The	probabilities	of	these	leaks	are	designed	to	be	low	and	their	occurrences	

are	assumed	to	be	at	probability	levels	on	par	with	household	refrigerators.	At	the	factory,	we	

believe	that	leakage	from	a	braze	joint	sufficiently rapid	to	cause	an	explosive	accumulation	of	

gas	is essentially nil.	We	 would	 not	 be	 able	 to	 effectively	 evacuate	the	 thermosiphon	 to	 the	

required	vacuum	level	if	it	had	such	a	leak.	However,	during	transportation,	the	VLT	freezer	is	

subjected	 to	 transport	loads	 that	 are	 only	 statistically	 understood.	 Some	 of	 these	loads	may	

subject	the	braze	joints	to	excessive	loads	or	fatigue	situations	that	could	result	in	damage	and	

therefore	 leakage. The	 process	 of	 sealing	 the	 fill-tube	 by	 the	 pinch-off	 method	 has	 been	

identified	as	a	potential	source	of	non-conforming	leak.	 If	 the	inside	of	 the	copper	 fill-tube	is	

not	 perfectly	 clean	 or	 has	 residual	 oil,	 it	 is	 possible	 that	 the	 pinch-off	 will	 not	 be	 100%	

effective.	We	test	every	pinch-off	seal	for	leakage	by	use	of	a	soap	solution	(residue	is	cleaned	

with	alcohol).	If	a	leak	is	detected,	it	will	be	a	slow	leak	and	it	will	be	into	the	surrounding	air	

since	 the	 freezer	 machinery	 compartment	 will	 not	 be	 closed.	 The	 freezer	 will	 also	 not	 be	

operating,	so	its	ignition	sources	will	not	be	energized. From	our	own	experience,	such	a	leak	

occurs	about	once	in	100	freezers.	For	a	less	experienced	assembler,	this	leak	may	occur	once	

in	 50	 freezers.	 In	 any	 case,	 leakage	 to	 atmospheric	 pressure	 is	 in	 excess	 of	 one	 hour	 and	 a	

second	pinch-off	is	executed	within	three	minutes	of	the	first	since	that	is	how	long	it	takes	to	

determine	the	effectiveness	of	the	seal.	Rarely	do	we	see	a	need	for	more	than	two	attempts	to	

pinch	off	the	fill-tube.

Leaks	due	to	transportation	loads	are	rare	but	do	occur.	We	will	estimate	this	event	based	on	

the	Arthur	D.	Little	report	even	though	the	thermosiphon	has	far	fewer	joints	subject	to	such	

loads	 than	 household	 refrigerators	 have. Of	 course,	 the	 freezer	 is	 not	 operational	 during	

transport	and	therefore	none	of	its	ignition	sources	will	be	relevant	during	transport.
Confidential Global Cooling Inc. Page 19 June 2013

Non-conforming	leaks	during	servicing	are	also	anticipated	at	the	pinch-off	tube and	with	the	

same	 likelihood	 as	 during	 manufacture.	 Braze	 joints	 made	 during	 servicing	 will	 be	 verified	

during	 the	 evacuation	 process	 and	will	 not	 provide	 the	 required	 vacuum	level	if	 they	 suffer	

from	any	significant	leaks.

6 RISK	ASSESSMENT

6.1 Introduction

Statistical	 leak	 data	 is	 assumed	 to	 be	 identical	 to	 that	 identified	 by	 household	 refrigerator	

manufactures	 for	 similar	 conditions	 and	 circumstances. ADL (Arthur	 D.	 Little	 1991) did	

consequence	modeling	to	define	refrigerant	charge	release	and	duration	that	would	constitute	

a	catastrophic	release	of	flammable	refrigerant.	They	found	that	combustion	of	50g	of R-152a	

would	achieve	an	overpressure	of	0.2bar (3psi) in	a	typical	kitchen	of	a	floor	size	of	180	sq.	ft.	

and	ceilings	of	8	ft. The	kitchen	is	also	assumed	to	have	a	single	door	(3ft.	by	7ft.)	and	a	single	

window	(2ft.	by	3ft.).	R-152a	is	in	the	ASHRAE	safety	group	A2	and	its	flammability	limits	are	

4.8	to	17.4%	by	volume	with	a	heat	of	combustion	at	17.4MJ/kg. Table	2,	taken	from	the	ADL

report,	lists	the	biological	and	damage	consequences	for	different	overpressures. Fig.	7	shows	

their	simulation	results of	overpressure	versus	R-152a	mass	for	a	confined	typical	kitchen.

Table	2	Biological	and	Structural	Damage	Consequences	from	Explosion	(Arthur	D.	Little	1991)

Overpressure 

[psi]*

Biological 

Damage Structural Damage

70 99% Fatality Total structural damage

50 50% Fatality Total structural damage

35 1% Fatality Total structural damage

15 Lung damage Severe structural damage

7-8 Shearing and flexure failure of brick wall panels, 8 to 12 

inches thick (not reinforced)

5 Eardrum rupture Shattering of concrete wall panels, 8 to 12 inches thick (not 

reinforced)

2-4

Non-reinforced cinderblock walls shattered; 50% destruction 

of brick buildings; steel frame building distorted; light industrial 

building ruptured.

1-2 Failure of wood siding panels. Shattering of asbestos siding 

and corrugated steel and aluminum panel failure.

0.1-1 Shattering of glass windows.

*Note that the total overpressure may be achieved by reflection of an incident wave of about half of the stated 

values.
Confidential Global Cooling Inc. Page 20 June 2013

Fig.	7 Overpressure [bar] from	Combustion in	a	Typical	Kitchen versus	R-152a	Mass [kg]

(Arthur	D.	Little	1991)

In	order	to	apply	this	data	to	R-170,	we	note	from	Table	1	that	the	heat	of	combustion for	this	

refrigerant	is	51.9	MJ/kg	versus	17.4	MJ/kg	for	R-152a.	We	will	assume	that	the	pressure	rise	

is	 directly	 proportional	 to	 the	 heat	 of	 combustion	 (this	 assumption	 does	 not	 include	

dissociation	energy	so	it	is	approximate).	That	suggests	that	R-170 will	have	roughly	3	times	

more pressure	generation	than	R-152a for	the	same	mass	of	charge.	Bio-storage	facilities	and	

laboratories	 have	 at	 least	 10ft	 ceilings	(Carnegie	 Mellon	 University	 2007) and	 are	 typically	

open	 spaced	 construction.	 We	 will	 consider	 a	 worst-case	 scenario	 where	 VLT	 freezers	 are	

placed	side-by-side	in	a	laboratory	hallway 10ft	wide	by	30ft	long	with	no	windows.	Assuming	

that	the	pressure	rise	is	essentially	adiabatic,	we	can	scale	the	effect	of	the	larger	room	by	the	

adiabatic	law.	This	gives	a	pressure	rise	of	about	0.2bar	(3psi)	 for deflagration of	R-170	in	a	

laboratory	environment,	almost	identical	 to	 the	R-152a	situation	in	a	kitchen. Of	course,	 this	

assumes	50g	mass	as	was	used	in	the	kitchen	study.

The	rate	of	pressure	increase	is	an	important	factor	in	an	explosion and	is	determined	by	the	

deflagration	index	KG (NFPA	2013).	Comparative deflagration	index	data for	n-Butane	and	R170	(ethane) are	shown in	Fig.	8	(Holtappels	2002). For	comparison,	the	maximum	value	of	KG

of	propane	is	100	bar-m/s	(NFPA	2013). Data	for	ethane	from	a	second	source	is	shown	in	Fig.	
Confidential Global Cooling Inc. Page 21 June 2013

9.	In	this	case,	the	abscissa	is	the	equivalence	ratio	Ø.	The	maximum	KG is	similar	in	both	cases

at	about	130	bar-m/s and	is	therefore	taken	as	confirmation	of	this	number.

Fig.	8	Deflagration	Index KG for	Ethane	and	n-Butane

					

Fig.	9	Deflagration	Index	KG for	Ethane	versus	Equivalence	Ratio	(Kunz	1998).

																	

Fig.	10	Peak Combustion Pressures	for	Ethane-Air	Mixtures	(Kunz	1998)

The	 peak	 pressure	 generated	 by	 constant-volume	 combustion	 of	R-170 is	 shown	 in	 Fig.	 10.	

These	 numbers	 are about	 5%	 lower	 than	 what	 would	 be	 expected	 at	 AICC	 conditions.	 Data	

from	technical	University	Delft	(Holtappels	2002) show	similar	results	and	confirm	the	general	

magnitude	 of	 the	maximum	generated	pressure though,	NFPA	 show	a	maximum	pressure	 of	

Initial Conditions: 1bar 

and 295K (22°C)

Initial Conditions: 1bar 

Initial Conditions: 1bar 

and 295K (22°C)
Confidential Global Cooling Inc. Page 22 June 2013

only	7.8bar	(NFPA	2013) for	R-170. The	fundamental	burning	velocity	of	ethane	is	(47cm/s),	

which	is	similar	 to	propane	(46cm/s)	and	n-butane	(45cm/s). Determining	 the	overpressure	

in	 a	 practical	 situation,	 say	 the	 machine	 compartment,	 from	 a	 deflagration	 event	 requires	

complicated	three-dimensional	modeling.	However,	we	believe	that	the	conditions	modeled	by	

Unilever	 for	 ice	 cream	 cabinets	 closely	 resemble our	 situation (Unilever	 2008).	 Unilever	

modeled	 a	 combustion	 event	 under	 stoichiometric	 and	 perfect	mixing	 conditions	 within	the	

compressor	compartment	of	an	ice	cream	cabinet assuming	R-290	(propane)	as	the	refrigerant.	

This	is	not	too	different	from	our	machine	compartment,	which	is	equipped	with	vents	for	inlet	

and	 exhaust	 cooling	 air and	 is	 located	 at	 the	 top	 of	 the	 VLT	 freezer. The	 Unilever	 model	

calculated	 an	 overpressure	 of	 0.1bar	 (1.5psi).	 A	 pressure	 of	 this	 magnitude	 would	 lift	 the	

machine	 compartment	 cover	 but	 would	 dissipate	 rapidly.	 In	 any	 event,	 a	 stoichiometric	

mixture	in	the	machine	compartment	volume	 (0.14m3)	would	represent	about	38g	of	R-170.

From	the	KG value	for	R-170,	the	maximum	rate	of	pressure	rise	in	this	compartment	would	be	

67.5 bar/s.

A	second	source	of	damage	or	injury	occurs	when	 the	 thermal	 radiation	 from	combustion	is	

sufficiently	large	and	 for	a	sufficiently	extended	time.	Fig.	11,	also	taken	 from	the	ADL study,	

shows	 the	 time required	 for	 pain	 sensation	 due	 to	 exposure	 to	 thermal	 radiation. ADL

concluded	 that	 thermal	 radiation	 levels	 of	 about	 12.5	 kW/m2 are	 sufficient	 to	 ignite	

combustibles	and	cause	pain,	but	are	not	sufficient	to	cause	serious	injury	after	exposures	of	

one	second	or	less.

Fig.	 11 Time	 [s]	 Required	 to	

Reach	 Pain	 Threshold	 Due	 to	

Exposure	 to	 Thermal	 Radiation	

[kW/m2]	(Arthur	D.	Little	1991)
Confidential Global Cooling Inc. Page 23 June 2013

We	are	assuming	that	adiabatic	flame	temperature	is	a	measure	of	the	radiated	energy	of	the	

flame.	Table	3 lists the	adiabatic	flame	temperature	for	some	common	fuels	including	R-152a	

all	 initially	 at	 1bar,	 20°C	 and	 stoichiometric	 for	 the	 fuel-air	 mixture. From	 this	 table	 and	

knowing	that	thermal	radiation	is	a	strong	function	of temperature,	it	is	clear	that	ethane	has	

the	lowest	potential	for	thermally	radiated	burns.

Table	3 Adiabatic	Flame	Temperatures	for	Various	Fuels (Wikipedia	2013)

Fuel Oxidizer Adiabatic Flame Temperature [°C]

Butane (C4H10) Air 1970

R-170 Ethane (C2H6) Air 1955

Propane (C3H8) Air 2392

R-152a Air About 2600 (Bennett 2011)

6.2 Probability of	Leaks

The	ADL report	provides	leak	data	from	three	manufacturers	of	household	refrigerators	over	

the	first	five	years	of	service.	By	assuming	that	the	manufacturer	tests	all	refrigerators	before	

shipment,	 they	 determined	 that	 units	 that	 arrived	 at	 their	 customers	 with	 insufficient	

refrigerant	must	have	lost	their	charge	during	shipment.	This	was	deemed	the	best	measure	of	

catastrophic	leak	potential.	We believe	that	this	is	the	best	indication	of	transport	damage	to	

braze	 joints	 and	 the	integrity	 of	 the	 thermosiphon. Arthur	 D.	 Little	 also	 argues	 that	 50%	 of	

non-functioning	refrigerators	occur	prior	to	shipping.	This	does	not	fit	our	assessment	since	all	

VLT	Stirling	 freezers	 are	 final	 tested	in	 three	 categories,	 namely	 a.	 Initial	 pull-down	 time,	 b.	

steady-state	 energy	 consumption	 and	 c.	 temperature	 recovery	 after	 door	 openings.	 This	

process	takes	about	12	hours	to	complete.	During	these	tests	the	thermosiphon	is	subjected	to	

high	 and	low	 pressures	 and	 temperatures	 from	 ambient	 to	 about	-100°C.	 If	 any	 gas	leakage	

were present,	it	would	be	at	an	extremely	low	rate	(less than	5%	loss	of	charge	in	12	hours)	to	

allow	 the	 VLT	 freezer	 to	 pass	 final	inspection.	Therefore	 for	 purposes	 of	 estimating	leakage	

due	 to	 transportation	 damage,	 we	 will	 assume	 that	 the	 number	 for	 household	 refrigerators	

that	 are	 dead	 on	 arrival	 represents	 a	 best	 estimate	 for	 this	 failure	 mode.	 This	 number,	

according	to	Arthur	D.	Little’s	investigation,	is	0.1%.

Arthur	 D.	 Little	 estimate	 that	 there	 are	 between	 11	 – 14	 tubing	 connections	 in	 household	

refrigerators with	 4	 joints	 inside	 the	 cabinet cold	 space.	 Two	 of	 these	 joints	 are	 copper	 to	

aluminum	 and	 were	 identified	 as	 the	 most	 likely	 to	 leak.	 All	 other	 joints	 are	 brazed.	 In	

household	refrigerators,	50%	of	all	catastrophic	leaks	occur	within	the	cabinet	cold	space.	The

VLT	 freezer thermosiphon	 has 7 braze	 joints	 and	 all	 are	 copper	 to	 copper.	 Three	 of	 the	

thermosiphon	joints	 are	within	 the	 foamed	 section	 of	 the	 cabinet.	All	 other	joints	 are	in	 the	

machine	 compartment.	 Where	 the	 thermosiphon	 enters	 the	 Stirling,	 the	 joints	 there	 are	
Confidential Global Cooling Inc. Page 24 June 2013

stainless	 steel	 to	 copper	 and	 those	 are	 vacuum	 brazed together	 with	 the	 Stirling	 cold	 head	

assembly.	The	1/8-inch copper fill-tube	extension and	 the	pinch-off	joint are silver	soldered.

No	 joints	 are	 inside	 the	 VLT	 freezer	 cabinet	 space.	 These	 joints	 are	 shown	 in	 Fig.	 12.	 A	

catastrophic	 leak	 into	 the	 cabinet	 could only	 occur	 if	 the	 16-gauge (about	 1.6mm)	 stainless	

steel (or	powder	coated	steel) inner	liner	were	punctured	sufficiently	and	at	the	right	place	to	

penetrate	the	3/8	inch	copper	thermosiphon	tube.	VLT	freezers	are manually	defrosted	but	ice	

does	 not	 adhere	 to	 the	interior	 freezer	 walls,	 rather	it	 accumulates	 on	 the	 thermal	 breaker.	

Due	to	the	very	low	temperature	of	VLT	freezers,	water	vapor	tends	to	freeze	out	in	the	form	

of powder	snow.	Removal	of	ice	from	the	breaker	is	done	with	a	plastic	scraper	while	the	snow	

in	 the	interior	is	 brushed	 out.	Damage	 to	 the	plastic	 thermal	 breaker	would	not	damage	 the	

thermosiphon	 since	 the	 thermosiphon is	 placed	 well	 back	 from	 the	 breaker	 surface	 and	 is	

encased	in	polyurethane	foam.	We	are	unable	to	identify	any	plausible	event	that	may	achieve	

simultaneous	 penetration	 of	 the	 liner	 and	 thermosiphon	 tube	 that	 could	 result	 in	 a	

catastrophic	failure.	In	addition,	there	are	no	ignition	sources	within	the	cabinet	cold	space	of	

VLT	 freezers.	We	will	 therefore	 assume	 that	all	 catastrophic	leaks	 occur	within	 the	machine	

compartment,	which	we	feel	makes the	numbers	more	realistic	for	our	situation.

We	 have	 reconstructed ADL’s household	 refrigerator	 data to	 include	 our	 belief	 that	 all	 fast

leaks	occur	during	transportation	(defined	as	factory	dock	to	installation	at	customer)	and	that	

they	are	half	as	likely	to	occur	(no	copper	to	aluminum	joints)	and	that	all	slow	leaks	occur	in	

use	at	the	end-user’s	site. This	is	summarized	in	Table	4.

Table	4 Modified	Leak	Rate	Data	Adjusted	from	Arthur	D.	Little	1991	Report

% of refrigerators

Manufacturer 0-7 days (DOA) Year 1 Total Years 2-5 Total Years 0-5

A 0.09 0.22 0.055 0.44

B - 0.5 0.125 1.00

C 0.1 - - -

Worst Case 0.1 x 0.5 = 0.05 0.05 0.125 1.00

Shipping leak 0.05 0.05 - 0.05

Slow leak 0 0.45 0.125 0.95

Fast leak 0.05 0.05 0 0.05

Average Case* 0.09 x 0.5 = 0.045 0.22 0.055 0.44

Shipping leak 0.045 0.045 - 0.045

Slow leak 0 0.175 0.055 0.395

Fast leak 0.045 0.045 0 0.045

*Average case is assumed by ADL to be the lower set of leak rates

Catastrophic	 leakage	 during	 transportation	 could	 happen	 by	 the	 following	 mechanism.	 The	

tubes	are	insulated	in	 the	machine	compartment	by	 flexible	material	and	are	attached	 to	 the	

Stirling	 engine,	 which	 is	 soft	 mounted	 to	 the	 cabinet.	 Transportation	 loads	 could	 therefore	

subject	 the	 VLT	 freezer	 to	 vibrations	 that	 allow	 small	 relative	motions	 of	 the	 thermosiphon	
Confidential Global Cooling Inc. Page 25 June 2013

tubes	and	cause	 fatigue	damage.	Since	 the	 freezer	is	not	powered	during	 transportation	and	

both	 the	 shipping	 container	 and	machinery	 compartment	 are	 not	 gas	 tight,	 we	 feel	 that	 the	

likelihood	 of	 accumulated	 gas	 is	 small.	 However,	 it	 seems	 possible	 under	 certain	

circumstances	that	some	gas	could	remain	in	the	machinery	compartment	up	the	point	when	

the	VLT	freezer	is	first	switched	on.	A	simultaneous	failure	of	the	two	exhaust	fans	(L10	life	is	

70,000	 hrs.	 for	 each	 fan)	 would	 exacerbate	 this	 situation.	 We	 will	 attempt	 to	 quantify	 this	

scenario	by	assuming	 that	 the	ignition	source	would	be	a	high-energy	 failure	of	 the	H-bridge	

on	the	inverter	board	of	the	electronics	section.	We	have	seen	this	 failure	in	non-conforming	

parts.	 Since	 this	is	 a	 failure	 that	 occurs	 on	initial	 power	 and	 each	 board	is	 pre-tested,	 these	

failures	are	always	caught	before	the	product	leaves	the	factory.	However,	we	will	assume	a	1	

in	 1,000	 chance	 of	 an	 electronic	 failure	 at	 the	 delivery	 site	 (based	 on	 2	 sigma	 normal	

distribution	 on	 current	 units	 in	 the	 field).	 Since	 there	 are	 at	 least	 10	 discrete	 sections	 that	

must	 all	 function	 properly	 for	 the	 electronics	 to	 start	 up,	 we	 will	 assume	 that	 an	 H-bridge	

failure	is	a	1	in	10	chance.

Slow	leakage	 from	imperfect	braze	joints	are	possible	but	 they	would	have	 to	be	 sufficiently	

small	as	to	allow	the	evacuation	process	of	the	thermosiphon	to	proceed	normally.	To	test	this	

scenario,	we	installed	capillary	tubes	to	all	braze	joints	within	the	VLT	cabinet	and	requested	

UL	to	test	for	potential	refrigerant	leakage	into	the	cabinet.	We	do	not	have	their	official	report	

as	of	this	writing,	but	in	speaking	to	the	UL	manager	of	the	project,	interior	space	leakage	has	

not	been	a	problem.

Fig.	12 Inner	Liner	of	VLT Stirling	Freezer Showing Thermosiphon	Joints

Joint 2 and 3 on 

either side here

Joint 1

Inner Liner

Joint to Stirling Engine 

(completed after 

cabinet is foamed)
Confidential Global Cooling Inc. Page 26 June 2013

6.3 Failure	Rate	Estimates

Failures	that	could	ignite	a	catastrophic	leak	within	the	machine	compartment	are	as	follows:

a. A	bad	power	switch	where	an	arc	with	sufficient	energy	is	present.

b. An	energetic	failure	of	the	electronics	at	initial	power	on.	We	believe	that	this	is	only	likely	

on	the	H-bridge	section	of	the	inverter (Fig.	6).	All	electronics	are	pretested	and	run	for	at	

least	 12	 hours	 before	 the	VLT	 freezer	is	 shipped.	We	 have	 not	 seen	 such	a	 failure	in	 the

field	but	nevertheless,	we	will	assume	a	field	failure	of	the	electronics	at	1	in	1,000	and	a	

likelihood	 of	 the	 failure	in	 the	H-bridge	as	 1	in	10	and	 further,	 that	 the	 failure	is	always	

energetic	enough	to	ignite	the	gas	mixture.

c. A requirement	is	that	the	gas	mixture	be	resident	in	the	machine	compartment.	For	this	to	

happen,	 both	 fans	 must	 have	 failed	 simultaneously.	 	 The	 fans	 are	 also	 tested	 before	

shipment	but	nevertheless;	 their	L10	lives	are	70,000	hours	(about	8	years).	That	means	

that	 10%	 of	 the	 fans	 fail	 in	 that	 time	 or	 0.0125/yr.	 For	 two	 fans	 simultaneously	 this	

becomes	1.56	x	10-4/yr.

We are	excluding	the	following items	as	potential	ignition	sources	at	start	up:

a. A	 locked	 rotor	 or	 armature	 condition.	 This	 is	 characterized	 by	 the	 moving	 components	

becoming	locked	within	the	Stirling	engine.	The	applied	power	therefore	heats	the	engine	

and	perhaps	 to	a	high	enough	 temperature	 to	ignite	 the surrounding gas	mixture.	 In	 this	

situation,	 the	 thermal	 fuse	 would	 have	 to	 fail	 to	 allow the	 Stirling	 engine	 to	 heat	 up.	 In	

addition,	with	a failed	 thermal	 fuse,	 the	Stirling	would	 take	a	number	of	hours	 to	reach	a	

high	 enough	 temperature	 so	 we	 feel	 that	 it	 is	 unlikely	 for	 that	 the	 gas	 not to have	

dissipated	by	then.

b. The	gasket	heater	relay. We	have	replaced	this	item	with	a	solid-state	switch	based	on	UL’s	

recommendation. There	will	be	no	gasket	heater	relay	in	SU780Us	using	R-170.

c. Door	alarm	switch.	This	circuit	is	only	energized	after	the	initial	pull	down	to	temperature.	

d. Dry	contact	relay.	This	is	for	an	external	alarm	circuit	and	is	energized	after	the	initial	pull	

down	to	temperature.

Table	 5 summarizes	 the	 failure	 rate	 estimates for	 potential	 ignition	 sources	 within	 the	

machine	compartment.	
Confidential Global Cooling Inc. Page 27 June 2013

Table	5 Failure	Rate	Estimates for	an	Internal	Event

Event Description Frequency of Probability Source / Discussion

1.1 Sudden release 0.0005/5 = 1x10-4 /yr From Table 3

1.2 Leak occurs at delivery 30/(7 x 24 x 60) = 3x10-3 Assume one week for delivery with

probability of fault occurring in last 30 

minutes

1.3 Two exhaust fans fail 

simultaneously (0.1/8)2 = 1.6 x10-4

/yr L10 life = 70,000 hrs (8 yrs)

1.4 Electronic component failure 5,000/1,000 = 5 Failure of electronics at 1 in 1,000.Maximum estimated delivery is 

5,000/yr.

1.5 H-Bridge failure 0.1 H-bridge failure as 1 in 10

1.6 Sufficiently energetic to ignite 1 Energy liberated is greater than MIE

1.7 Arc in switch 3x10-8 From ADL 1991 report

1.8 Sufficient energy to ignite 9.1x10-2 From ADL 1991 report

For the	situation	where	a	catastrophic	leak	within	 the	machine	compartment	is	ejected	 from	

the	VLT	freezer	by	the	fans,	it	is	possible	that	ignition	sources	outside	of	the	freezer	may	ignite	

the	 mixture.	 According	 to	 ADL,	 citing	 UL	 research,	 a	 release	 of	 refrigerant	 outside	 of	 a	

refrigerator	is	dispersed	below	the	lower	flammable	limit	in	about	one	minute.	We	will	assume	

that	the	same	situation	will	apply	to	the	VLT	freezer	when	the	fans	eject	a	flammable	mixture.

Again	 we	 are	 assuming	 that	 the	 thermosiphon	 tubes	 catastrophically	 leak	 in	 the	 last	 thirty	

minutes	of	delivery.	 In	 this	case,	 the	VLT	 freezer	is	switched	on,	operates	normally	(with	 the	

exception	 of	 the	 thermosiphon)	 and	 therefore	 discharges	 an	 explosive	 mixture	 into	 the	

immediate	surrounds	of	its	placement	by	the	fans. Ignition	sources	may	be	door	switches,	dry	

contact	relays,	gasket	heater	relays	and	/	or	other	energetic	sources	in	the	immediate	vicinity	

of	the	freezer.	A	worst-case	laboratory	situation	would	have	VLT	freezers	lined	up	next	to	each	

other,	spaced	 ten	inches	apart.	 It	is	quite	conceivable	 that	an	adjacent	 freezer	could	ingest	a	

discharge	 of	 a	 flammable	 gas	 mixture.	 We	 will	 assume	 four additional	 freezers	 in the	

immediate	vicinity,	operating	normally. Each	freezer	would	ingest	the	charge	and	expel	it	to	its	

neighbor.	We	will	assume	that	the	charge	is	equally	potent	for	each	freezer	but	is	sufficiently	

dispersed	 by	 the	 fourth	 freezer	 not	 to	 present	 any	 remaining	 risk.	In	 this	 case,	 the	 ignition	

sources	would	be	as	follows:

a. Gasket	 heater	 relay.	 This	 would	 be	 functioning	 in	 the	 adjacent	 VLT	 freezers	 since	 they	

would	likely	be	at	temperature.	The	relay	is	closed	twice	in	every	hour. We	are	including	

the	relay	here because	they	could	exist	in	non	R-170	units.

b. Door	alarm	switch.	The	door	of	an	adjacent	freezer	could	be	opened.

c. Dry	contact	relay.	This	could	be	energized	if	a	fault	is	detected	in	an	adjacent	freezer.
Confidential Global Cooling Inc. Page 28 June 2013

Table	6 Failure	Rate	Estimates	for	an	External	Event

Event Description Frequency of Probability Source / Discussion

2.1 Sudden release 0.0005/5 = 1x10-4 /yr From Table 3

2.2 Leak occurs at delivery 30/(7 x 24 x 60) = 3x10-3 Assume one week for delivery with

probability of fault occurring in last 30 

minutes

2.3 Adjacent freezer door is 

opened Handled as a switch event

2.4 Adjacent freezer gasket 

heater relay is energized Handled as a switch event

2.5 Adjacent freezer dry contact 

relay is energized Handled as a switch event

2.6 Arc in switch 3x10-8 From ADL 1991 report

2.7 Sufficient energy to ignite 9.1x10-2 From ADL 1991 report

6.4 Fault	Tree	Analysis

Fig.	13 shows	the	fault	tree	for	a	fire	or	explosion	in	the	machine	compartment	for	the	worstcase	 data.	 The	 most	 serious	 ignition	 source	 is	 an	 energetic	 failure	 of	 the	 H-bridge	 on	 the	

inverter board	 (see	 Fig.	 6). The	 frequency	 of	 a	 fire	 or	 an	 explosion	 inside	 the	 machine	

compartment	is	2.4 x	10-11 /yr. under	the	worst-case assumptions.

Fig.	13 Worst	Case	Scenario	for	Explosive	Event	in	Machine Compartment

1.0

5 X E-1

2.4 X E-11 /yr

Failure of Electronic

Component Causes

Ignition

9.1 X E-2

Leak Occurs at 

Final Delivery

3 X E-8 0.1

Switch Causes

Ignition

Both Exhaust

Fans Not 

Functioning

Sudden Release Leak Ignites

Arc in Switch Sufficient Energy

to Ignite H-Bridge Failure

Electronic

Component

Failure

Fire or Explosion Inside

Machine Compartment

5

0.5 X E-1

5.4 X E-9

2 Switches

1 X E-4 /yr 3 X E-3 1.6 X E-4 /yr

Sufficiently

Energetic
Confidential Global Cooling Inc. Page 29 June 2013

Fig.	14 shows	the	fault	tree	for	having	a	fire	or	an	explosion	outside	of	the	VLT	freezer	within	a	

typical laboratory	space	using	worst-case	data. Any	of	four	adjacent	VLT	freezers	are	assumed	

to	 be	 likely	 ignition	 sources	 to	 the	 gas	 cloud.	 The	 ignition	 sources	 are	 assumed	 to	 be	 from	

switching	events	such	as	relays	and	door	switches.	An	energetic	electronics	failure	only	occurs	

on	start	up	so	it	is	not	considered	as	an	ignition	source	in	the	adjacent	freezers. The	frequency	

of	 a	 fire	 or	 an	 explosion	 caused	 by	 adjacent	 freezers	 is	 1	 x	 10-14 /yr. under	 the	 worst-case	

assumptions.

In	 contradistinction	 to	 household	 refrigerators,	 the	 chance	 of	 a	 fire	 or	 explosion	 is	 much	

higher	within	the	VLT	freezer	machine	compartment.	

Fig.	14 Worst-Case	Scenario	for	an	Explosive	Event	in	an	Adjacent	Freezer

7 RISK	MITIGATION

Given	the	risk	of a	fire	or	explosion	in	the	machine	compartment	as	2.4	x	10-11 /yr.	and	noting	

the	 estimation	 of	 the	 currently	 installed	 base	 of	 VLT	 freezers	 at	 500,0005 the	 potential	 of	

added	annual	 fires	is	negligibly	small.		Nevertheless,	it	is	prudent	 to	consider	risk	mitigation	

measures.	We	believe	that	the	following	items	are	most	likely	to	return	good	results:

1. Check	for	leak	integrity.	We	already	do	this	but	clearly	the	urgency	of	this	task	is	magnified.

 5 Internal	estimation

Sudden Release Leak Occurs at 

Final Delivery

3 X E-8 9.1 X E-2

Fire or Explosion Inside 

Adjacent Freezer 

Machine Compartment

1 X E-4 /yr 3 X E-3

Sufficient Energy 

to Ignite Arc in Switch

Leak Ignites

3.3 X E-8

3 X E-8 9.1 X E-2

Sufficient Energy 

to Ignite Arc in Switch Arc in Switch Sufficient Energy to Ignite

Dry Contact Relay 

Causes Ignition

1.1 X E-8

4 Freezers

3 X E-8 9.1 X E-2

1 X E-14 /yr

1.1 X E-8 1.1 X E-8

4 Freezers 4 Freezers

Door Alarm Switch 

Causes Ignition

Gasket Relay 

Causes Ignition
Confidential Global Cooling Inc. Page 30 June 2013

2. Consider	 transportation	 loads	 and	 means	 to	 prevent or	 limit movement	 of	 the	

thermosiphon	tubes	during	shipping.	The	Unilever	study	also	identified	regular	movement	

of	ice	cream	cabinets	in	the failure	of	tubing	joints	(Unilever	2008).

3. Reduce	 ignition	 sources	 either	 by	 redesign	 or	 by	 removing	 the	 possibility	 of	 flammable	

gases	coming	into	contact	with	an	ignition	source (sealing,	 for	example).	We	believe	 that	

we	 are	 addressing this	 concern	 by	 pre-testing	 all	 electronic	 components	 used	 in	 the	

machine	compartment.

4. Conforming	to	UL-471	specifically	now	that	this	standard	includes	VLT	freezers.	

8 DISPOSAL	ISSUES

Safe	disposal	of	discarded	units	must	be	taken	into	account.	Currently	there	is	a	framework	to	

reclaim	refrigerants	 from	VLT	 freezers	at	end	of	life. With	the	use	of	R-170,	it	is	vital	to	alert	

the	service	or	reclaiming	entity	that	flammable	refrigerants	are	present.	Unless	other	guidance	

is	 provided,	 we	 would	 suggest	 following	 the	 same	labeling	 requirements	 now	 advised	in	 40	

CFR	Part	82	for	hydrocarbon	refrigerants.

9 BIBLIOGRAPHY

1. ANSI/ASHRAE.	Safety	Standard	for	Refrigeration	Systems. Standard,	Atlanta:	ASHRAE,	2010.

2. Arthur	D.	Little.	"Risk	Assessment	of	Flammable	Refrigerants	for	Use	in	Home	Appliances."	

Report,	Division	of	Global	Change,	EPA,	1991.

3. ASHRAE.	Position	Document	on	Natural	Refrigerants. Report,	Atlanta:	ASHRAE,	2011.

4. Bennett,	Casey.	Laminar	Burning	Speed	and	Flame	Structure	of	1,1-Difluoroethane	(HFC152A)/Air	and	Difluoromethane	(HFC-32)/Air	Mixtures. MSc	Thesis,	Mechanical	and	

Industrial	Engineering,	Northeastern	University,	Boston:	Northeastern	University,	2011.

5. Berchowitz,	D.	M.,	and	Y.	Kwon.	"Environmental	Profiles	of	Stirling-Cooled	and	CascadeCooled	Ultra-Low	Temperature	Freezers."	Sustainability (mdpi)	4,	no.	11	(2012):	2838-

2851.

6. Carnegie	Mellon	University.	Construction	of	Biosafety	Containment	Laboratories. 2007.	

http://www.cmu.edu/ehs/facilities-construction/biosafety-containment-laboratories.html	

(accessed	June	23,	2013).
Confidential Global Cooling Inc. Page 31 June 2013

7. Holtappels,	K.	Report	on	the	Experimentally	Determined	Explosion	Limits,	Explosion	

Pressures	and	Rates	of	Explosion	Pressure	Rise	- Part	2:	Ethane,	Ethylene,	Propane,	n-Butane,	

Ammonia	and	Carbon	Monoxide. Report,	TU	Delft	and	BAM,	TU	Delft,	2002.

8. IPCC.	"Fourth	Assessment	Report:	Climate	Change."	Report,	2007.

9. Kaufeld,	M.	Availability	of	Low	GWP	Alternatives	to	HFCs. Report,	Environmental	

Investigation	Agency,	Washington:	EIA,	2012,	53.

10. Kunz,	Oliver.	Combustion	Characteristics	of	Hydrogen- and	Hydrocarbon-Air	Mixtures	in	

Closed	Vessels. Diploma-Thesis,	Luft	und	Raumfahrttechnik,	University	of	Stuttgart	and	

California	Inst.	of	Technology,	Pasadena:	Explosion	Dynamics	Laboratory,	1998.

11. Mitu,	Maria,	Venera	Giurcan,	Domnina	Razus,	and	Dumitru	Oancea.	"Temperature	and	

Pressure	Influence	on	Ethane–Air	Deflagration	Parameters	in	a	Spherical	Closed	Vessel."	

Energy	&	Fuels 26,	no.	8	(2012):	4840-4848.

12. NFPA.	Standard	on	Explosion	Protection	by	Deflagration	Venting. Report,	National	Fire	

Protection	Association,	Quincy:	NFPA,	2013.

13. Unilever.	"EPA	Submission	- Use	of	R290	in	Ice	Cream	Cabinets."	2008.

14. Wikipedia.	Adiabatic	Flame	Temperature. June	13,	2013.	

http://en.wikipedia.org/wiki/Adiabatic_flame_temperature	(accessed	June	23,	2013).