COP and Superheat - does is it help?

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I try some writting her (Im not native English - so excuse for misspellings word and grammar)

If we take AC for cars in desert area with 2 degree C of evaporator and 70 degree C in condenser temperature no SH or SC on pipes before condenser/evaporator

compressor assume 1 in isentropic effectivity for more easy calculation (ie. ideal situation...)

(i know - is ugly way to put numbers)

R134a	Temperature C	Pressure Mpa	Enthalpy kJ/kg	Entrophy kJ/kg	Density kg/m^3
without SH exchanger
	2	0,31	251,62	0,93	15,46			Cold gas from evaporator
	77,04	2,12	291,04	0,93	106,5	-39,42		work to the compressor
	70	2,12	156,14	0,54	996,25	134,9		heat from condenser
	2	0,31	156,14	0,58	29,66			After ex-valve quality 0,51551
	2	0,31	251,62	0,93	15,46	-95,48		Cooling power from evaporator
						3,42		Heating-COP
						-2,42		Cooling-COP
						266,86		kg/hour refrigerant for 10 kW heat
						17,26		m^3 i suction volume per hour for heat
						95,87		cc efficiency volume on compressor on 50 Hz (3000 rpm) for 10 kW heating
						267,87		liter liquid refrigerant before pressure loss in TXV for 10 kW heat
						377,04		kg/hour refrigerant for 10 kW cooling
						378,46		liter refrigerant/hour for 10 kW cooling power
						24,38		m^3 i suction volume per Hour for 10 kW cooling
						135,46		cc efficiency volume on compressor on 50 Hz (3000 rpm) for 10 kW cooling

and for HFO1234yf (new refrigerant for Car to replace R134a)

HFO1234yf	Temperature C	Pressure Mpa	Enthalpy kJ/kg	Entrophy kJ/kg	Density kg/m^3
without SH exchanger
	2	0,34	201,99	0,74	18,83			Cold gas from evaporator
	70	2,04	232,02	0,74	137,88	-30,03		work to the compressor Q = 0,95 => steam in vapor out from compressor
	70	2,04	141,07	0,53	882,47	90,95		heat from condenser
	2	0,34	141,07	0,57	29,86			After ex-valve quality = 0,62384
	2	0,34	201,99	0,79	18,83	-60,92		Cooling power from evaporator
						3,03		Heating-COP
						-2,03		cooling-COP
						395,82		kg/hour refrigerant for 10 kW heat
						21,02		m^3 i suction volume per hour for heat
						116,76		cc efficiency volume on compressor on 50 Hz (3000 rpm) for 10 kW heating
						448,54		liter liquid refrigerant before pressure loss in TXV for 10 kW heat
						590,94		kg/hour refrigerant for 10 kW cooling
						669,64		liter refrigerant/hour for 10 kW cooling power
						31,38		m^3 i suction volume per Hour for cooling 10 kW
						174,31		cc efficiency volume on compressor on 50 Hz for 10 kW cooling

You lose COP going from R134a to HFO1234yf and volume flow on liquid line almost doubling compare to R134a for same cooling power and system need bigger pipe with more volume inside or accept higher flow rate and compressor have almost 30% bigger pumping suction volume for same cooling power.

If done HFO1234yf system witch suction SH with liquid line, is possibly make better COP and lower mass and volume flow (but how this work in car in hot temperature and maximum fan to cooling down compartment short after start and suction gas already hot from evaporator before SH-exchanger, this is different story)

HFO1234yf	Temperature C	Pressure Mpa	Enthalpy kJ/kg	Entrophy kJ/kg	Density kg/m^3
with SH- exchanger
	2	0,34	201,99	0,74	18,83			Cold gas from evaporator
	2(g) -> 53 (g)	0,34	252,37	0,91	14,96	50,36		heat from SH- exchanger
	53 (g) -> 110 (g)	2,04	293,37	0,91	92,37	-41		work to the compressor
	110(g) -> 70 (liq)	2,04	141,07	0,47	882,47	152,3		heat from condenser
	70(liq) -> 39(liq)	2,04	91,33	0.32	1048	-50,36		heat to SH- exchanger
	39(liq) -> 2 (liq/g)	0,34	141,07	0,57	29,86			After ex-valve quality = 0,317
	2	0,34	201,99	0,79	18,83	-110,66		Cooling power from evaporator
						3,71		Heating-COP
						-2,7		cooling-COP
						237		kg/hour refrigerant for 10 kW heat
						12,6		m^3 i suction volume per hour for heat
						70,03		cc efficiency volume on compressor on 50 Hz for 10 kW heating
						226,48		liter liquid refrigerant before pressure loss in TXV for 10 kW heat
						325,32		kg/hour refrigerant for 10 kW cooling
						310,4		liter refrigerant/hour for 10 kW cooling power
						17,3		m^3 i suction volume per Hour for cooling 10 kW
						95,97		cc efficiency volume on compressor on 50 Hz for 10 kW cooling

With good heating transfer SH-exchanger seem HFO1234yf working very good - better than R134a without SH-exchanger and almost all this is depend to make cooler liquid line before TXV and quality of evaporated mass going from 0.62 of liquid mass for self-cooling to only 0.317 of liquid mass and have almost double of usable liquid mass in evaporator to absorb heat.

But i real world you have heating from compressor easily exceed maximum 110 degree C of hot gas temperature from compressor, SH-exchanger not work perfectly and we have also starting process before we have any cool suction gas from evaporator to feed SH-exchanger...


Interesting notice how this look in using DiMetylEther (short DME) in car-using situation - this single refrigerant is very similar to R12 an R134a in pressure at same temperature

DME	Temperature C	Pressure Mpa	Enthalpy kJ/kg	Entrophy kJ/kg	Density kg/m^3
with out SH - exchanger
	2	0,29	522,82	1,93	6,26			Cold gas from evaporator
	84,87	1,81	613,14	1,93	35,6	-90,32		work to the compressor
	70	1,81	264,72	0,91	578,47	348,42		heat from condenser
	2	0,29	264,72	0,99	15,56			After ex-valve quality = 0,49706
	2	0,29	522,82	1,93	6,26	-258,1		Cooling power from evaporator
						3,86		Heat-COP
						-2,86		Cool-COP
						103,32		kg/hour refrigerant for 10 kW heat
						16,5		m^3 i suction volume per hour for heat
						91,65		cc efficiency volume on compressor on 50 Hz for 10 kW heating
						178,62		liter liquid refrigerant before pressure loss in TXV for 10 kW heat
						139,48		kg/hour refrigerant for 10 kW cooling
						241,12		liter refrigerant/hour for 10 kW cooling power
						22,27		m^3 i suction volume per Hour for cooling 10 kW
						123,73		cc efficiency volume on compressor on 50 Hz for 10 kW cooling

DME have high latent heat compare to most other refrigerant except ammonia, so not need so much liquid refrigeration mass to give lot of cooling power

(i hope is not made to much mistake on all calculation and numbers above and make wrong conclusion depend of them in later text here.)

Some compare:

for 10 kW cooling at 2 degree C for evaporator and 70 degree C on condenser	R134a	HFO1234yf	HFO1234yf with SH exchanger and 110 degree C hot gas from (ideal) compressor	DME (dimetyl ether)
mass flow kg/hour	377	591	325	139
volume flow liq liter/hour	379	669	310	241
cc on compressor at 50 Hz (3000 rpm)	135	174	96	123,7
suction volume m^3/hour	24,38	31,4	17,5	22,3
cooling-COP	2,42	2,03	2,7	2.86

DME not winning to have SH exchanger

DME have almost same burning heat as methanol/etanol in air - around half of energy per kilo compare burning propane/butane - Need also higher lever of leak gas blend in air before lowest explosion level compare to HC (around 3.5 percent in air, HC have 2 percent limit) and if make design right on for example car-AC with smaller pipes (going from 8 mm to 6 mm on liquid line ex.) etc. it needs only 150 - 180 gram DME refrigerant compare to standard small/medium car with 470 gram R134a load for same cooling power..

ie. load is close to allowed maximum HC-refrigerate mass inside fridge and freezer for indoor using (max 150 gram HC) with free placing and no demand of extra ventilation.


With HFO1234yf you need going up some size on pipes and bigger load of refrigerant to take more mass-flow and if using SH-exhanger with some usable area inside, take also lot of liquid volume. I expect around 30% more refrigerant with HFO1234yf compare to R134a for same cool-power.

But.. last time i look in new car with new HFO1234yf filled - newer see any trace of SH-exhanger in cooling loop, evaporator seems not enforced/rugged. TXV still inside compartment area with O-rings-sealing as standard in many car, and build pretty same as ordinary R134a-system, but now we have flammable refrigerant with poison fumes if burning...

So they either take down cooling capacity a bit or work with high mass flow and lower COP in system, so after pretty tales about carbon oxide foot-print and so on, seems in the end cheating and only using cheapest solution provided and payed from refrigerant makers in time had possibility to make serious move to non synthetic and nature and more effective existed refrigerant...

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Is very lively discussion over year if people slightest thinking to using HC as propane and isobutane-blend in car-AC - but now is flammable refrigerant is provided from 'right side' - seem now OK even if refrigerant burning make poisonous hydrofluoric gas in some percent in smoke (0.1 percent hydrofluoric gas in breathing air is deadly even in very short time as couple of seconds).

As consumer as me want have some choice between refrigerant in car in case of burning and for me is very easy to select between car with 700 grams of HFO1234yf and giving poison smoke if burning compare 150 gram of DME for same cooling power and with burning have same smoke as over Trangia stove burning ethanol...