Some information on the aplication of suction gas/liquid line heat exchangers

There seems to be quite a bit of confusion on the effect of a suction/liqiud line heat exchanger.

It isnt possible to give a general answer on the effect such a heat exchanger will give. It depend mostly on the refrigerant used, and also on other parameters.

Refrigerants with high spesific latent heat compared to the spesific heat of the wapor is not suited to the application of a SG/LL exchanger.

Refrigerants with low spesific latent heat compared to the spesific heat of the wapor is well suited to the application of a SG/LL exchanger.

When i talk about suitability i have in mind the level of increase in cooling capasity and COP. I dont initially consider the eventual problem of excessive high discharge gas temperature when i'm talking about the suitability.

Look at the line for constant entrophy on a log p-H chart. If this line is near parallel with the saturated wapour line, as it is with R404a and R134A, then it is a significant gain to acheive with the application of a SG/LL, both in cooling capasity and COP. These refrigerants have low spesific latent heat.

On the oposite, if there is a considerable angle, like 15 to 30 degres, between the wapour saturation line and the constant entrophy line, like with R22, R410a, R408a and the extreme case R717 ammonia, then this refrigerant is not suited for the installation of a SG/LL heat exchanger. These refrigerants have a high spesific latent heat.

This is the fastest way to determine the aproximate suitability of a spesific refrigerant to the application of a SG/LL heat exchanger.

The variation of the compressor suction temperature has no measurable influence on the power consumtion of the compressor when it is running. The power consumtion of the compressor is only determined by the suction and discharge pressures, and to a lesser extent to the physical properties of the refrigerant gas. As an example, at the exact same pressures, R22 gives a slightly higher power consumtion than R404a. If Air Duster had still been around here, he could probably have explained mathematically why the suction temperature have no measurable influence on the power consumtion of the compressor.

If we assume constant evaporating and condensing temperatures, then the power consumtion will be constant regardless of what the suction gas temperature is. From that we see that any gain from the installation of a SG/LL heat exhanger has to come from increased cooling capasity.

It is fundamentally important to distinguish between the power consumtion of a specific compressor when it is running, and the power consumtion necessary to maintain the proper temperature in, lets say, a coold room. If we, with the aplication of a SG/LL heat exchanger, is able to increase the capasity of the compressor and the system as a whole, then the compressor dont have to run as long as it ordinarily has to do. And the compressor will have a reduced power consumtion over time even if it still consumes the exact same power when running.

When we install a SG/LL exchanger, there is noticeable two different things that happen as seen from the compressor point of wiew. First the increase in SG temperature, and then the reduction in the mass flow rate.

As the temperature increases, the gas expands. Since the compressor compress a constant volume for every stroke, the mass of the refrigerant compressed every stroke actually decreases. The mass flow rate decreases. As stated previously, the SG temperature has no influence on the power consumtion when the compressor is running. The power consumtion is unchanged after the installation of the SG/LL heat exchanger.

These two things works in opposite directions. The reduction in the mass flow rate actually contribute to decreased cooling capasity, and the increased SG temperature contributes to increased cooling capasity.

With this information we can now have a look at what happens at the evaporator. The easiest way to do this is to consider the evaporator and the SG/LL exchanger as one unit. We have no need to know or to calculate the liquid temperature leaving the SG/LL exchanger, and no need to know the liquid temperature entering the expancion device either. Nor do we need to know the evaporator outlet temperature or the gas temperature entering the SG/LL exchanger. We can pretend ourselves standing outside the cool rom and not knowing whats happening inside. We se the liquid enter the coold room with the same temperature as earlier. We see the same suction and discharge pressures. We know the mass flow rate is reduced. And we se the considerably increased SG temperature leaving the evaporator-SG/LL exchanger-coold room.

We now have to do some math to determine if we have acheived a gain or a loss of cooling capasity.


First one example without SG/LL exchanger:
Refrigerant R404a

Condensing _______40C________Pressure 18,3 bar/abs
Subcool __________0K
Entering SG/LL ____ 40C _______Enthalphy 263,8
Evaporation ______-10C _______Pressure 4,41 bar/abs
Superheat ________5K
SG _____________-5C _________Enthalphy 366,7
Mass flow rate ____0,1 kg/s
Spesific volum gas _0,0464 m3/kg
Volum flow/comp displacement 4,64 l/s

This gives a cooling capasity of 366,7-263,8x0,10=10,3kW

Now with SG/LL exchanger:

Condensing __________40C ___________Pressure 18,3 bar/abs
Subcool _____________0K
Entering SG/LL _______40C ___________Enthalphy 263,8
Evaporation _________-10C ___________Pressure 4,41 bar/abs
Superheat ___________40K
SG _________________30C ___________Enthalphy 397,7
Mass flow rate _______0,085 kg/s
Spesific volum gas ____0,0545 m3/kg
Volum flow/comp displacement 4,64 l/s

This gives a cooling capasity of 397,7-263,8x0,085=11,4kW

We see that the SG/LL exhanger leads to an increase in cooling capasity of 10% which lead to a decrease in actual power consumtion of 9% du to reduced running time.


Now an example with R22.

First one example without SG/LL exchanger:
Refrigerant R22

Condensing ____________40C ___________Pressure 15,3 bar/abs
Subcool _______________0K
Entering SG/LL _________40C ___________Enthalphy 249,7
Evaporation ___________-10C ___________Pressure 3,54 bar/abs
Superheat _____________5K
SG ___________________-5C ___________Enthalphy 405
Mass flow rate ________0,069 kg/s
Spesific volum gas _____0,0671 m3/kg
Volum flow/comp displacement 4,64 l/s

This gives a cooling capasity of 405-249,7x0,69=10,7kW

Now with SG/LL exchanger:

Condensing ________40C _____________Pressure 15,3 bar/abs
Subcool ___________0K
Entering SG/LL _____40C _____________Enthalphy 249,7
Evaporation _______-10C ____________Pressure 3,54 bar/abs
Superheat _________40K
SG _______________30C _____________Enthalphy 429
Mass flow rate ______0,059 kg/s
Spesific volum gas ___0,0782 m3/kg
Volum flow/comp displacement 4,64 l/s

This gives a cooling capasity of 429-249,7x0,059=10,6kW.

We see that the SG/LL heat exchanger actually leads to a smal decrease in cooling capasity wit R22. In addition we get a considerably higher discharge gas temperature which is not desireable with R22.


Now an example on a freeze system with R404a and a long suction line and/or a suction gas accumulator with a considerable heat influx.

First one example without SG/LL exchanger:
Refrigerant R404a

Condensing ____________40C ___________Pressure 18,3 bar/abs
Subcool _______________0K
Entering SG/LL _________40C ___________Enthalphy 263,8
Evaporation ___________-30C __________Pressure 2,1 bar/abs
Superheat evaporator ___5K ____________Enthalphy 354
SG compressor _________-0C ___________Enthalphy 374
Mass flow rate ___________0,0438 kg/s
Spesific volum gas at comp 0,106 m3/kg
Volum flow/comp displacement 4,64 l/s

This gives an evaporator cooling capasity of 354-263,8x0,0438=3,95kW.
The heat influx into the suction line is 374-354x0,0438=0,88kW.
Total compressor cooling capasity is 3,95+0,88=4,83kW.

Now with SG/LL exchanger:

Condensing ____________40C ___________Pressure 18,3 bar/abs
Subcool _______________0K
Entering SG/LL _________40C ___________Enthalphy 263,8
Evaporation ___________-30C __________Pressure 2,1 bar/abs
SG compressor _________25C ___________Enthalphy 396
Mass flow rate ________0,0397 kg/s
Spesific volum gas at comp 0,117 m3/kg
Volum flow/comp displacement 4,64 l/s

This gives cooling capasity of 396-263,8x0,0397=5,25kW.

The increase in evaporator cooling capasity is impressive 33%. The heat influx to the suction line and/or suction gas accumulator is eliminated with the installation of the SG/LL exchanger. Thus the evaporation capasity and the total compressor capasity is equal.
This translates to a reduction in power consumtion of 25%.


It is two factors i have not considered in this simplified explanation. One is the pressure loss on the suction side of the heat exchanger. This pressure loss give a loss in cooling capasity in the examples with heat exchanger. The other factor is the heat flow in to the cold suction line in the example without heat exchanger, except this last example. This heat flow is obviously greatly reduced or totally eliminated with the installation of a SG/LL heat exchanger. This heat flow gives various amounts of decrease in the cooling capasity in the examples without heat exchanger.

I have also not considered the volumetric efficiency of the compressor. All examples is given with the same volumetric suction gas flow.

Capasity changes in a real system will of course deviate some from these theoretical calculations. However the overall picture stands firm also in real system aplication.

An exelent method to learn about the performance effects of SG/LL heat exchangers is to download a god compressor calculation software and experiment with different operation conditions, spesifically with different SG temperatures.

I had posted a paper earlier on this subject. But you have prepared a one more paper.

Quote:

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Originally Posted by smpsmp45

I had posted a paper earlier on this subject. But you have prepared a one more paper.

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I dont think i have seen it. Have to look it up.

http://www.irc.wisc.edu/file.php?id=49

I got it. Have look at this paper.

Sr is very interesting this , first sorry for my english , im a spanish speaking.
i have the same system with a split a/c unit (12 000 btu/h) and i used a solar panel in the suction line , but so far my amperage no change, i have 22F degree in super heating , suction pressure at 59psig but i see no saving . i no sure what kind of gas have the unit because it was precharge from factory , but it could be R 22.
could you give me a hand with this problem ?
best regards
javier

we use them everywhere because we can control the superheat at the compressor and have cold liquid going to the rooms. When conditions in cooler stay the same (however the liquid is colder thus better capacity in cooler (less flash gas better performance etc.)
we can also put the coolers on 0 superheat as any liquid will evaporate in the exchanger. thus the cooler is full giving 200% better performance. thus the evaporator pressure can be higher etc.
put your calcs in coolpack and you will see.