Suction gas <-> liquid line heat exchange and performance gains

[superheat]

Some HCFC retrofits beg for some type of heat exchange. I have seen them with no subcool and no superheat. The only way to run them is with heat exchange.

Is this board in any agreement as to application of heat exchangers. I put them in all low temp walk-ins. Would most people say that is the only place to put them? Would many people say don't even put them in any low temp boxes?

[mohamed khamis]

Some HCFC retrofits beg for some type of heat exchange. I have seen them with no subcool and no superheat. The only way to run them is with heat exchange.

Is this board in any agreement as to application of heat exchangers. I put them in all low temp walk-ins. Would most people say that is the only place to put them? Would many people say don't even put them in any low temp boxes?

Hi everybody

Don not oppress the SG<>LL heat exchanger, it sometimes gives an improvement in the system efficiency and sometimes impairs it according to the application field in which it will be installed. In a short, it is favorably installed in the application of low evaporating temperature under -1°C but if it is installed in other applications that for a safety considerations in terms of preventing compressor from flood backing and to prevent the liquid line from gas flushing and likely to excess vibration in liquid line. So the question may be raised how this heat exchanger help the system efficiency or ruin it this comes from the following: Firstly it should be establish a good foundation for some relationships:

System efficiency (COP) = Qe / Wcomp

Qe = refrigerant mass flow * (evaporator outlet enthalpy - evaporator inlet enthalpy)

Wcomp = refrigerant mass flow * (compressor outlet enthalpy - compressor inlet enthalpy)

Mass flow through the TXV = cd*Ao*Sqrt (2*pressure difference* inlet refrigerant density)

Mass flow thru the compressor = Cylinder size * RPM/60*volumetric efficiency

The mass flow rate at all point in the cycle must be the same otherwise the system is unbalance.


Firstly, as it is known the SG<>LL heat exchanger job is to increase the subcooling and superheating degree.

The increase in subcooling degree (lower inlet refrigerant temperature at TXV) results in an increase in refrigerant density at TXV inlet, therefore the mass flow rate inevitably is increased thru the TXV for the same other parameters. On the other hand, as the inlet enthalpy to the evaporator must be increased without any doubts. The net product is the existence of SG<>LL heat exchanger will certainly enhance the evaporator cooling capacity as it is done in the mechanical subcooling system case. Now to ensure the action will be occurred to the system COP we have to look at the compressor power.

The increase in superheating increment on P-h diagram does not tell what will be happened to the compressor enthalpy difference “compressor work”. I expected without any modifications in the inlet and outlet pressure the compressor enthalpy difference will be maintained the same if u draw on h-s diagram and I have verified that by using CATT program and I found the increment in compressor enthalpy difference is very slight 2%. However, this is superficial looking, the existence of heat exchanger leads to increase the pressure drop (unless it was soldered LL with SG) the excessive of pressure drop can lead to a significant increase in compressor work not power. Alternatively, the increase in suction gas temperature will lead to a decrease in its density and in turn the volumetric efficiency will indeed be dropped as a result of two sides (drop in refrigerant density and suction pressure drop). Therefore, the mass flow rate thru compressor has tendency to drop but there is a rule of mass conservation which tells the mass flow rate must be constant along with the cycle. Thus, the compressor must mince itself to offset the excess of TXV feeding by raising the drawing electrical current to handle the excess in TXV feeding. The net outcome is the compressor power in also increased. Therefore, two competing effects at a work. The increment in cooling capacity attempts to enhance COP but the increment in compressor power plays against that. Therefore, the system COP can be improved or hurt in conjunction with the installation of SG<>LL heat exchanger.
However, the judgment sentence can be given in case of the low evaporating temperature applications except in Ammonia or R-22 application. The reason of benefits in low Tev is as long as Tev is low the latent heat to convert liquid to vapor is large and u have to assist the evaporator by adding mechanical superheating such as in accumulator or using SG<>LL heat exchanger without harming the compressor power because at this case the density of the refrigerant is in fact very low and compressor refrigerant capacity is very high so the superheating has no significant effect at this moment. However, in ammonia and R-22 it is not preferred due to the increase in suction gas temperature means increase in discharge temperature and therefore the compressor oil viscosity will be negatively effect due to the high discharge temperature especially in NH3 and R-22, the polytropic index is quite big and any rise in suction temperature will has severe influence on the discharge temperature. Sorry for long post

Cheers

[mohamed khamis]

Hi everybody

Don not oppress the SG<>LL heat exchanger, it sometimes gives an improvement in the system efficiency and sometimes impairs it according to the application field in which it will be installed. In a short, it is favorably installed in the application of low evaporating temperature under -1°C but if it is installed in other applications that for a safety considerations in terms of preventing compressor from flood backing and to prevent the liquid line from gas flushing and likely to excess vibration in liquid line. So the question may be raised how this heat exchanger help the system efficiency or ruin it this comes from the following: Firstly it should be establish a good foundation for some relationships:

System efficiency (COP) = Qe / Wcomp

Qe = refrigerant mass flow * (evaporator outlet enthalpy - evaporator inlet enthalpy)

Wcomp = refrigerant mass flow * (compressor outlet enthalpy - compressor inlet enthalpy)

Mass flow through the TXV = cd*Ao*Sqrt (2*pressure difference* inlet refrigerant density)

Mass flow thru the compressor = Cylinder size * RPM/60*volumetric efficiency

The mass flow rate at all point in the cycle must be the same otherwise the system is unbalance.


Firstly, as it is known the SG<>LL heat exchanger job is to increase the subcooling and superheating degree.

The increase in subcooling degree (lower inlet refrigerant temperature at TXV) results in an increase in refrigerant density at TXV inlet, therefore the mass flow rate inevitably is increased thru the TXV for the same other parameters. On the other hand, as the inlet enthalpy to the evaporator must be increased without any doubts. The net product is the existence of SG<>LL heat exchanger will certainly enhance the evaporator cooling capacity as it is done in the mechanical subcooling system case. Now to ensure the action will be occurred to the system COP we have to look at the compressor power.

The increase in superheating increment on P-h diagram does not tell what will be happened to the compressor enthalpy difference “compressor work”. I expected without any modifications in the inlet and outlet pressure the compressor enthalpy difference will be maintained the same if u draw on h-s diagram and I have verified that by using CATT program and I found the increment in compressor enthalpy difference is very slight 2%. However, this is superficial looking, the existence of heat exchanger leads to increase the pressure drop (unless it was soldered LL with SG) the excessive of pressure drop can lead to a significant increase in compressor work not power. Alternatively, the increase in suction gas temperature will lead to a decrease in its density and in turn the volumetric efficiency will indeed be dropped as a result of two sides (drop in refrigerant density and suction pressure drop). Therefore, the mass flow rate thru compressor has tendency to drop but there is a rule of mass conservation which tells the mass flow rate must be constant along with the cycle. Thus, the compressor must mince itself to offset the excess of TXV feeding by raising the drawing electrical current to handle the excess in TXV feeding. The net outcome is the compressor power in also increased. Therefore, two competing effects at a work. The increment in cooling capacity attempts to enhance COP but the increment in compressor power plays against that. Therefore, the system COP can be improved or hurt in conjunction with the installation of SG<>LL heat exchanger.
However, the judgment sentence can be given in case of the low evaporating temperature applications except in Ammonia or R-22 application. The reason of benefits in low Tev is as long as Tev is low the latent heat to convert liquid to vapor is large and u have to assist the evaporator by adding mechanical superheating such as in accumulator or using SG<>LL heat exchanger without harming the compressor power because at this case the density of the refrigerant is in fact very low and compressor refrigerant capacity is very high so the superheating has no significant effect at this moment. However, in ammonia and R-22 it is not preferred due to the increase in suction gas temperature means increase in discharge temperature and therefore the compressor oil viscosity will be negatively effect due to the high discharge temperature especially in NH3 and R-22, the polytropic index is quite big and any rise in suction temperature will has severe influence on the discharge temperature. Sorry for long post

Cheers

Hi there

There are some notes on the previous post which are:

Mass flow thru the compressor = Cylinder size * RPM/60*volumetric efficiency* refrigerant density at compressor inlet

... because at this case the density of the refrigerant is in fact very low and compressor refrigerant capacity is very high so the superheating has no significant effect at this moment.

Cheers

there is a correction which is

...because at this case the density of the refrigerant is in fact very high (as a result of decreasing in TEV)and compressor refrigerant capacity is high so the superheating has no significant effect at this moment.

Cheers