THE OPERATING CHARACTERISTICS OF THE FLASH GAS CONDENSER

 

Flash gas and refrigerant slip are two side effects that are created by the metering device along with the desired pressure drop to make vapor compression refrigeration function.  These two side effects hamper the heat removal efficiency of the system more than most realize.

Flash gas

 

From what we are led to believe from various educational material on the subject, the effects created by flash gas on system performance are minimal. This is due to the fact that the information is portrayed in terms of weight and since gas is so much lighter than liquid, the overall effect seems very minimal. If you refer to a brochure created by Parker/Sporlan, Page 2/ Race Catalogue 20-10 UK, it explains that based on a typical R-410 Air conditioning application that 100*F liquid refrigerant entering the TEV, and 50*F evaporating temperature, liquid refrigerant represents 87% of the flow by weight, though it only represents 29% of the flow by volume. Therefore, between the necessary restriction to create the pressure drop and only 29% of refrigerant remaining liquid by volume you get a true picture of the inefficiency that exists.

 

Another visual example can be seen in a study done by Purdue University called 2012 Flash Gas Bypass Method for Improving Performance Of An A/C System With A Micro Channel Evaporator. This shows clear tubing exiting the TXV and you will notice the trickle of liquid refrigerant flowing in the bottom of the tubing.

 


 

 

 

 

 

 

 

 

Refrigerant Slip

 

If you continue reading, Parker/Sporlan, Page 2/ Race Catalogue 20-10 UK it also explains some of the issues caused by refrigerant slip. It explains that once the refrigerant passes thru the TEV that it becomes a two phase refrigerant and that the liquid and vapor travel at different velocities because of the effects of gravity are greater on liquid. I would like to take that one step further to truly give you a sense of the difference in velocity comparison between the liquid and vapor as it is leaving the TXV, because after reading that information from Parker/Sporlan the first thing that comes to mind is steam leisurely floating away from a pot of boiling water.

This is not the case, because the other variable that comes into play is the fact that the refrigerant flash is expanding from a liquid to a vapor as it boils off leaving the orifice of the TXV. At 50*F one pound of R410A liquid consumes 0.0141 ft3 of space and at 50*F one pound of R410A vapor consumes 0.3816 ft3 of space which is 27.06x more space. Confined to the limits of volume of the inside of the refrigerant tubing this means the vapor would be forced to bypass the liquid at a much higher velocity.

The information listed here does a lot to explain the reason for the design of the refrigerant distributor by; mounting it directly to the outlet of the TXV and increasing the size of the inlet over the size of the tubing feeding the TXV to reduce the effect of refrigerant slip, mixing the vapor and liquid together to speed up liquid delivery to the evaporator and splitting up the mixture to deliver it equally to the evaporator circuits to minimize the effects of refrigerant flash. The refrigerant distributor does a good job of utilizing the remaining liquid capacity after pressure drop, but the inefficiency still remains.

 

Flash Gas Condenser (FGC)

What if there was a device that not only would help to reverse the effects of flash gas, but also reduce the amount of it when it occurs? What if this same device could utilize refrigerant slip to increase the flow rate of the liquid refrigerant and then reduce the amount of vapor as it passes thru the device?

The FGC was designed to reduce flash gas by removing heat from the refrigerant vapor as it passes through the phase change material (PCM) chamber due to its location in the system and the close proximity to the outlet of the TXV and evaporator coil causing the FGC to effect the system in other positive ways. 1) It lowers the line temperature at the outlet of the TXV, which reduces the amount of flash gas created when the liquid refrigerant travels thru the orifice of the TXV. 2) The reduction in flash gas at the outlet of the TXV and the removal of heat from the vapor traveling through the PCM, the chamber increases the amount of liquid refrigerant in the tubing and by lowering the line temperature toward the evaporator, it minimizes any increase in saturation temperature before the liquid reaches the evaporator coil. The boil off from the pressure drop (flash gas) pushes the liquid condensing in the FGC faster than it would travel (because normally the amount of liquid in the line would not increase and the flash gas would just bypass it (refrigerant slip)), but as it gets closer to the FGC it slows down because the vapor is starting to condense and fill the line, in turn the flash gas forms behind it is pushing this once flash gas that is starting to condense faster. This process continues until the PCM temperature and the line temperature reach equilibrium, which is toward the end of the compressor cycle. By the time this point in the compressor run cycle is reached the effect of the FGC has lowered the line temperature and the valve temperature so much that the system is seeing similar effects to increased sub cooling the liquid before the valve (reducing the amount of flash gas from forming as it exits the valve) which also increases the systems capacity.

I have depicted this in an illustration I have included. Illustration 1 depicting a system without a FGC attached, Illustration 2 depicting a reduced amount of flash gas exiting the TXV and illustration 3 depicting an increased volume and velocity of liquid refrigerant traveling out of the FGC on its way to the evaporator coil.

 

 

 

 

 

 

 

The FGC is assembled with a Phase change material (PCM) that has a latent change point between the starting temperature and the ending temperature during compressor run at the exit of the TXV this is what makes the effects of the FGC sustainable.

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