One of the most read post on the site is the one Posted on April 6, 2012 titled WHY USE HOT GAS BYPASS? Because this is one of the most read postings (over 1,900 views to this one alone), I thought it would be good to revisit this subject and also show some additional information and options on controlling “capacity” in an air conditioning system.
HGBP has been used for years to create a false load on the evaporator in order to keep it running during low load conditions. There are a few ways to do this.
For many years, the most widely known way is to pipe hot gas from the discharge line of the compressor in the outdoor unit to the inlet of the evaporator coil after the TXV. This requires that the TXV be equipped with a hot gas side port distributor piped back to the discharge line on the last stage running (Y1). In a 2 pipe condensing unit, this requires a 3rd pipe to the evaporator. The HGBP valve is field installed in the condensing unit as close to the compressor discharge line as possible. The HGBP line should be insulated with minimum ½ wall thickness. Rule of thumb for sizing — use the same size as the liquid line. Remember, in most cases, we are only bypassing approximate 65%. If we would be bypassing more than that, we would have to use a larger line size than the liquid line size as refrigerant in gas form needs larger tubing.
Here is the typical lay out of a system with Hot Gas By-pass valve and side port distributor:
For more information on this, read my previously mentioned post.
The most recent development to help create a load on the system when “low load conditions exist” is to use a field supplied and installed Rawal Valve. A lot of contractors have been installing these for a few years and are familiar with the valve. The APR Rawal valve mounts inside the condensing unit and eliminates the 3rd pipe going to the evaporator. This saves labor and the expense of additional copper pipe/fittings. Contact Rawal direct ( www.rawal.com ) Rawal will assist in the application and sizing of the valve.
The APR Control is a capacity-control device that modulates a system’s air conditioning or refrigeration capacity to match variations occurring in the BTU quantity of the existing load. The device accomplishes this capacity modulation by “reading” the heat content of the system return air, which changes the pressure of the suction gas. Using its hot gas bypass valve, the APR Control then bypasses a portion of the hot gas discharge from the compressor through the device’s desuperheating chamber and into the suction inlet of the compressor.
The hot gas enters the desuperheating chamber at an angle creating a circular hot gas path, wiping the walls of the chamber and giving up excess system heat. The gas then travels up the “mixed gas lift tube,” losing energy as it returns to the system’s compressor suction inlet. The increase in suction pressure resulting from the hot gas and suction mix reduces the compression differential in the compressor, causing a decrease in required compressor horsepower.
Through the evaporator coil, the APR Control senses the enthalpy of the return air. A reduction in the enthalpy of the return air causes a drop in coil temperature that causes a drop in the suction pressure. This triggers an instant response in the APR Control hot gas bypass valve and causes it to bypass some of the compressor discharge through the APR Control desuperheating chamber to the compressor suction return. This results in a decrease in hot gas to the condenser coil and ultimately less liquid refrigerant to the evaporator coil.
At the evaporator (in an R-410a system), the APR Control attempts to hold the DX coil at 105 p.s.i. by changing the rate of flow of liquid refrigerant into the DX coil and adjusting the dehumidification window of the evaporator coil. Simultaneously, the difference in system BTU capacity and the current BTU load is continually being bypassed in the form of hot gas through the APR Control’s desuperheating chamber and into the suction line at the compressor.
The overall effect is a reduction in cooling capacity of the DX coil corresponding to the reduction in the load. Most importantly, the APR Control will not decrease the maximum or designed capacity of the system when it is needed!
So, now you have an option to running the hot gas by-pass line from the outdoor unit to the indoor unit. As far as I know, almost all manufacturer’s approve of the Rawal valve. As I mentioned earlier, Rawal is also available to assist with sizing the valve for your application. Be sure to check out the link provided above for additional information.
Keep these both in mind if you have a need for an application to prevent condensate from freezing on the surface of the evaporator coil when the system is operating at extremely low load conditions. We have used them locally and they do work well.
(Some information was courtesy of Ed Schmidt – CM, NATE, Johnson Controls CPS and Bruce Porter Manager UPG Field Service. Bruce has written many application bulletins for UPG and is well-respected for his expertise in the HVACR field. He also teaches HVACR at Metro Technologies Center HVAC. Also, information on Rawal Valves was obtained from their web site (and noted in this post above). Thanks to all for allowing me to use some of their materials and ideas!)