Hot Gas Bypass – Revisited

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 ) Rawal will assist in the application and sizing of the valve.

RAWALThe use of the Rawal device has been a great solution to loading problems, humidity issues and long line set applications.

Operational Description

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!)


About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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9 Responses to Hot Gas Bypass – Revisited

  1. Ron Taylor says:

    The Rawal valves helped the humidity problem we had in a pharmacy in Corpus Christi Texas.

  2. Allan Kluttz says:

    The elimination of that third pipe helps to also return oil, which Rawal seems to be lacking. Also must mention that Rawall is somewhat limited on unloading because of that.

  3. Tim says:

    As you know every systems in the world has advantages and disadvantages. I would like to know what are the disadvantages of the APR valve?

    • Tim in all the applications that I used RAWAL Valves (APR) I never found a problem. I always used their enginering staff to assist in the sizing and application. I would suggest visiting their site and discussing this with them. I’m sure there are cases where they are not applicable but I do not want to speak for them.

  4. Nate says:

    Is there another company out there manufacturing a control like the APR or they the only game in town?

    • There probably are but in my experience, RAWAL VALVES have a proven track record. Also, their “sales” department will help you engineer just what you need. The beauty of any APR is no need to run the long hot gas by-pass line between the condenser and the evaporator.

      • Nate says:

        I don’t doubt they are great, but they already have two distribution points in my area and refuse to set me up. I can’t sell them to my customers.

      • don’t know what else to tell you. You could try searching for other APR manufacturers to see if there are others who could assist you. Beyond that, whatever you can do with Rawal is between you and them. My blog is here to provide information on these types of things and to provide technical support on heating and A/C products and functions.

      • Nate says:

        I can totally appreciate that. I reread my last comment and I honestly didn’t mean to seem snarky. I just didn’t know if there were other mfgrs. out there. Thanks.

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