WHY USE HOT GAS BYPASS?


The main purpose for adding Hot Gas Bypass (HGBP) to any system is to prevent condensate from freezing on the surface of the evaporator coil when the system is operating at extremely low load conditions.  The HGBP valve is controlled by suction pressure, and its set point should be adjusted as low as possible to limit the amount of gas being bypassed and to minimize the negative effect of HGBP on the operating cost of the system.  A set point between 50 and 55 PSIG for R-22 and 88 and 98 PSIG  (26 to 30 F) will prevent the finned surface of the evaporator coil from dropping below 32 F.  Once the HGBP valve starts to open, the cooling output of the unit will remain constant, but its compressor KW will continue to increase.

Although HGBP will never hurt the reliability of the system, it shouldn’t be added to every unit. There is always a chance of contaminating a system during the installation, and HGBP may be ineffective on systems where:

  1. The supply air CFM is constant and above 320 CFM per nominal ton.
  2. Most of the air is being re-circulated.
  3. The indoor design temperature is 70 F or higher.

On these systems, the evaporator temperature will never approach 32 F.  The evaporator temperature will drop as the internal load and outside temperature drop, but the supply air will also drop and this lower temperature air will satisfy the room thermostat before the HGBP valve starts to open.

Adding HGBP for capacity reduction on these systems will not be practical because the set point of the HGBP valve may have to increase to 68 PSIG for R-22 and 118 PSIG for R-410A (40 F) or higher to get it to open, and this will increase the operating cost of the system.

Hot Gas Bypass (HGBP) is more commonly used on systems with:

  1. 100% Outside Air.
  2. A variable supply air CFM (Frequency drives or VAV systems)
  3. An indoor design temperature below 70 F.
  4. A supply air below 300 CFM per nominal ton.

On the first two of these systems, the cooling load will vary over such a wide range that the evaporator temperature could easily drop below 32 F.

On the last two of these systems, the evaporator temperature will be low at full load and could easily drop below 32 F with a small change in operating conditions. This only applies to  units with 1 stage of cooling or face split coils. Units with interlaced DX coils will seldom require any HGBP.

Please note that a system should never be required to bypass more than 30% of its compressor’s lowest stage of capacity.  If too much gas is bypassed, too little refrigerant will go into the condenser coil and the unit may experience low head pressure problems.

 Also note, HGBP is always piped into the first stage of cooling. It will require a TXV metering device and head pressure controls, and the HGBP line must be routed to the OUTLET of the TXV at the evaporator coil.  The HGBP tubing should be insulated with minimum ½ wall thickness insulation for its entire length.  The “Rule of Thumb” for sizing is to use the same size as the liquid line.

Attached at the end of this posting is a “typical” HGBP piping layout.

(Most of the information provided for this post was taken from an application bulletin written by George Simonson York UPG Technical Service retired who taught me a lot.)

HGBP piping layout

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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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23 Responses to WHY USE HOT GAS BYPASS?

  1. oladele adeyemo says:

    Thanks a lot, l,m really enjoyed your tips. And l believed your courage, techology advance in area of hvac ll serve as motivation, for young engineer

  2. arnik jain says:

    thank u for the info

  3. Sahyog says:

    We are designing a chiller for 450RT. Turndown Ratio is 10% Min. Now during shutdown of the plan, only AHU will be using Chilled Water and the maximum Load is 45RT which means that most of the time AHU Load will be around 20-22RT. Since the turndown ratio is 10% which is 45 RT, how can we use the chiller during shutdown period when the load is less than 45 RT.

  4. Hossam says:

    i have fresh package unit which having four compressor (two stage) evry stage is two compressor
    how many hot gas by pass i have to add to the unit ?

    for stage number 1 there is two compressors running togather
    do i have to make hotgas by pass for the two compressors?

  5. Steve Carlino says:

    We have aaon units at our church. Have had a lot of problems finding people who know how to work on them. Recently technicians came out and added 47 lbs of R-22 to one unit. They also found all hot gas bypass connections unsealed and sealed them. They have worked on these units before but never detected this. Would these connections contribute to major gas loss or are these independent issues?

  6. Hifzi says:

    Hi.. i have an issue here when there are oversized on HVAC selection. The area/room capacity required only 70KWR but the system has been sized about 95kWR. This beacuse heat dissipition provided by consultant was wrong and caused system calculated oversized.Now compressor has problem because always running at part load. What is solution to overcomes this issue? Is it necessary to change the compressor to corrected capacity required? But the cost is higher…The client asked to use HGBP valve….is it working..?I think by using HGBP may stress the compressor and kW of compressor also higher. I am planning to produce the heat by using duct heater…is it work?please advise.

  7. Bruce Hintzen says:

    Are hot gas bypass’s also used for dehumidification. I have a couple of AAON units in my labs with 100% outside air and have trouble keeping the RH down in this labs. Should I also be reheating the discharge air.

    • Hot Gas By Pass is only to keep the coil from freezing in low load situations. Humidity control is usually accomplished using hot gas reheat through a separate coil or some form of heat in the duct work down stream

      • Bruce Hintzen says:

        I did some more research on my AAON units and found that they do have a HGRHV’s and a seperate coil. I just need to make sure they are working. Thanks for the information

  8. Dan Eiserman says:

    I was taught that the bypass valve should be set at what the suction is on the engine room monitor. At this point my suction pressure is 9. But when i adjust the valve to 9 the unit freezes. Why is that. 10.000 pound Bh3 system

  9. Thanks for creating this blog. Reading the correspondence between end users with actual issues and yourself is of huge value to those who wish to continue to learn so that we can better assess and provide proper solutions in the industrial HVAC market. Do you post on the Hazardous HVAC market and the challenges they face?

  10. mahmadou says:

    thanks a lot your blog has been very help full.it really helped understand hgbp.

  11. Bilal says:

    Dear Sir,
    I would like to ask one question.

    when we have 100% Fresh air Unit and we have the unit with One compressor ( Hermetic Scroll Compressor ), do we need to provide HGB in the unit and Why?

    • The main purpose for adding Hot Gas Bypass (HGBP) to any system is to prevent condensate from freezing on the surface of the evaporator coil when the system is operating at extremely low load conditions. If your system is not going to operate under low load conditions and the evap coil will not get near 32 degrees F, then you probably will not need HGBP installed on it.

  12. Reza Rostam says:

    I really enjoyed ready your blog. It is very helpfull.
    R

    • Thank you! I am retired now after working 50 years at this trade and I started this before I retired as a way of “paying it forward”. The technology changes I’ve seen in my time really don’t even compare to what is coming up down the road in our industry.
      All I can do is encourage everyone to attend classes and keep reading so you don’t get left behind!
      Thanks again for the support and comment!

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