Hot Gas Bypass for DX systems 1

[ChrisConley]

I recently read an article going on and on about the evils of hot gas bypass for capacity control. We have specified hot gas bypass on numerous systems for increased capacity control several times. Does anyone have any comments about hot gas bypass?

 

[quark]

For critical applications where the evaporating temperature is to be maintained constant (for ex. compressed air dryers) I found this system to be the best. We faced lot of problems with cyclic air dryers. But if the systems are large and energy consumption is more, it is better to see some other option.

Though I don't have experience with large systems, I heard the life of compressor gets reduced with hot gas bypass.

Regards,

 

[pennpoint]

We use hot gas bypass in conjunction with suction cooling to keep return gas at the compressor within recommended temperature (65F or lower).

pennpoint

 

[MintJulep]

If designed and maintained correctly, hot gas bypass works just fine.

Its very easy to do both poorly however.

 

[ChrisConley]

Do you have any tips on the proper design MintJulep? Specifically the application I'm dealing with is cooling a computer room. I've specified an 8 ton unit, with a 3 ton circuit and 5 ton circuit with hot gas bypass on the 3 ton circuit. Is there anything in particular I should watch for?

 

[cme]

I'd suggest low ambient control to 0 deg F if its in a cold climate. Are you using a liebert or similar?

I had HGBP designed on a 30 ton split system for a library. The contractor provided an APR valve for low capacity control. Anybody have experience with these?

 

[imok2]

With hot gas by-pass to the evaporator you want to make sure you have a by pass line solenoid valve and a by pass regulator. In addition, the refrigerant distributor must be a venturi type design or if it is the pressure drop type distributor it must be equipped with side outlet connection, downstream of distributor orifice or nozzle.

With this system the hot gas is mixed in evaporator and, therefore, not only raises the low side pressure, but also provides a false load, defrosts the coil, increases refrigerant velocity within the evaporator, and generally eliminates the need for de-superheating with a liquid injection valve.
The expansion valve feeding the evaporator has its remote bulb and equalizer connected to the suction line leaving the evaporator. Thus, when superheat increases, it will open and feed liquid refrigerant into the evaporator along with hot gas until such time as the load increases to permit bypass valve to close. Hope this can help you.

 

[ClydeMule]

One of the "evils" is the energy efficiency. You basically create a false load and run the compressor all of the time. It really depends on the application. If you have to have very accurate temperature control, have low thermal mass (air dryer), or need lots of unloading then it is the price you have to pay.

For medical cooling applications (chiller cooling and MRI) we do not use HGBP because the chiller is at 10-20% load MOST of the time, so HGBP is very inefficient. It is easier to just use two compressors. They also do not have tight temperature control requirements.

Look up the Copeland Digital Scroll. It is an interesting answer to HGBP. It has been used mostly for air dryers and from what I have heard has been successful.

Good Luck,

Clyde