setting up hot gas bypass valve for first time

[nick1986]

hello everyone this is my first post. im setting up an external hot gas bypass valve for the first time tomorrow. i understand how they work but i dont know what pressure to set it to open at. the system is direct expansion and is on r407 the target temp is 20*c. can anyone tell me the procedure for setting one up and if there is any calculations you use to set them up (if any) for future reference that i could use incase i come across another that needs setting up for a different refrigerant etc.
cheers for your help.

[RANGER1]

nick1986 ,
Doyou mean downstream pressure regulator ?

Downstream pressure regulator is like a false load valve to keep unit going all the time for numerous reasons .

Can you give a bit more info of system and what you think its purpose is .

[Denco Technical]

Typically set a pressue equal to 1.5c to 2.5c coil tempreture.

A correct gas charge (superheat, subcooling withe the HGBP closed first.

Its

[lana]

Hi there,

I assume that this valve is for capacity control.

Set it for design suction pressure.
If capacity at the evaporator falls then suction pressure will decrease. This valve injects hot gas into the evaporator inlet (or outlet) to increase the load.

Selecting this valve is very important. If its too big or too small you wont get the correct suction pressure.

Also there are different types with or without pilot pressure.

If you inject the hot gas at the outlet of the evaporator (compressor suction) there is a chance of compressor overload. Also, in this case oil entrainment is not good as the velocity of the refrigerant decreases in the evaporator.

The best way is to inject the hot gas into the inlet of the evaporator. In this case velocity is the same as in design condition (HGBV closed) and also the Chance for compressor overload is minimized.

What I did few years ago is this : I used a small TEV with liquid branch from the liquid line to cool down the hot gas a little bit and then injected to the evaporator inlet. I used Danfoss HGBV with pilot pressure and worked perfectly.

Another problem is when the system is over sized. In this case HGBV will open immediately at start up.

Cheers

[nick1986]

Typically set a pressue equal to 1.5c to 2.5c coil tempreture.

A correct gas charge (superheat, subcooling withe the HGBP closed first.

Its

Hi thanks for reply. So you set it to open at that pressure to stop frost forming on evap coil and to keep a load on the evap in low load conditions. How do you work out what pressure the system will be at when the evap coil temp is at 1.5c to 2.5c given the charge is correct?

[Denco Technical]

I am afraid its trial and error and a lot of experience needed, once the unit is up, charged and running okay you can start setting the HGBV. one this i will say is open the valve a little bit at a time, leave it for 10-15 mins then test again. Also keep a eye on the compressor temperature (body)

If you are really concerned does you company have someone who has done it before?

[nick1986]

Yeah there is a senior engineer going to meet me on site now. What I was told to do is when the system is running and the suction temp drops below 0c which converts to 67psi for r407c on manifold gauge temp pressure scale the valve must open to keep the evaporator temp above 0c to stop the evap freezing up also providing load to the evap to keep the room at a constant temp?

[frank]

I always fit a CPR valve in conjunction with HGB to protect the compressor from excessive inlet pressures during Hot Gas Injection.

As mentioned, oil return can suffer if the HGB valve is operating for long periods, especially when injecting into the evaporator outlet. As Lana says, inject into the evaporator inlet if possible.