Computer room AHU

[pactor]

Hello I need a bit of advice please.
The Denco computer AHU's at the place I am working have a by-pass valve / press regulator valve between the discharge and the evaporator, we had a unit and the capillary on the tev had worn through the expansion valve had closed but the the unit does not stop on LP because the by-pass valve opens the keep the pressure up. The compressor runs red hot and eventually stops on an internal hi temp cutout but then restarts when it gets cold. Is this by-pass valve needed I've seen lots of other denco ahu's and this valve is not fitted. I can see it might be useful if you were chilling water but this is only cooling air returning at about 25 c. The trouble is you never get a signal to the BMS that a unit is short of gas all that happens is the by-pass valve opens and keeps it going. Is is set wrong maybe. How should it be set up. I was thinking of winding it down so it does not operate and just let the unit stop on LP at least it would then indicate to the BMS there was a fault.
Thanks for any help

[dudthefridge]

this by pass valve sounds like the hot gas bypass, it is used when the unit runs in dehum.the compressor runs to bring the hum down in the room the temp also drops as a result,so to give load the hot gas is taken from the compressor discharge to the evaporator to stop the L/P switch dropping out.
hopethis helps in working out what tepsyou will take next.

[Gary]

I can see it might be useful if you were chilling water but this is only cooling air returning at about 25 c.

An air conditioning system doesn't "only" cool the air... it also dehumidifies the air. That hot gas valve is there to maintain a minimum coil temperature, thereby keeping the humidity from getting too low, which is important in a computer room.

[Denco Technical]

There is a reason why the HG valve is fitted as its not standard, so is there a reason why, i tend to agree with the above poster and there must be close tolerances with the humidity levels on site?

On the other hand the LP should still work regardless of the charge, the LP measures from a true suction on the system so would detect a low suction pressure situation.

[goshen]

Hello I need a bit of advice please.
The Denco computer AHU's at the place I am working have a by-pass valve / press regulator valve between the discharge and the evaporator, we had a unit and the capillary on the tev had worn through the expansion valve had closed but the the unit does not stop on LP because the by-pass valve opens the keep the pressure up. The compressor runs red hot and eventually stops on an internal hi temp cutout but then restarts when it gets cold. Is this by-pass valve needed I've seen lots of other denco ahu's and this valve is not fitted. I can see it might be useful if you were chilling water but this is only cooling air returning at about 25 c. The trouble is you never get a signal to the BMS that a unit is short of gas all that happens is the by-pass valve opens and keeps it going. Is is set wrong maybe. How should it be set up. I was thinking of winding it down so it does not operate and just let the unit stop on LP at least it would then indicate to the BMS there was a fault.
Thanks for any help
Hi i agree with what was said in response ,but u must check why this specific unit was orderd with a hgbp.this hgbp should only kick in when necessary.low coil temp ?dehumidfing?
most have a solonied valve that activates the hgbp .
bottom line- check all settings :eek:

[Gary]

An air conditioning system doesn't "only" cool the air... it also dehumidifies the air. That hot gas valve is there to maintain a minimum coil temperature, thereby keeping the humidity from getting too low, which is important in a computer room.

Rule of thumb: In order to achieve 50%RH on any A/C system, the evap coil air off temp (measured close to the coil) must be 11K/20F lower than the room temp.

[Denco Technical]

We also install HGBP to in prove close Torrance's, It quite common that units "over run" when in the cooling phase.

In my day to day job 80% of technical calls are regarding HGBP valves :-)