old system with hot gas bypass. help!

[joapysoe]

Hi All,
very new to this, infact my first ever post on a forum to be truthful. brief intro, I am not a fully fledged frosty but am an engineer offshore. and have been for a while.I have a general knowledge of the fridge cycle but not enough obviously.

I have an old air drier which has recently failed and was sort of hoping you guys could bail me out.

system was tripping on HP and this was due to lack of cooling through sea water cooled condensor, this problem was addressed but on re instating system there appears to be no flow accross either the expansion valve or liquid flow/moisture indicator sight glass. the gas inlet to the condensor is cool to the touch. there is a clear gas/liquid level in the condensor sight glass.

it appears to be that the gas flow is passing through the hot gas bypass valve thus shortening the circuit and missing out the cooling process and resulting in a very hot thermal tripped compressor.

I plan to change out the filter drier and if need be the expansion valve later but just wanted to ask if anybody has had any experience with a similar system and could pass on any snippets of info that could help. especially with regard to the hot gas bypass.

the system is R22 and my LP press is 3.5bar, normal running is 4bar. there is a level in condensor so have ruled out lack of gas so far. ooh and whe i close the hot gas loop the comp stops on LP.

any help much appreciated, bit of long one for a first post but hope it wont put you lot off.

[Spoc]

You've no gas in the system. get it pressure tested and repaired properly and vac / recharge with correct amount of R22

[Plank!]

Why is the hot gas bypass open when the system is running?
Is the liquid line to the expansion device open?
If you have liquid in the condensor/reciever then it should be able to make it to the expansion device and back to the suction - check everything in the liquid line, including the filter drier.
Any restriction there would cause the LP fault.

[Pooh]

The reason the hot gas bypass is open is that the suction pressure is low due to a shortage of gas or the discharge pressure is too low after the cleaning of the condenser. The hot gas valve is fitted to stop the evaporator freezing so will inject hot gas if the suction pressure drops. If you have less than 5 Bar difference between the suction and discharge pressure the TEV will not work and closes.
What is your discharge pressure should be a minimum of 10 bar to be safe.

Ian

[joapysoe]

Cheers for the advice Ian and all.
will let you know any update.

all the best
Joe

[noushad123]

Hi joapysoe
The hot gas bypass is open for evaporator freezing, are you check anti freezing thermostat may be this is fault and giving command to open the EXV or Solenoid, second

[The Red Krawler]

The unit is short of gas.

When a dryer with a hot gas bypass valve becomes short of gas, the hot gas valve starts to allow more and more hot gas from the discharge line into the suction line to maintain a constant backpressure (low gas = low pressure = needs more hot gas to stay 'high').

Dryers like to leak from their evaporators, so chuck an electronic leak detector on the air outlet side (alternatively, the auto drains discharge) and sniff the gas. If you get a reading, then you may as well bin that dryer - 9 times out of 10 the labour charge and parts cost more than a new dryer.

Cheers
TRK

[The Red Krawler]

And a quick side note... If you want to put a safety control to prevent damage if it ends up short of gas again, stick a manual reset high temperature safety on the discharge pipe at about 20°C higher than 'normal' discharge temperatures. As the unit becomes short of gas, the suction temps go up = the discharge temps go up.

Putting an LP is a waste of time, as the hot gas valve will bleed more and more hot gas through and keep the backpressure above the point of the LP control.

[get the gauges]

Another side note,

system was tripping on HP and this was due to lack of cooling through sea water cooled condensor, this problem was addressed but on re instating system there appears to be no flow accross either the expansion valve or liquid flow/moisture indicator sight glass. the gas inlet to the condensor is cool to the touch. there is a clear gas/liquid level in the condensor sight glass.

I doubt the unit was tripping on H/P, it was was more likely the L/P trip in the same reseting switch assy,and the seawater condensing water valve did not appear to be working due to not reaching the opening pressure as the unit was short on gas .