Compressor Air Dryer problem

[tvrao]

Hi,
Can somebody help me, i am in a big trouble.

I have a refrigerant air dryer running with R134a and fitted with hot gas bypass pr.regulator. It is tripping on high pressure (setting is at 225psi). I have done many things to find the problem but not yet solved.

I have completely dismantled all the parts, flushed out and reassembled. (filter drier and bypass pr.regulator replaced, liquid & suction accumulators cleaned, comp. oil replaced, Faulty txv replaced on suspect)

notes-
Condenser is in good condition and heat transfer is excellent.
original TXV 2.8tr was replaced with a 6tr one. Adjustment is done to control the liquid.

on starting HP was maintaing at 165psi and LP was at 40psi (pr.regulator set for)
Temp- subcooling is below 10degC (i.e temp at filter drier), Evp temp 4-5degC, Suction temp is 12degC, Conden inlet temp- around 75degC

ambient is around 23degC, Heat load is not much

after 3to 4 hours of running, HP on condenser inlet is gradually increasing to above 225psi and tripping with HP switch. After stopping the system also the HP is not coming down (like trapped).

Some oil is also flushing out from the compressor to the system it seems.

can somebody tell me what could be the reason for increasing high pressure, pl suggest
Also pl tell me the desire operating pressures & temps of the 134a system for the dryer.
regards

[mikeref]

I've had R134a break down once or twice and run some stupid pressures with very poor cooling.
Your situation sounds like an overcharge... though why change the TX from 3 to 6? Was the orifice on original valve discoloured? Ramping up of discharge pressure over a period of time means your system is no longer balanced. Heat of rejection is incorrect as you said there was 10*C subcooling.
Less condenser tubing for condensing forcing discharge pressure up.

[tvrao]

I've had R134a break down once or twice and run some stupid pressures with very poor cooling.
Your situation sounds like an overcharge... though why change the TX from 3 to 6? Was the orifice on original valve discoloured? Ramping up of discharge pressure over a period of time means your system is no longer balanced. Heat of rejection is incorrect as you said there was 10*C subcooling.
Less condenser tubing for condensing forcing discharge pressure up.

Thanks for your reply.
There is no overcharge for sure. I hv also the same opinion that my system is not correctly balanced. but when i touch the condenser outlet, feel piping is cooled.
can you pl explain how to balance the system.
I have changed the txv from 3 to 6 as i don't hv spare.
your valuable guidance is appreciated pl.

regards

[tvrao]

I've had R134a break down once or twice and run some stupid pressures with very poor cooling.
Your situation sounds like an overcharge... though why change the TX from 3 to 6? Was the orifice on original valve discoloured? Ramping up of discharge pressure over a period of time means your system is no longer balanced. Heat of rejection is incorrect as you said there was 10*C subcooling.
Less condenser tubing for condensing forcing discharge pressure up.

Thanks for your reply.
There is no overcharge for sure. I hv also the of the same opinion that my system is not correctly balanced. but when i touch the condenser outlet, feel piping is cooled.
can you pl explain how to balance the system.
I have changed the txv from 3 to 6 as i don't hv spare.
your valuable guidance is appreciated pl.

regards

[PaulZ]

Hi tyrao
You have changed from a 3 tr to a 6 tr TXV, I think this will cause you a problem. Have you checked the superheat setting on the TXV. With an air dryer fitted with a hot gas bypass valve you may be maintaining the correct evap pressure but the evap could be full of liquid and over a period of time with the change of compressed air temp it could be getting pumped out and this will cause a lack of condenser space due to to much Liquid. If there is a plate stating the amount of gas in the system, or did you charge by pressure? As mike has said 10oC subcooling is not correct.
Regards
Paul

[tvrao]

I am getting your views and agree with it. what I should adjust to bring the parameters for effective operation in my case. the qty of charge on name plate is 17kg whereas the present could be around 13kg. I cannot charge more because of this HP problem.

do adjusting (closing side) txv further will solve this?
how the temps & prs. are to be maintained at each point ? pl describe.

since i am only elect engineer and not familiar with refrigeration system design, kindly help
regards

[serviceguy]

Correct me if i'm wrong,
Your startup is with an evaporation temp of 6 degC and a condensing temp of 48 degC. A subcooling of 10 K and a superheat of 6 K. Your ambient temp is 23 degC.

normally you can say that the difference between ambient temp and condensing temp should be around 10 - 15 K (not 25) and that the pressure regulator is set at 1 degC so that the cooler does not freeze up. and the superheat should be around 5 - 7 K.

Wen the TXV is to large and your pressure regulator is opening to early you are bringing to much load at the compressor so the head pressure will rise. (because the capacity of the condensor is not enough).

But also if your system is overcharged the condensor capacity will decrease because it is filled with liquid.

If the nameplate says it needs 17 kg and you only filled 13 kg then maybe there are non condensible gasses in your system, this makes your condensing pressure go high and your subcooling becomes high.

regards

[tvrao]

Thanks for the reply.
I am wrong in calculating the values of sub cooling and superheat. better i would give the measured parameters for your clear understanding. Today I have opened the 6tr txv further (adjusted to 50% opening) and find no tripping on hp but the system seems to be inefficient. pl look into the following values recorded after 10hrs of operation.

Condenser inlet - 70deg C, 190psi
condenser outlet - 29deg C
Evaporator temp - 9 deg C
Compressor suction - 35psi (bypass Pr regulator operating intermittently)
Suction temp- 22degC,
Amb-Temp- 24degC

Is the gas charge not enough? Upto what level I can increase the pressure on hp side with 134a system.

regards

[serviceguy]

It looks like youve got non condensible gas in your system. because your subcooling is 21K. Your superheat is 27K so there seems to be lak of refrigerant.
First you have to check for non condensibles. block in the condensor and put on the fans. The condensingtemp in the condensor should be thesame as ambient temperature. so when ambient 24 degC the pressure should be 93psi.