cold stor maintenance

[Noori]

Hello every one.
After I maintained a cold store( using refrigerant R22 and Re-maintained reciprocating Copland compressor with capacity 7.5KW) the temperature for the inlet air to the evaporator reduced to -27C and stay at this temperature about 50 minutes then rising till -22C and then go back to-27C and so on.Please can any one help me to know the reason for this cyclic operation.
Noori

[Brian_UK]

Sounds like the compressor cycling off and on again in response to the set point.

What is the room temperature?

[Noori]

Dear Brain.
Air temperature -27C is near the suction evaporator that’s mean the room air temperature may be more with five dregs (actually I did not measure it in front off the evaporator).The set point is -30C.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /><o:p></o:p>
Thanks <o:p></o:p>

[nuntachai]

Noori....

I thing .... You have to fine adjusting of the expansion valve in order to define refrigerant flow rate.

[750 Valve]

Sounds like the system could be cycling on LP

[nike123]

If your set temperature is -30°C and your evaporator air inlet temperature is -27°C to -22°C than few questions are coming on my mind!
Where is placed termostat bulb (or probe)? What is set differential of thermostat?
Does compressor work non-stop during this temperature fluctuations or it stops in meantime?
Does this cold store is single on that compressor?
How many evaporators are in that cold store and how are they arranged?
What is control method of condenser fan speed?
What is evaporator (or total) superheat and is it fluctuating simmilar as temperature fluctuations?
What is subcooling and does it fluctuate also?
Does sight glass have occasional flow of bubbles?
What is cut in and cut out pressure of low side pressure stat?

[Noori]

Dear Nike 123
1- Temperature probe was placed in the suction air to the evaporator.
2- The compressor still running during fluctuation.
3- Yes it is single cold store for the compressor.
4- One evaporator and the store inside dimension are 6.72* 4.72*3.27 with sandwich walls (10 inch in thickness).
5- The condenser has 4 single speed fans and controlled according to condenser pressure to starts one by on.
6- SST (saturated suction temperature) is -40 and actual suction gas temperature is approximately -15.I did not read its fluctuation during fluctuation period. The ice is cover the suction line and the inlet compressor area.
7- Saturated temperature at the liquid receiver outlet is 32C while actual liquid temperature is 29.
8- For the bubbles some time yes and some time no according to condenser pressure and it is not completely filled with liquid during fluctuation period.
9- For the Lp the cut out is o.5psig and I set it as protection ( ie-the unit shout be manual reset for starting).
10- All reading at outside temperature 22C and condenser pressure is 150psig with two condenser fans is running and condenser outlet air temperature is 29 and inlet air temperature to the evaporator is -27 .
Best regard
Noori

[Gary]

7-Saturated temperature at the liquid receiver outlet is 32C while actual liquid temperature is 29.

The liquid line subcooling is 4K.

32C - 29C = 4K

Add refrigerant until the subcooling is about 8.5K.

[nike123]

<o:p></o:p>

8- For the bubbles some time yes and some time no according to condenser pressure and it is not completely filled with liquid during fluctuation period.<o:p></o:p>


Occasional occurrence and absence of bubbles in sight glass indicate at change in subcooling!
Your condenser measurements states:
Tai (temperature air in)=22
Tao (temperature air out)=29
Tc (temperature of condensation)=150psig(r22)=28°C
Tao cannot be higher than Tc! These measurements are not good. Check them again.
Also:

7- Saturated temperature at the liquid receiver outlet is <st1:metricconverter productid="32C" w:st="on">32C</st1:metricconverter> while actual liquid temperature is 29.

Saturated temperature at liquid receiver cannot be higher than saturated temperature at condenser if you don't have additional heat source from condenser to receiver or from pressure measuring point (probably on compressor discharge port) to receiver port .
If you measured condensation pressure for previous condenser measurements in different conditions and time, or your measuring equipment is bad, or you made wrongly conversion from pressure to temperature, than it is possible.

As <st1:place w:st="on"><st1:City w:st="on">Gary</st1:City></st1:place> pointed, you should charge additional amount of refrigerant to increase subcooling to point where it is enough to overcome pressure drop of liquid line from condenser to expansion device. That could indicate sight -glass without bubbles in all working conditions ( especially at full load and high ambient)
I would also check temperature difference at filter-dryer that is not more than 1K before charging additional amount.
Also, I would, during charging, switch all fans ON (to avoid fan cycling) and cover portion of condenser till i reach 35-<st1:metricconverter productid="40°C" w:st="on">40°C</st1:metricconverter> steady condensation temperature.

Also, I would adjust fan regulation (or cover portion of condenser if that could not be adjusted wit such outdoor temperatures) to maintain 35-<st1:metricconverter productid="40°C" w:st="on">40°C</st1:metricconverter> condensation temperature.<o:p></o:p>

[Noori]

Dear Nike 132Thanks a lot for your nice and accurate answer.
1-150psig is the condenser pressure at the out let of liquid receiver while at the outlet of compressor should be more with approximately on bar that is due to pressure drop When the gas passing the ondenser tubes and elbows and the gas temperature at this point (compressor discharge point) is 75C.
2-Yes there is an additional heat to the liquid receiver coming from the condenser fan so I will separate it form this additional heat.
3-I will measure the temperature difference across the filter.
4- Do u mean I should keep the condensation temperature for all the operating condition with the rage 35-40C even in the night. Actually I can reach this point if I adjust one condenser fan running When ambient temperature go down to 22C.
Best regard.
<O:p> </O:p>

[Noori]

Dear Cary.
THanks so much for your answer
Is 8.5K subcooling is astanderd for cold stor.
Best regard
Noori

[nike123]

4- Do u mean I should keep the condensation temperature for all the operating condition with the rage 35-40C even in the night. Actually I can reach this point if I adjust one condenser fan running When ambient temperature go down to 22C.
Best regard.
<o>:p> </o>:p>

Yes, because lowering condensation pressure means lowering deltaP across TEV and reducing his capacity.
Add to that cycling of fans and subcooling fluctuations which result in bubbling and refrigerant gas at TEV which further reduce TEV capacity, and there you are, with lowered cooling capacity and fluctuations in coldroom temperature!