cold display

[marco blanco]

system running with R134,low back is about half bar which very low,sight clear, going down to temperature set at 2 c, low pressurer switch set at 15 psia, there is two solonoid liquid valve, and two controllors , one for top evaporato and one for for the bottom two door little fridge. can anyone tell me why the pressurer is very low.

[Brian_UK]

You say that the sight glass is clear so that's a good sign.

Is it reaching the set temperature of 2°C ?

Was the unit working before at a different temperature/pressure?

We would need some more information before a judgement could be made on your question as to why is the pressure low, it may be correct?

[Electrocoolman]

Sight glass could be clear due to lack of refrigerant?

Is there any activity in SG when unit starts up?

What is expansion device?

[Gary]

This sounds familiar. I wonder if this display case is having the same problems:

http://www.refrigeration-engineer.com/forums/showthread.php?t=7085

[marco blanco]

temperature 2c , but pressuere is bit low . system was always running like this according to the customer.

[Gary]

What makes you think the pressure is low?

[icebox]

Does it have expansion valves or capillaries?

[marco blanco]

evaporating pressure on r 134 should be 1bar. but reading on my gauges pressure is about half bar .
system has got an expansion valve...
as i said before is going down to the desire setting no problem with that.

[lana]

evaporating pressure on r 134 should be 1bar. but reading on my gauges pressure is about half bar .
system has got an expansion valve...
as i said before is going down to the desire setting no problem with that.

Hi there,

For R134a, at pressre of 1bar(g) means that evaporating temp. is arounf -10°C. At 0.5bar(g) the evaporating temp. is -17°C.
I suggest you check the followings :
1- TEV filter (if is has any).
2- Any blocking in the liquid line, like filter or ...
3- Evaporator. Fan is working properly, any frost on the evaporator, ...
4- Check the refrigerant charge.

Hope this helps a little.
Cheers:)

[icebox]

Compressor might be a bit oversized for the appliance.What evaporating pressure do you have when only one solenoid cuts out at setpoint & the other is still working?Are you running the unit at a lower temperature than the cabinet was originally designed for?

[Gary]

evaporating pressure on r 134 should be 1bar. but reading on my gauges pressure is about half bar .
system has got an expansion valve...
as i said before is going down to the desire setting no problem with that.

Is that the pressure with both solenoids open or one closed?

Did you check out the link I provided in my previous post? It may be the same problem.

[Dan]

I tend to agree with Lana's comments. But since you have a TEV, I would look toward adjusting superheat if there are no other obvious causes. And since you are attempting to operate dissimilar loads with a single compressor, I think you have to peek into the link that Gary provided to see the problems that causes. First, let's make sure the evaporators are feeding fully. Check the superheat.

If the suction pressure is at one bar, and the leaving air is 2deg C... there is a good chance that your leaving refrigerant temperature is at or around 2 deg C.

Pardon me while I think out loud in familiar units... you are operating at 15 deg F saturated suction, and you are maintaining a box temperture of 36 deg F... I am willing to bet that you have a superheat of 20 deg F, (5 deg C) which is too high. Adjust the TEV to provide a coil leaving temperature of -3 Deg C and you should see an improvement in operation. Ultimately you want to see a superheat of no more than 5 deg C. After adjustment I would expect your suction pressure rising to 1.4 bar, which seems closer to most designs, you should see a leaving suction temperature of -10 deg C. Which should provide a 2 deg C air temperature with considerably less operation of the compressor.

It is possible that my guesses are wrong, but I usually find evaporators that are underfeeding (high superheat) to closely match the returning air temperature. You want the leaving pipe to be colder than the return air temperature. I hope I did my conversions okay. :)

Now let's consider what I call the "master" and "slave" setup. One load is usually the primary load and the other secondary. In your case, you seem to have a rear storage compartment which will be easily satisfied, and may tend to overcool the product if the other load is an open display and requires more constant refrigeration.

If my assumption is correct, you should use a thermostat to control the "master" load, which also stops and starts the refrigeration for the "slave" load. This will insure the highest frequency of compressor loading. The rear service portion with doors should just be a slave, and you can adjust the superheat or airflow to keep it as a live load that doesn't overcool while everything runs together, until the master load is satisfied. This will improve your suction pressure because you are not completely stopping refrigeration everytime the thermostat for the rear service is satisfied.

[lana]

... I am willing to bet that you have a superheat of 20 deg F, (5 deg C) which is too high.

I couldn't agree more. BUT 20°F superheat is 11°C.;)
Just SI guy talking.:D

[Gary]

If the suction pressure is at one bar, and the leaving air is 2deg C... there is a good chance that your leaving refrigerant temperature is at or around 2 deg C.


I'm thinking you meant to say "entering air", not "leaving air".

[Gary]

Now let's consider what I call the "master" and "slave" setup. One load is usually the primary load and the other secondary. In your case, you seem to have a rear storage compartment which will be easily satisfied, and may tend to overcool the product if the other load is an open display and requires more constant refrigeration.

If my assumption is correct, you should use a thermostat to control the "master" load, which also stops and starts the refrigeration for the "slave" load. This will insure the highest frequency of compressor loading. The rear service portion with doors should just be a slave, and you can adjust the superheat or airflow to keep it as a live load that doesn't overcool while everything runs together, until the master load is satisfied. This will improve your suction pressure because you are not completely stopping refrigeration everytime the thermostat for the rear service is satisfied.

If we move the master solenoid valve from it's branch liquid line to the main liquid line, then the compressor will pump the system down and shut off whenever the master thermostat is satisfied. The slave thermostat and solenoid valve will keep the slave load from overcooling.

[marco blanco]

thank you guys for help . i willl try to adjust tev.

[Dan]

I couldn't agree more. BUT 20°F superheat is 11°C.

I can only laugh at myself. I knew I would blow it somewhere along the line.:)