Max low side pressure in R404A system

[Wilade]

I'm working on a project to involving commercial custom undercounter refrigerators and having them listed by Underwriters Laboratories (UL). We are currently doing our pre-testing and are having troubles getting under the maximum low-side pressure as given in UL standard 471. Essentially it states the ultimate pressure of the low-side components must be at least 3-times the max as recorded during the pressure and temperature test, or 5-times for a Canadian UL listing. It further goes on to say the maximum low-side pressure cannot exceed 174 PSIG.

For the test we must condition the unit for at least 12-hours at 104F with all the doors propped open, and then begin recording the temperatures and pressures (low and high-side). The test runs until the unit achieves stabilization, somewhere about 10-hours or so, and the pressures and temperatures must stay within bounds. It is even allowable for the compressor to trip on its high limit for the first 8-hours.

Where our problem is coming in is in what appears to be our choice of refrigerant. We are using R404A with "off-the-shelf" Emerson compressor/condensers and Heatcraft evaporators. The maximum low-side pressure we are seeing is 248 PSIG. This is happening right at startup and drops immediately - but not before we fail for the pressure being to high.

I understand why the pressure is what it is - that is right from the temperature/pressure chart for R404A. My question is: If this is a legitimate problem, and I don't see how it could be, how could anyone hope to pass the UL test for Canadian approval using R404A?

Any thoughts or insights would be welcome. My test lab manager and I have been pulling our hair out for a few days on this.

Thanks, Wilade

[Gary]

Got a liquid line solenoid?

When it gets down to temp, what is your receiver outlet subcooling?

[Wilade]

Thanks for the reply. No solenoid in the system and we did not have any thermocouples on the receiver.

Are you saying we (or anyone using R404A) would need to have a pump-down system in place to avoid the problem?

[chillin out]

Are you saying we (or anyone using R404A) would need to have a pump-down system in place to avoid the problem?
I would say so.

Chillin:):)

[Gary]

Thanks for the reply. No solenoid in the system and we did not have any thermocouples on the receiver.

Are you saying we (or anyone using R404A) would need to have a pump-down system in place to avoid the problem?

Without a solenoid, the bulk of the charge will migrate to and condense to liquid in the evaporator in the off cycle. If the suction line is downhill from the evap to the compressor, then liquid will travel by gravity to the compressor inlet.

This can be minimized by installing a reverse trap at the evap (suction line goes up to the top of the evap and then down to the compressor), but the liquid could still slug back when the compressor starts.

A solenoid would stop the migration. It doesn't necessarily have to be a pumpdown. The solenoid could simply close when the compressor stops running and open when the compressor starts.

The subcooling tells you if there is excess refrigerant in the system. The more refrigerant in the system, the more refrigerant to migrate on the off cycle and the greater chance of floodback on startup.

[lowcool]

gday wil have come across cheap stuff that says do not go above 170 psi on a low side component,so whats the point of having bottle pressure when we have design pressure lower than it.static pressure is static pressure.i have also pondered these inconceivable things allowed by the public representatives

[chemi-cool]

I agree with Gary and add that you can also use a TXV with low MOP.
It can also help you avoid high low side pressure.

Not using an SV can cause other problems such as flood back of compressor at start up......

[Wilade]

I'm still being plagued by this problem, for over 2-months now!

We gave up on the UL side and approched the manufacturer. I brought our purchasing manager into the fray to keep up with the phone tag. I sent our PM the following e-mail outlining the problem that was then forwarded to Emerson's Application Engineering department, "
Below I have outlined the issues with UL & Emerson as I know them today.



For our refrigeration project to get a CUL listing, we need to meet the test criteria set forth in UL standard 471.




In a refrigeration system there is a low-side and high-side. UL is concerned with the maximum pressure the components of both "sides" of the system can withstand without failure.



For one of the tests we must place the unit in an environmentally controlled chamber and keep it there, with all the doors open, at 104F (70C) for at least 24-hours.




For any refrigerant used in a system, it's pressure is determined by its temperature.



The refrigerant in our unit is R404a. At 104F its pressure is 248.5 PSIG.



After the 24-hour conditioning is done, we start the unit and take a series of pressure and temperature and pressure readings over a period of 6-8 hours. At no time during this test period can any of the readings exceed the UL limits.



In our testing the high-side pressures are fine.



For UL the maximum low-side pressure allowed is; 5-times the marked low-side working pressure OR 5-times the maximum pressure recorded during the test, whichever is higher.



UL states the Emerson compressor shells in the low-side of the system have only been tested to 1000 PSIG. So for the first case; 174 PSIG marked working pressure (174x5=870 PSIG). Everything is fine.



However, during our test the pressure recorded, before the unit is energized, is 248.5 PSIG (248.5x5=1242.5). We fail.



I hope this helps to clarify the issue somewhat.



If you have any questions prior to our conference call let me know."


Today I received their final answer, "Our Application Engineers believe that the problem you are encountering, is a result of the system being over charged?

We have never experienced a problem like this with any OEM before."

I find it had to believe I have overcharged the system in question. While adding a pump-down system I recharged it to around 75% of the receiver capacity, and there was still bubbles in the sight glass. When we tried running the unit in the 104*F room it tripped on the high limit 3-4 times then would not come down to temperature to cycle on the control (33*F setpoint) only got to about 40*F. I then added another 4-ounces of refrigerant and just cleared the sight glass, total now about 80% of the receiver capacity. Re-ran the same test and the unit ran like a champ - never tripped on the high limit and got down to temperature and began cycling on the control within 1-hour.

Due to time constraints with other projects I have not had the oppertunity to check the superheat or sub cooling - I plan to do so in the morning. In the mean time, anyone have any comments or suggestions? Do we need to abandon our plan of using R404a and use R134a? Any and all help will be greatly appreciated!

[Gary]

The refrigerant in our unit is R404a. At 104F its pressure is 248.5 PSIG.

However, during our test the pressure recorded, before the unit is energized, is 248.5 PSIG (248.5x5=1242.5). We fail.


If there is both liquid and vapor in the evaporator, then the pressure must correspond to the temperature... and you fail before you begin.

Only by pumping down the system in the off cycle, so that there is only vapor in the evaporator, can you possibly pass the test.