Back pressure flood back concerns

[blair]

Hi,

I have a question regarding Evap pressure control valve,Some back ground on the system is:R22
3x26kw Copland re-sip compressors
20 evaps in 10 rooms
condensing temp 38c (10 fans controlled with vsd's)
sub-cooling 2k
evap temp -15c to -7c
super heat 4k to 6k.

Each room has 2 evaps 2 TXV's(one per evap) 2 LLS with common suction with danfoss (PCV) solenoid operated back pressure valves for hot gas defrost set to +10c.
The liquid solenoid's is controlled from the air off to ensure product can't be frozen.
Room set point is 0.5c with +/-0.5c tolerance

The issue is with the rooms being controlled off the air off the load comes and goes which causes the compressors to stop and start more often than desired.

I have trialled on half the rooms,engaging the PCV to control the evap temp to -3 so the air off floats on the TXV from 0.1c too 0.4c and therefore not triggering the LLS's and providing some constant load which enable's the compressors to run for longer periods of time.The LLS's are set to cut out if the air off reaches -0.1c and cut back in at +0.4c and the PCV was set when the air on was +1c.

My concerns are that the evaps might fill with liquid and flood back?

Hopefully from the above info someone can give me some advice,if more info is required I can provide . Thank's

[aramis]

I have a question regarding Evap pressure control valve,Some back ground on the system is:R22

You ended up with a completely different question.

Correctly sized and adjusted EPRs are all you need to prevent freezing your product, provided your compressors operating envelope, number and control can cope with the low pressures that the system may experience when most of the rooms are cold.



Each room has 2 evaps 2 TXV's(one per evap) 2 LLS with common suction with danfoss (PCV) solenoid operated back pressure valves for hot gas defrost set to +10c.

This will also produce some floodback and your compressor should have a correctly sized suction accumulator.



The liquid solenoid's is controlled from the air off to ensure product can't be frozen.
Room set point is 0.5c with +/-0.5c tolerance

Controlling air off with such a narrow differential is not a good idea. The system will always cycle frequently.

You can probably get by using EEVs but contrary to many posts I’ve read, EEVs control AVERAGE superheat, so you can still get an instantaneous variation greater than you want but on AVERAGE you will get closer to a stable control.



The issue is with the rooms being controlled off the air off the load comes and goes which causes the compressors to stop and start more often than desired.


This is why it's not a good idea.



I have trialled on half the rooms,engaging the PCV to control the evap temp to -3 so the air off floats on the TXV from 0.1c too 0.4c and therefore not triggering the LLS's and providing some constant load which enable's the compressors to run for longer periods of time.The LLS's are set to cut out if the air off reaches -0.1c and cut back in at +0.4c and the PCV was set when the air on was +1c.


This leads to freezing your product and not getting a stable control. You should set your EPR’s so that your product has no chance to freeze (might be a saturation pressure equivalent from 0°C down to -2°C for most products.

Then set your thermostat so that the product gets the cooling the product needs.


My concerns are that the evaps might fill with liquid and flood back?

The system must be protected with a suction accumulator. Your only concern is how much refrigerant can flood back. This depends on number of evaporators entering defrost simultaneously and how many of the other cooling evaporators can have problems of change of load or block or …

If your protection is sized correctly not to trap oil at minimum load and not overflow with liquid refrigerant at maximum floodback, you don’t need to worry.

[blair]

Thanks Aramis

Your reply has been very helpful and yes the system has a suction accumulator with heating elements and the program only allows one room to defrost at a time with approx 6 Min between each so I feel the system will most likely be fine.

Another 2 questions if you will; how could I determine if the protection is correctly sized?

And what is the theory behind "not trapping oil at minimum load" is it if the suction accumulator is too big the oil will be trapped in it at low load?

Thanks
Blair

[aramis]

Thanks Aramis
…how could I determine if the protection is correctly sized?

The purpose of the suction accumulator is to prevent liquid refrigerant reaching the compressor at runtime.

It must to this preventing oil trapping, high pressure drop and excessive superheat.

You can measure if it is working correctly by keeping track of your compressor suction superheat. If it is positive all the time and specially through defrost cycles and low thermal load, the accumulator is doing its job.

If you have seen sketches of its internals you will see an orifice designed to return oil and eventually liquid refrigerant in manageable amounts to the compressor. This hole is not at the bottom where it could easily get plugged, so the accumulator is always trapping some oil.

In order to return this oil you need a minimum refrigerant velocity to suck the oil from the shell side of the accumulator. It is similar to an oil raiser.

So to see if the accumulator is doing its job efficiently you need to check oil return, excessive suction superheat witch and excessive pressure drop through the accumulator too.

There is more to it, like not plugging the return hole and several design variations, but these are the main concerns.


And what is the theory behind "not trapping oil at minimum load" is it if the suction accumulator is too big the oil will be trapped in it at low load?

Yes, as you can now imagine reading above.

This is why in very tiny letters, manufacturers warn in some lost page at the end of their catalog state that suction accumulators could trap oil if the flow drops down to 33% of their nominal capacity.

Normally when not insulated, you can tell if the accumulator has oil trapped because of a change in the normal condensation at the bottom on the outside wall.

[aramis]

Oops! I forgot to mention something important for your first question, it is most important to size one correctly:

Linux users call this the RTFM directive!

In this case you have to follow the manufacturer’s instructions to size one correctly.

[blair]

Thank-you Aramis you advice is much appreciated.