Two temperature display case with low suction presure

[kengineering]

Hello everyone and Happy New Year. I have posted this in this forum because it may be similar to a rack set-up found in supermarkets.
This case is an open self select "air screen" on top with a sliding door closed refrigerator on the bottom. A single 1 Hp. condensing unit feeds both evaporators. The top, open section, has a 3/4 ton TEV while the smaller bottom, closed section, has a 1/5 Ton TEV. The refrigerant is R-22 and each circut is controlled by individual t-stats and solenoid valves with a low presure control shutting down the compressor after pumpdown should both sections reach temperature. The open is currently set for 36*f and the closed unit for 38*f.
The open section requires most of the run time but on occasion the top is satisfied when the bottom calls. When this occurs the suction presure drops down and imediately short cycles. If left to pull down it goes right into a vacuum. If the top is running when the bottom calls it runs fine. The case actually maintains the desired temperatures for both sections but it just doesn't seem right that the bottom won't run without the top.
Any thoughts or suggestions?
Thanks as always, Ken

[The Viking]

Depending on the pipe layout and so on,
Oil collecting in the lower evaporator?
Charge OK?
Superheat OK?
Head pressure control OK?
TEV with MOP that's failed?

Sorry, this list could get long, my best bet is the oil but then, I haven't seen the unit.

[Gary]

The best solution would be to unload the compressor when the pressure drops, but this may not be enough and further steps may need to be taken.

[US Iceman]

The compressor capacity is greater than the heat load of the smaller coil. That is why the suction pressure is going into a vacuum Ken.

Since the compressor probably does not have unloaders (for capacity control) you are faced with one of three possibilities I see now.

1) hot gas bypass to unload the system, and load up the compressor
2) If the compressor is 3 phase, you could use a VFD to slow down the compressor when the smaller coil is ON
3) Us two compressors, each sized for the appropriate capacity of each coil.

What you have is a design problem...

[old gas bottle]

just as a matter of interest, when the bottom coil calls and the top is in the off cycle, is it freezing while in a vac or is the valve shutting down,have you tried a new valve and drier in the bottom section,may well be a bad design or oil but have a play down bellow as they say.

[kengineering]

I believe you have all confirmed my suspision of a bad design. First, the upper TEV was changed and the coil was driping with oil. Iceman you are correct in that the hermetic does not have capacity control. So if I were to try 1) hot gas bypass to unload the system, and load up the compressor. I inject say a1/4 inch hot gas line on the low side of the valve? And this would raise the pressure thereby unloading the compressor and loading the smaller evap? Is this controlled say activated by a HG solenoid simutanious with the liquid? If not how would it be accomplissed? Just to note I have no intention of redesigning this case So I will not be able to give you the results, but would like to know for my own knowlege base. Thanks to all, Ken

[US Iceman]

Ken,

Download this file and have a look... This has some good information you might want to consider (maybe not for this job, but just in general).

http://www.sporlan.com/90-40.pdf (1.02 MB)

Basically, hot gas bypass serves to keep the volume of gas constant to the compressor, so the suction pressure does not decrease.

This keeps the evaporating pressure contstant so you maintain the coil temperature constant (and the discharge air temp)

I like the method that injects the hot gas into the distributor, this way you get good mixing (of the gas and liquid) through the evaporator.

This way you do not need a de-superheating liquid injeciton valve (which just complicates the whole affair) to keep the suction gas temperature down to something reasonable.

[Brass Monkey]

Ken,

The hot gas solenoid needs to be wired in parrellel with the liquid line solenoid for the smaller coil. But before the hot gas solenoid there must be an interlock contact that is switched closed when the self serve solenoid coil is de-energised, used a relay. This will stop the system becoming low on capacity when both the solenoids are energised. Cause when the self serve solenoid is energised the hot gas solenoid will not be able to be energised due to the relay contact (normally closed).

Peter.

[Dan]

If the top is running when the bottom calls it runs fine. The case actually maintains the desired temperatures for both sections but it just doesn't seem right that the bottom won't run without the top.
Any thoughts or suggestions?

Ken, Iceman is right. This is bad design. But bad design is what you have to deal with.

The open section requires most of the run time but on occasion the top is satisfied when the bottom calls. When this occurs the suction presure drops down and imediately short cycles. If left to pull down it goes right into a vacuum.

The open sector is primary. This should be the "Master" load. The closed back portion should not have a temperature control. It is a closed section and will survive as a "Slave" load. You can use an SORIT valve to raise the pressure in the back part, if you need to prevent freezing. You should only have a single expansion valve, to contain cost if not for any other reason.

Think how you can share the two loads in simple fashion such as air flow sharing.

[Gary]

Possibly a second solenoid valve could be added to the closed case and wired parallel with the open case solenoid valve. This way the closed case would only feed refrigerant when both thermostats are calling and would still have it's own shutoff to prevent freezing.

Or better yet, simply move the open case solenoid valve to the common liquid line, before it tees off to the two case sections.

[750 Valve]

Poor design, trying to do too much with too little control - ie no chance to capacity control the comp easily. I have seen this design many times on cheaply manufactured merchandisers, usually they don't run t'stats and sols though, if they did they don't work well like that.
SST is the go here, like Dan says the open section is the main load and will run a lower sst than the doored section to achieve their desired conditions. Putting an EPR on the higher temp case and flicking the sol setup is my advice. Run the whole setup off an LP that is set to satisfy the load of the open case only - cutout with both cases at temp - this will likely be a little low in sst, say -10C to -12C due to the fact that the epr will be shut and comp will basically be doing duty of one coil only. Set cut in to maintain temp via sst as well, say 2 to 3C for a bit of an off cycle, or 0 to 1C if the products critical like meat or deli.
Check comp for short cycling and adjust a bit if needed and be sure to wire in a defrost timer as setup will run low sst's a fair bit but will keep temp and won't ice up, best of all its REAL simple.

[Cofreth]

Use 2 compressor,one for top and one for bottom.