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Thread: extreme winter operation
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17-01-2009, 05:46 AM #1
extreme winter operation
we have been having a prolonged cold snap in our area of late( minus 44 f yesterday morning) I was wondering if anyone had any tips on system operation during the winter season. My two stage system has been operating with a average discharge pressure of 80 psig for the last few days with no adverse effects on cooler temperatures or loading dock heating.
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17-01-2009, 07:36 PM #2
Re: extreme winter operation
You may want to talk with your refrig consultant, engineer, or contractor first, but has a client in northern Idaho with 2-stage screw installation that found it possible to operate his plant system single stage during January and February, and save wear ane tear on the booster plus save some power dollars...
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23-01-2009, 04:10 AM #3
Re: extreme winter operation
I'd guess that your energy savings are substantial running such a low head pressure.
Are you using NH3 to cool your compressor oil (thermosyphon / oil cooilers)? If so, are you noticing low oil temps in your compressors?
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23-01-2009, 06:11 PM #4
Re: extreme winter operation
Are you hot gas defrost? That low of a head could cause problems with defrosts.
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23-01-2009, 09:36 PM #5
Re: extreme winter operation
we use liquid injection oil cooling on our frick RWB-40's and closed loop glycol on our Vilter 450's. We use hot gas for defrost,dock heat and under floor heat in the freezers. low discharge pressure has been a issue when ambients get to -25 f and lower. the system is pumped liquid except the liquid supply for the liquid injection oil cooling.
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25-01-2009, 11:00 PM #6
Re: extreme winter operation
chilldis I have run one of my engine rooms for short time periods at 90 psi head pressure. I was told dont do it why I asked ? I got reasons from everwere 1. NH3 would condense in the oil separators 2. defrost problems etc etc I had a professional engineer tell me last year not to go below 100 psi Why who knows. It sounds like you are not having problems . So keep on saving energy. C.D.
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26-01-2009, 03:11 PM #7
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26-01-2009, 03:16 PM #8
Re: extreme winter operation
Do you run your screws at 80psig head pressure? Very often screw with liquid injection oil cooling have a problem at head pressure lower than 100psig. It isn't easy to defrost coils at 80psig. Do your coils have back pressure regulators or liquid drainers for defrosting?
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26-01-2009, 03:27 PM #9
Re: extreme winter operation
Originally Posted by CHIEF DELPAC
It has to do with how the system was designed. Some systems do not react very well when the discharge pressure falls below 125 psig, while others may operate OK at 90 psig.
Oil cooling methods like liquid injection could also be one of those topics that prevent low discharge pressure.
The end result is; you need just enough pressure differential to make the valves (liquid injection, defrost regulators, etc.) work. The problem is valve capacity. In most cases, the outlet pressure of the valve is determined by what the valve is trying to accomplish. However, the valve capacity required to meet that purpose is determined by pressure differential across the valve port.
What the professional engineer should have told you was how to get your discharge pressure lower than 100 psig!If all else fails, ask for help.
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26-01-2009, 03:29 PM #10
Re: extreme winter operation
I found many professional engineers have limited knowledge about operation at low head pressure. Usually, they design plants for the hottest ambient conditions, but operation at low head pressure is the different story. You mentioned couple reasons.
1. NH3 will never condense in oil separator, because it is superheated.
2. Majority of evaporator coils can be defrosted at head pressure lower than 100psig. They should be adjusted properly.
Ask your professional engineers about the reasons of operating at head pressure higher that 100psig. We will discuss them at this forum.
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26-01-2009, 03:41 PM #11
Re: extreme winter operation
I am having problems agreeing with the statement that nh3 will never condense in an oil separator because it is superheated.
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26-01-2009, 05:05 PM #12
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26-01-2009, 08:04 PM #13
Re: extreme winter operation
I have seen nh3 condense in an oil separator, the machine had cycled off for a period of time and the oil heaters were not functioning.
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26-01-2009, 09:07 PM #14
Re: extreme winter operation
It would better if we talk about properly functioned equipment. However, in you case I think that nh3 won't condense in oil separator. Nh3 will condense in discharge line when compressor is idle for a while. This condensate will drain to oil separator through leaking check valve. At lower condensing pressure less likely nh3 will condense in discharge line. At 90psig head pressure nh3 won't condense if temperature in compressor room higher than 60degF. It means that at lower head pressure less condensation in discharge line.
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26-01-2009, 10:21 PM #15
Re: extreme winter operation
Refrigerant condensation in the oil separator would only occur when the oil temperature is less than the saturation temperature of the refrigerant (at whatever pressure being considered).
This could happen even with an operating sump heater. If the sump heater does not produce sufficient heat to keep the separator warm in a cool engine room the oil can cool down.
Oil separators have a lot of surface area and can reject a fair amount of heat.
However, if the engine room is warm the usual sources of liquid are from: liquid slugging or condensation in the discharge line.If all else fails, ask for help.
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27-01-2009, 12:36 AM #16
Re: extreme winter operation
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27-01-2009, 02:57 AM #17
Re: extreme winter operation
to answer a few of the questions, my discharge pressures range from a low of 70psig to a high of 100psig with average (by time) of 80psig. the condenser water pumps are controlled by a thermostat that locks out operation below 28f. the condenser fan controls are set to 135psig with a 5psig differential. my evaps have defrost pressure regulators set a 75psig.
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27-01-2009, 03:47 AM #18
Re: extreme winter operation
Your condensers are cooled by natural convection or by wind. When head pressure is lower than 130psig, you don't have a control of this pressure. I prefer to have control of head pressure. When head pressure 80psig or lower, you will have problem to defrost you evaporators. For now, probably you are lucky and head pressure during defrosting was in the range 80-100psig. Why do your fan control set to 135psig? I think that fan set point of 100psig will save you more energy.
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28-01-2009, 01:02 AM #19
Re: extreme winter operation
good points Segei, I am in the process of fine tuning my system for maximum energy conservation. the 100psig setpoint seems to be a very logical step. as far as defrosts, with the low ambients come very low humidity levels that in turn allow 80psig discharge pressures to work well for this time of the season.
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28-01-2009, 01:10 AM #20
Re: extreme winter operation
We have been having same winter here. When the plant here is down that is the only time we have head pressure issues. So we just valve off the condensors, to combat the low head pressure. If it is still low at with these steps we then will start a freezer to get it back up to defrost. We also will just open the liquid returns this helps if closing the whole thing would be to much. Hope this helps.
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28-01-2009, 01:50 AM #21
Re: extreme winter operation
Your coils won't defrost at 80psig. Probably, they don't have a frost at all. To have a frost on the coil, the temperature of this coil should be lower than dew point of surrounding air. Major source of the frost is ambient air. When outside temperature is -30F, dew point will in the range of -35F ~-40F. If your coldest coil has temperature -35f or higher, frost won't form on this coil.
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28-01-2009, 01:58 AM #22
Re: extreme winter operation
Major reason of low head pressure during cold weather is direct wind. Some kind of shield will prevent plant from low head pressure. You won't need to valve off condensers or to run freezers. Sometimes people shut off some fans and put curtains on them(to protect condenser from direct cold wind).
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28-01-2009, 02:26 AM #23
Re: extreme winter operation
I know why we get low head pressure. Curtains would be nice but i dont think my boss would spring to outfit 15 condensors with them so the simplest way is to close the valves. What we have been doing for the last 30 years or so.
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28-01-2009, 03:03 AM #24
Re: extreme winter operation
But it isn't the safest way. Usually, condensers don't have relief valves. One day somebody forgot to open valves and it can be problem at higher ambient temperatures. I think that it is easier to spend several hundred dollars to buy material for the curtains. Permanent shields will work even better.
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28-01-2009, 05:49 AM #25
Re: extreme winter operation
I guess i was under the assumption that ALL condensing units had to have srv valves. All of the ones we have do even are really old ristos and for not opening a valve that is supposed to be opened comes with the training i guess.
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28-01-2009, 11:58 AM #26
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28-01-2009, 01:30 PM #27
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28-01-2009, 01:32 PM #28
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28-01-2009, 01:41 PM #29
Re: extreme winter operation
Originally Posted by PoodzyIf all else fails, ask for help.
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28-01-2009, 01:43 PM #30
Re: extreme winter operation
Evaporators will most certainly defrost at ~75 psig if the regulators and pipe mains are sized correctly.
The higher discharge pressures are required when the above is not properly done.If all else fails, ask for help.
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28-01-2009, 02:30 PM #31
Re: extreme winter operation
It depends.
Hot gas defrosting is the triple process.
1. Hot gas supply.
2. Condensation at certain temperature(pressure) level.
3. Condensate draining.
75psig tell us that we have correct condensation level. But it tell us nothing about hot gas supply and condensate draining.
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28-01-2009, 03:01 PM #32
Re: extreme winter operation
No argument from me on those points. I fully concur.
For a hot gas defrost system to work well at these lower pressures the ability to provide an adequate supply of hot gas is determined by the hot gas supply pipe capacity. If this pipe cannot supply enough hot gas at a low pressure loss then you can have some problems.
The next big issue is being able to drain the condensate from the coils during defrost as fast as it forms. This is dependent on the coil design (the coil circuits have to be free draining) and the method of controlling the defrost. Defrost relief valves work only on pressure and do not know the difference between liquid or vapor, only pressure. This is where drainers work very well, if properly applied.
The last point is pressure. If you try to defrost at low pressures (just high enough for defrost to occur) you cannot have high pressure losses in the hot gas supply piping.If all else fails, ask for help.
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29-01-2009, 02:17 PM #33
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29-01-2009, 02:32 PM #34
Re: extreme winter operation
In the past no one paid any attention to this issue. I believe the coil manufacturers are now doing a better job of engineering the coils to achieve this, although .... I am surprised that we as an industry continue to do the same old thing without thinking about how hot gas defrost actually works.
Here is how to get gravity drainage. If you can drop a marble down any of the coil circuits, the marble should roll out of the circuit.If all else fails, ask for help.
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16-12-2009, 04:07 AM #35
Re: extreme winter operation
There was a concern regarding overpressure if the valves were closed to limit wind driven condensation in cold weather.
Does closing a valve on the inlet to the coil only adequately limit the condenser capacity in extreme cold adequately ?
By only closing the inlet valve the discharge side is open to the HP receiver.
I suppose the condenser could draw a large of amount of liquid back up but with a sufficiently sized HP receiver will that not work effectively?
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16-12-2009, 04:14 AM #36
Re: extreme winter operation
If the condenser coils fill up with liquid and have closed valves on the condenser coils, you certainly have a potential bad issue. In this case you would be right to be concerned about over-pressure from liquid lock-up in the coils.
Leaving the coil drain valve open will still allow the refrigerant to migrate back to the condenser coils.
I've had a new system operating at about 70 psig discharge the last few weeks with below 0°F dry bulb temperatures. It starts up and runs just fine. The customer has had numerous start-up problems with his other systems though at various locations.If all else fails, ask for help.
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17-12-2009, 02:34 PM #37
Re: extreme winter operation
I think that closing inlet of the condenser will significantly reduce capacity of this condenser. However, closing condenser outlet will reduce capacity of this condenser to 0%. During winter operation we have opportunity for significant energy savings that we don't have in summer time. Many refrigeration plants can operate at condensing pressure below 100 psig and people should use this opportunity.
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19-12-2009, 03:17 PM #38
Re: extreme winter operation
sorry I have been away from the forum for so long(new grandson living with us) After some fine tuning my system has been operating very well with a average discharge pressure of 95 psig. This whole change in system operating procedures has been a learning process for me. Every system operator should remember when you are the only person operating a system DO NOT try to make all the changes the "experts" suggest at one time. Small changes can have a large impact on system performance. I have tuned my system to operate well from ambients of 115 f. to -25 f. with a average energy savings of 23% over the past year. Thanks to all for the feedback and suggestions.
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19-12-2009, 10:32 PM #39
Re: extreme winter operation
Forgive me for my ignorance as our climate is much different to yours .
Just want to learn how its done
Where is the load coming from under such low ambient conditions ?
Do you insulate hot gas lines as well as have them free draining back to condenser ?
I guess you may have float at low points + drain back to suction .
Where do you get heating for chillers or load out docks , is it hot gas ?
We have cool stores with similar head pressures of 600kpa (86psi) in winter with 10 deg C ambients !
I would have thought liquid injection at these pressures would be ok if sized for your climate as you can have it on boosters .
It may only be affected by low discharge pressures starving it I would have thought .
If 2 stage system could you single stage system under these conditions ?
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20-12-2009, 04:10 AM #40
Re: extreme winter operation
Originally Posted by Ranger1
Originally Posted by Ranger1
You can still get hot gas, but you have to get it to condense for heat recovery since there is not much temperature available. For defrost, it depends on the defrost regulator pressure settings and how large the hot gas pipe is. If the pipe is too small it restricts the mass flow and slows down the defrost. The smaller pipe also adds pressure loss, which affects the defrost cycle also.
Originally Posted by Ranger1If all else fails, ask for help.
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21-12-2009, 10:33 PM #41
Re: extreme winter operation
Hi,
Is overcompression (fixed VI ) at low discharge pressure a problem or waste of energy ?
grtz
sorry for my bad english
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22-12-2009, 03:41 AM #42
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02-01-2010, 02:51 AM #43
Re: extreme winter operation
personal stuff sorry
Last edited by jaysephus; 04-02-2010 at 05:49 PM. Reason: personal stuff sorry
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02-01-2010, 03:55 AM #44
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02-01-2010, 02:38 PM #45
Re: extreme winter operation
personal stuff sorry
Last edited by jaysephus; 04-02-2010 at 05:50 PM. Reason: personal stuff sorry
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02-01-2010, 04:47 PM #46
Re: extreme winter operation
Definitely, your plant is unique.
FES recommendation about 100 psi liquid pressure is for 100% loaded compressor. Your compressor is most likely unloaded.
To prevent low head pressure. I would leave operating one smallest condenser or one cond. fan. Install wind shields. You won't get low head pressure due to air natural convection. Only wind can bring it so low. Leave one operating evaporator per cold room, this prevent fluctuation of the refrigeration load. Suction pressure can be increased. Before cold night, freezer set point can be reduced by 1 F and it can be reseted back during the day.
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02-01-2010, 05:17 PM #47
Re: extreme winter operation
personal stuff sorry
Last edited by jaysephus; 04-02-2010 at 05:50 PM. Reason: personal stuff sorry
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02-01-2010, 06:09 PM #48
Re: extreme winter operation
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