extreme winter operation

[chilldis]

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.

[VGV]

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...

[Oregon Jim]

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?

[nh3wizard]

Are you hot gas defrost? That low of a head could cause problems with defrosts.

[chilldis]

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.

[CHIEF DELPAC]

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.

[Segei]

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.
As far as I understand your condenser pumps and fans are off. Condensers are cooled by natural air convection. 80psig is the average head pressure. What is the range of head pressure fluctuation?

[Segei]

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.
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?

[US Iceman]

... 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


There is no reason why a refrigeration system will not work at these pressures, but there could be some limitations which prevent you from doing this in your specific system.

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!

[Segei]

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.
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.

[nh3wizard]

I am having problems agreeing with the statement that nh3 will never condense in an oil separator because it is superheated.

[Segei]

I am having problems agreeing with the statement that nh3 will never condense in an oil separator because it is superheated.
I mentioned about operating compressor and oil separator. If you have different opinion, let's discuss it.

[nh3wizard]

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.

[Segei]

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.
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.

[US Iceman]

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.

[Segei]

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.
Certainly, condensation can happen in cold engine room. However, this amount isn't significant, because we don't have a lot of nh3 vapor to produce significant amount of liquid. When people see liquid in sight glass, this is liquid from discharge line.

[chilldis]

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.

[Segei]

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.
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.

[chilldis]

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.

[Poodzy]

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.

[Segei]

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.
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.

[Segei]

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.
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).

[Poodzy]

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.

[Segei]

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.
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.

[Poodzy]

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.

[brian_chapin]

Your coils won't defrost at 80psig.

Odd that. We have every hot gas defrost in our building set at 70(+/-5)psig and they defrost just fine.

I assure you they form frost, and the defrost cycle removes it.

[Segei]

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.
You have a choice.
1. Install wind shields and forget about low head pressure.
2. Continue to open and close condenser valves.

[Segei]

Odd that. We have every hot gas defrost in our building set at 70(+/-5)psig and they defrost just fine.

I assure you they form frost, and the defrost cycle removes it.
I mentioned about condensing pressure. As far as I remember you have defrosting at 110psig or higher.

[US Iceman]

... for not opening a valve that is supposed to be opened comes with the training i guess.


No guess work needed. This should be part of your SOP's if you are doing this as an Operational Requirement.

[US Iceman]

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.

[Segei]

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.
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.

[US Iceman]

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.

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.

[Segei]

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.
Unfortunatly, many coils don't have free condensate draining.

[US Iceman]

Unfortunatly, many coils don't have free condensate draining.

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.;)

[Inniswhe]

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?

[US Iceman]

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.

[Segei]

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.

[chilldis]

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.

[RANGER1]

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 ?

[US Iceman]

Where is the load coming from under such low ambient conditions ?


A production facility should have a relatively constant load. A cold storage warehouse can be quite different if no product cooling load is evident.



Do you insulate hot gas lines as well as have them free draining back to condenser ?


I do. And use what you describe in the next quote below. I like inverted bucket traps form steam systems for this. The disc traps don't have enough capacity I think.

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.




I guess you may have float at low points + drain back to suction .

[br1uno]

Hi,

Is overcompression (fixed VI ) at low discharge pressure a problem or waste of energy ?


grtz





sorry for my bad english

[Segei]

Hi,

Is overcompression (fixed VI ) at low discharge pressure a problem or waste of energy ?


grtz





sorry for my bad english
I think that lower condensing pressure will save you more energy than keeping Vi. It is easy to test. Compare compressor energy use at two different condensing pressures. What are your discharge pressures and suction pressure?

[jaysephus]

personal stuff sorry

[Segei]

I have to run a cold storage in the very cold winter months. After doing much fine tuning with my system I can actually get liquid to move with 45 psi discharge pressure. I use an FES screw with liquid cooling and it's suggested lowest psi to keep cooling is 100psi at least, but it does work at 45psi. What my main problem is that my cold storage low temp recirculator is filled with discharge pressure. It needs high discharge pressure to push liquid into the low pressure receiver and 45 seems to be the happy number. If I can't get 45 I have to start my process freezer compressors to get high side up again(my process freezer & cold storage freezer are common)
We actually do isolate the inlet to the condenser, i forget who brought that up, but yes it does work for me, to a point. When I have trouble with low head pressure, one by one I can isolate inlets to condensers (don't forget the equalizing lines) and I can get a rise in head pressure. The very last thing is to isolate a condenser. I have large condensers and in the winter it all goes up to them. So before winter comes it would be a good idea while all the ammonia is in the engine room to isolate a condenser, pump it down a little to try and get all the liquid out. There is no concern of overpressurizing trapped liquid for me, it's minus 10F all winter, and I have PRV's on the condensers just in case.
How do you supply liquid to the evaporators? Do you have CPR or liquid pumps? How do you do defrost?

[jaysephus]

personal stuff sorry

[Segei]

The liquid to the evaporators is liquid pump feed, two cornell pumps. I live in the northern US and when it is that cold out the air is so dry we have no need for it. The load is so light during these times it's sometimes colder outside than inside the cold storage. As for defrosts, maybe different for you but for me I straight up don't need them. The only door that opens in the winter time is the truck dock doors, going to the outside. By October all of the product is inside of the cold storage so the doors to the warm warehouse never open. I have 7 large evaporators and I check them out everyday and they look clean with not ice. I think one of the reasons why is that the coils are never asking to be feed with ammonia because there is almost no load to heat up the room. Setpoint is 0F and when the RTD reads 2F it turns on the valve to feed ammonia, the thing is it holds it's 0F temp so well. Defrosting is not needed all winter. All I need is my 45psi or more to help move liquid around and I'm ok. However, some days I'll come in and we had a minus 25F night and the engine room is lit up with alarms. At 2am it just couldn't do it anymore and either I get a low level liquid alarm or the FES screw shuts down with high oil temp from lose of pressure for liquid cooling. I start up my low side process refrigeration, blah blah blah and away I go. Great thing for me is I get a few months of shut down time, all of the product is out of the freezer and I get to do maintenance on everything.

This works well for me, and maybe just me and my system. My system may be nothing like yours, but at least you can maybe better understand mine and take the good with the bad. Good luck!!!
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.

[jaysephus]

personal stuff sorry

[Segei]

that is exactly what i do, you know your stuff. wind shields would be nice i do agree, but i doubt my manager would spend the money for that one. right, i can look at the past trends on the plc but i doubt the compressor ever loads up. we isolated all the way down to half of a condenser side out of 5 large condensers. we take the ones with the de-superheaters out right away, way to much surface area on those. may i ask, what are you keeping cold? what is your system like?
I don't have a plant right now. Currently, I help people to run plants efficiently. So I saw many different plants. Sometimes people use tarp and 1x1 wood(and strings) to make the curtains. Roll up for summer and roll down for winter. It isn't expensive.