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gwapa
19-10-2007, 12:55 AM
Hi to all
I would like to kwow your thought about the best form to control the temperature in a freezer working at -28C with a evaporation temperatura at -35C
The evaporator will work with amoniac overfeeded 4:1
The defrost cycle is clear up but the normal operaation during cooling I have my doubts
Please se attached file
The option are:

Open a close the inlet solenid valve 21SV01Or

Open and close the suction powered valve 21PC01Note that if we clse the inlet valve we will pump dawn the evaporator
If we lose the succion powered valve the pressure inside will rise and may be we will have high speed of the ammonia when it open again
Thanks in advance

PaulZ
19-10-2007, 06:56 AM
Hi Gwapa
The schematic looks like a standard hot gas set up with pressure bleed off valve. I am not sure of the configuration of the gas powered suction valve, gas to close or gas to open.
In cooling
Liq 21SV01 open
Suct 21PCV01 open
Hot Gas 21SV04 closed
Pressure Bleed 21SV05 closed
Defrost pressure reg 21PCV02 closed
and 21SV02 and 21SV03 either open or closed depending on the configuration of the suction valve.
For defrost
Liq 21SV01 closed
Suct 21PCV01 closed after pump down
Hot Gas 21SV04 open
Defrost pressure regulator 21PCV02 will control pressure in evap during defrost between 400 and 500 kpa
When defrost hot gas is finished pressure bleed valve 21SV05 will open usually in drain down time, this will stop sudden rush of gas and liquid down suction line when suction solenoid opens.
21SV02 and 21SV03 will be either open or closed depending on suction valve configuration ie opposite to cooling.
Hope this helps.
Paul

gwapa
19-10-2007, 02:20 PM
Thanks Paul

My thought is; what happen when you stop cooling?
Which valve (s) close?

If you close the inlet solenoid valve (liquid) you still have ammonia inside the evaporator. This liquid will evaporate leaving the evaporator tubes empty and you are going to have also a postcooling whithout control .When the cooling resume you have to fill again the evaporator (it take time) so you will have a small dalay and a surge in the separator tank.

In other hand. If you close the suction valve you stop the cooling almost instantaneally and you leave the evaporator full of ammonia liquid. This ammonia will increase the presure untill its temperature be close to the room temperature.When cool time resume , the succion valve will open . The presure will low and some ammonia will vaporise .This ammonia gas will produce a hight speed inside the tube of the evaporator and could produce vibration of the coil. You also would not have surge in the separator tank

What do you think about?
Reagars
Gwapa

PaulZ
19-10-2007, 03:10 PM
Hi Gwapa
When the liquid solenoid is closed the evap will pump out but not completely depending on pump down time. Even if there is liquid left it will not cause a problem and the amount of cooling will be minimal. The small bleed solenoid lowers the pressure in the evap gradually during the drain down time so when the main suction valve opens there is not a large pressure difference between evap and suction line.

The problem with shutting the suction line is you will have too much liquid in the evap and it will take a long time to defrost the evap as you will have to warm this liquid as well as the gas in the evap.

We have installed many hot gas defrost systems and have had no problems with high speed gas or liquid flows through the evap or suction lines.
Our normal sequence for defrosting is as follows
on initiation of defrost cycle is
* shut liquid sol and pump down about 10 mins
* shut suction sol and open hot gas sol valve
* defrost time from 15 - 30 mins depending on ice build up, generally try to keep this time as short as possible
* set defrost pressure regulator valve at about 400 - 500 kpa, when we install penthouse units we would set at 400 kpa relief
* drain down time about 10 mins and in this time open the bleed sol valve this allows the pressure to drop to nearly suction pressure before tye suction valve is opened
* open suction and liquid sol valves
* about 2-3 mins later turn on fans, this allows any moisture on fins to freeze and not be blown off coil
If you have the correct level in your accumulator there will be be some surge but not enough to cause a problem.

Another problem with not pumping the evap down is there could be a lot of liquid coming back to the vessel when the suction valve opens as some of the hot gas will condense in the evap filling it even more then normal and this could cause high level in accumulator.

Parker have a book on hot gas systems and how to set them up, if you can not find it on the net send me your email address and I will sned it to you.

Hope this helps
Paul

gwapa
19-10-2007, 04:48 PM
PaulZ thanks again,
Please mail me the parker paper
wpadilla@cantv.net

My concert is only in the cooling period.

Which valve should we use to control the room tempertaure?

Should I control the room temperature with the liquid solenoid valve opennig and shuting it or we should use de sucction valve to do the same.

Thanks for you thoughts

NH3LVR
19-10-2007, 08:38 PM
Which valve should we use to control the room tempertaure? Should I control the room temperature with the liquid solenoid valve opennig and shuting it or we should use de sucction valve to do the same.
If at all possible use neither. Unless you have other loads the Engine Room will be loading and unloading every time the valves operate. If it is a single evaporator system it will cause wild swings in suction pressure.
The best way is to control the engine room suction pressure, if that is feasible in your situation. Or a electrically operated suction pressure regulator.

nh3wizard
19-10-2007, 09:07 PM
PaulZ thanks again,
Please mail me the parker paper
wpadilla@cantv.net

My concert is only in the cooling period.

Which valve should we use to control the room tempertaure?

Should I control the room temperature with the liquid solenoid valve opennig and shuting it or we should use de sucction valve to do the same.

Thanks for you thoughts

You want to control the temp via the liquid solenoid.

gwapa
19-10-2007, 09:49 PM
This is a tuna facilities
There are three walk-in freezer
Each freezer has three evaporator of 18 Tr (63kW)each

PaulZ
20-10-2007, 09:32 AM
Gwapa
I presume all the rooms are at the same temp. A thermostat should be used to control room temp by shutting the liquid and suction valves.
You will also need a signal back to the plant room to turn the liquid pumpand compressors off if all rooms are satisfied. You will also need some sort of suction control to load / unload or stop / start compressors.
I will send the Parker info next week.
Paul

Sergei
22-10-2007, 10:01 PM
Usually, in industrial refrigeration we 2 types of temperature control.
1. When temperature is satisfied, liquid solenoid off, but evaporator fans continue to run.
2. When temperature is satisfied, liquid solenoid off and fans off.
Suction valve should be always open unless we have hot gas defrosting.

PaulZ
23-10-2007, 01:12 AM
Sergei
This system is hot gas defrost. I have found over the years that if the fans stay on you can get on top of any sudden load quickly and this would be my prefered option. It also stops hot spots in a large room.
As this is a pumped liquid system the suction solenoid will have pressure relief for defrost so shutting the suction will allow the liquid in the evap to remain so there is no delay in getting liquid into the evap when the room comes back on. This will also stop temp overrun when there are multiple rooms on the system.
There is an accumulator vessel so there is no risk of liquid back to the compressors when the room is called on.
Paul

Sergei
23-10-2007, 02:11 AM
Sergei
This system is hot gas defrost. I have found over the years that if the fans stay on you can get on top of any sudden load quickly and this would be my prefered option. It also stops hot spots in a large room.
As this is a pumped liquid system the suction solenoid will have pressure relief for defrost so shutting the suction will allow the liquid in the evap to remain so there is no delay in getting liquid into the evap when the room comes back on. This will also stop temp overrun when there are multiple rooms on the system.
There is an accumulator vessel so there is no risk of liquid back to the compressors when the room is called on.
Paul
1. I prefer fans off for energy savings. There is no difference for sudden load if fans off or on. Problem of hot spots in refrigeration rooms usually exaggerated. I saw room 3m x 3m x 3m and fan on and room 30m x 20m x 10m fan on and off. Both rooms don't have hot spots.
2. When room back after defrosting, practically no liquid in the coil. Coil will not defrost if it has liquid.

PaulZ
23-10-2007, 06:10 AM
Hi Sergei
I did not say there was liquid in the coil after defrosting I said when the room cycled on temp, you misunderstood what I said. The reference to defrost was only about the pressure relief on the suction sol valve. I know there would be very little refrigerant in the coil after defrost as there is a pump down cycle before hot gas, if you read one of my previous posts you would see this.
The fans issue is personal choice and no doubt there would be for and against for both ways.
In reference to hot spots you are probably right if the product put into the freezer is at or near the room temp but in a lot of instances where the product comes in a fair bit warmer or in some cases use holding freezers as pull down rooms, this is why I prefer the fans on all the time.
Paul

Sergei
23-10-2007, 03:59 PM
Hi Sergei
I did not say there was liquid in the coil after defrosting I said when the room cycled on temp, you misunderstood what I said. The reference to defrost was only about the pressure relief on the suction sol valve. I know there would be very little refrigerant in the coil after defrost as there is a pump down cycle before hot gas, if you read one of my previous posts you would see this.
The fans issue is personal choice and no doubt there would be for and against for both ways.
In reference to hot spots you are probably right if the product put into the freezer is at or near the room temp but in a lot of instances where the product comes in a fair bit warmer or in some cases use holding freezers as pull down rooms, this is why I prefer the fans on all the time.
Paul
Hi, Paul.
If you put warm product in holding freezer, the temperature in this freezer will go up and your evaporators will run nonstop anyway to pull down the temperature. I have one question. Why to close suction valve when temperature satisfied?

PaulZ
24-10-2007, 02:43 AM
Hi Sergei
What I have found over the years is that a lot of clients use their freezers for things other then what they were designed for. They could have a 600 pallet room and the will put a couple of pallets of product in that may only be -10 and pull it down to -18 or -20. If the fans are not running this product could be some distance from the thermostat probe and will take a long time to freeze at normal cycling, this is one of the reasons for leaving the fans on.
With regards to shutting the suction solenoid when the room is down to temp. If you have say 3 rooms with 3 evaps per room and you leave the suction sol open those evaps will pump out. If you get 2 or 3 rooms come on one after another the level in the accumulator will drop fairly quickly until you get some liquid coming back down the wet gas return. During this time the make up solienid is on and the reg valve is set for normal operation. If the amount of liquid being pumped to the evaps exceeds the amount of liquid being fed into the accumulator you could end up having low level faults or the liquid pump cavating. If the suction is shut a fair bit of liquid is retained in the evap so when the room cycles on there is liquid coming back fairly quickly and the vessel level doesn't drop as much.
There are probably several opinions on how a system like this should be controlled all I can say is what has worked for me. If you can keep a system as stable as possible you have less headaches and call outs.
Paul

Sergei
24-10-2007, 03:45 AM
Fan operation is starting point of our disagreement. I prefer evaporator fans cycling(with liquid solenoid) or fans with VFD. In that case liquid always present in the coil at evaporating temperature. When fans are off, liquid in the coil continue to evaporate, but at slower rate. Energy savings of this operation is significant. Evaporator fans using energy and release this energy into refrigerated space. This is additional(parasitic) refrigeration load. Usually, to remove the heat of 2HP fan we have to spend additional 1HP of refrigeration plant power. If we shut off this fan, we will save not 2HP but 3HP.
Regarding to 2 warm pallets in 600 pallets freezer. This isn't blast freezer. This is holding freezer. Majority of this pallets are cooled by natural convection. For them isn't important fan cycling or fan nonstop operation. Temperature of surrounding air is very important for them.

PaulZ
24-10-2007, 06:50 AM
I would be interested in other people's comments on this subject