Defrosting of NH3 evaporators

[SimonBrown]

Hi

What would be the best defrosting method for an ammonia system (with recirculation and liquid pumping) in which the evaporators operate around minus 5 to minus 10 degree C

simon

[US Iceman]

Hi Simon,

What is the air temperature in the room?

I'm assuming the -5C to -10C is the refrigerant evaporating temperature?

Best Regards,
US Iceman

PS. Welcome to the RE forums. Feel free to contribute. We simply cannot have enough ammonia guys!

[SimonBrown]

Air temperature is between plus 3 to 4 degrees

[US Iceman]

Air defrosting might be possible if the space temperature is about 4C. However, to accomplish this task with the evaporator temperature being around -5C I might suggest the use of an automatic valve to isolate the evaporator from the suction pressure during defrost.

Prior to defrost, the evaporator should be pumped down (liquid line solenoid closed) with the fans running. This should boil out any liquid ammonia still present in the coil.

A few minutes after this has been completed, the suction valve would close. This isolates the evaporator so that it can defrost using the 4C air. The suction valve prevents the coil pressure from being pulled down below 0C.

After some further time has passed the coil should defrost nicely.

Of course, if you use the suciton valve to isolate the evaporator coil from the suciotn pressure, the only thing you have to add to get quicker defrost is a hot gas line amd a few additional valves (depending on how you normally set up a hot gas defrost coil).

The hot gas would also be much faster than air defrost.

Myself, I would just go ahead and use hot gas defrost to ensure a clean coil in a shorter time frame.

[Tycho]

The hot gas would also be much faster than air defrost.

Myself, I would just go ahead and use hot gas defrost to ensure a clean coil in a shorter time frame.

I would go with the hot gas, that way, you will also be sure that you get rid of any oil left in the evaporator.

if you use air defrosting, or heating elements, there is always a chance the coil will fill up with oil over time.

My experience is that with hotgas you get most to all oil out of the evap and back to the liquid tank so you can drain it, but with heating elements or air defrost, any oil left in the evaporator will turn to a toothpaste consistency and you face numerous defrosts before the evaporator freeze properly again.

[US Iceman]

My experience is that with hotgas you get most to all oil out of the evap and back to the liquid tank so you can drain it, but with heating elements or air defrost, any oil left in the evaporator will turn to a toothpaste consistency and you face numerous defrosts before the evaporator freeze properly again.

I think that is very true. I also believe this is somewhat dependent on how we circuit evaporators and pipe the suciotn lines.

If a top-fed coil was used with gravity drainage the problem described would be much less prone to occur.

[NH3LVR]

In my experience anything below 3C requires something more potent than Air Defrost.
Even at that it always a good idea in case you ever want to move a degree or two lower. The plant I am servicing at the moment has a problem with dock workers leaving the door open. When this happens the operator must shut off the liquid and wait (sometimes for several hours) to defrost.
Better to apply Hot gas or Electric coils (Not my favorite, a maintenance headache) and have the sytem work properly.
Another common problem is when DX is applied. When the coil blocks up with frost the TX valves will overfeed and cause liquid to return down the suction line.
I do understand the desire to keep cost low. However what is called "Value Engineering" always is more expensive or does not operate properly in the long run.

[US Iceman]

I do agree with NH3LVR & Tycho that hot gas is the best way to go. It does provide a much more positive defrost in a shorter time frame.

I'm not really "pushing" or recommending the idea of air defrost in the temperature ranges mentioned. In fact, these temperature ranges are really getting towards the minimums where air defrost is possible, or could be recommended.

And... I did say I would use hot gas if I were designing the system myself.

[Josip]

Hi, SimonBrown

Hi

What would be the best defrosting method for an ammonia system (with recirculation and liquid pumping) in which the evaporators operate around minus 5 to minus 10 degree C

simon

Definitely automatic hot gas defrosting

Best regards, Josip

[kasperDK]

hi

I think it depents on,wath type of material you want to cool wet or dry, and the size of the evap. and fin spaceing, small evap, wet material, fin spaceing >12mm= hotgas, large evap,dry material good fin spaceing the option for air defrost is posibel

[nh3wizard]

hi

I think it depents on,wath type of material you want to cool wet or dry, and the size of the evap. and fin spaceing, small evap, wet material, fin spaceing >12mm= hotgas, large evap,dry material good fin spaceing the option for air defrost is posibel

[US Iceman]

I believe the idea KasperDk is suggesting is...it depends on if the product is wet or dry, and how you can select different evaporator configurations to possibly suit this application using air defrost.

[kasperDK]

Yes thank you US Iceman

i´ll try to be better and clear in my posts

[nh3wizard]

Sorry, Im easily confused

[tinglehand]

I'm agree with the hot gas defrost. In the case of air defrost there is another issue and is the product that depending of the packing will suffer because of the lost of moisture in the air.

But like this thread is almost dead. What about the evaporator pan defrost, is better hot gas or electric?

[US Iceman]

In the case of air defrost there is another issue and is the product that depending of the packing will suffer because of the lost of moisture in the air.

I'm not sure what you meant above. Can you provide some additional explanation please?

BTW, welcome to the RE forums.

[knighty]

mmm, I'm not a fridgie.... can you tell me (just quick) how a hot gas defrost works ?

guessing it's just like a heat pump ? what was cold gets hot and what was hot gets cold ?....

the bit that has me stuck... is hot GAS ? I don't really see how you could keep the refrigerant as a gas and hot at the same time... ?

[US Iceman]

knighty,

The term hot gas defrost comes from the use of the discharge gas off of the compressor(s). At certain times (when defrosting of coils is required) the hot gas is allowed to flow into the evaporator coil.

The hot gas begins to warm the evaporator coil. When the gas has cooled off to it's saturation temperature, the gas begins to condense into a liquid.

All of this heat energy melts the frost on the coil.

The hot gas defrost pressure in the evaporator coil is maintained at a specific pressure by a regulator valve. By maintaining a specific pressure we can control the temperature at which the hot gas condenses in the evaporator.

This pressure will be equal to a saturation temperature for the refrigerant. The saturation temperature is normally 45-55F (7.2 - 12.7).

Keeping the gas hot is not the problem. The problem is getting rid of the liquid that has condensed.

[taz24]

gets hot and what was hot gets cold ?....

the bit that has me stuck... is hot GAS ? I don't really see how you could keep the refrigerant as a gas and hot at the same time... ?[/quote]


Hot gas is just the term used for defrosting the evap using the systems own discharge gas which is hot and at high pressure. Because it is high in temp and because the heat transfer is from the evap out it can be more efficient than electric defrost. It does have its own problems though, one being stress caused by the sudden temp change and expansion / contraction. One way round this is to use the cooler saturated gas which is drawn from the top of the receiver to defrost systems.
Cheers taz.

[US Iceman]

One way round this is to use the cooler saturated gas which is drawn from the top of the receiver to defrost systems.

Can't say I've seen this on any system except supermarket packs.

[taz24]

Can't say I've seen this on any system except supermarket packs.

True it was predomenatly used on multi stub pack sytems but nowadays most systems have moved back to electric and off cycle. More cost effective they claim . No help what so ever for the original question I know but I get easily distracted. Ooh look a pritty buterfly

Cheers taz.

[US Iceman]

More cost effective they claim.

And I always thought it was because they had such a difficult time making a hot gas defrost system work and still remain cheap!

[taz24]

And I always thought it was because they had such a difficult time making a hot gas defrost system work and still remain cheap!

Well that and the constant repair to the suction and discharge solenoid valves. For some reason they did not enjoy going from -35c to +40c in 5 secs. Also while I'm on the subject why did they design packs with mag valves positioned in imposible places to repair and work on them

Cheers taz.

[Josip]

Hi, Taz24

Well that and the constant repair to the suction and discharge solenoid valves. For some reason they did not enjoy going from -35c to +40c in 5 secs. Also while I'm on the subject why did they design packs with mag valves positioned in imposible places to repair and work on them

Cheers taz.

Answer is very simple, because they did not think about repairing. Many unit's design/construction engineers never ever were in position to do (see) some impossible maintenance on their units and try to improve that.

There is nobody to listen feedback from site. It seems common problem all around the world

Chief of unit assembling department must be an outfield engineer with lot of experience regarding service works/problems.

Best regards, Josip

[Tycho]

Hi, Taz24



Answer is very simple, because they did not think about repairing. Many unit's design/construction engineers never ever were in position to do (see) some impossible maintenance on their units and try to improve that.

There is nobody to listen feedback from site. It seems common problem all around the world

Chief of unit assembling department must be an outfield engineer with lot of experience regarding service works/problems.

Best regards, Josip

Sometimes on could think they do it on purpose

I bet they have a website much like this where they share stories of how they, after much thinking and responses from others finally managed to put a critical component in the most unaccesible place on a compact unit

some of their posts probbably start like this:

""Oh man, you guys are gonna loooove this ........ You need to be double jointed and have really really small hands to reach it, I'm talking baby hands lol....... will post pictures tomorrow....... oh and as always, I'll post all "product improvement" suggestions I get here, so you'll have something to laugh over""

[US Iceman]

There is nobody to listen feedback from site. It seems common problem all around the world

Exactly right. There is huge disconnect between the field people and the designers. This is usually where the service department becomes involved as a warranty claim.

It's not the service guys who are at fault. It's the designers who have never had to work on the stuff they design.

If the designers (and the salesman) did their job better by asking more questions, there would be less work for the poor guys who have to work on the equipment.

As the process works now, it seems the problems become the issue for the next department... Sales, then designers, then fabrication, then startup, then warranty & service....then the customers!

It can work better, but everyone has to talk and listen to one another.

[taz24]

Yeh, you are right.

Designers ought to be made by law to work on the systems they design. Cost comes into as well I imagine. Smaller area and fewer components.

Oh well no point moaning eh I'll just dislocate my shoulders so I can work on this next valve

Cheers taz.