water defrost for low temperature coils

[ROCKY123]

Hello,
Is water defrosting for large low temperature coils really efficient...?
We have a tunnel freezer with almost 1500 liters of ammonia with a dedicated compressor (evaporating at -45°C). Defrost system is only water spray and it takes very long time to melt those ice and I guess the entire coil volume is slowly boiled off and back to the separator during the defrost cycle.
Compressor has to deal with all these gases this will be equivalent to roughly 100kw of refrigeration capacity.
Since the compressor is dedicated to this machine, it wont work during defrost cycle and therefore no availability of hot gas to defrost or to drain the coil. Is there a more efficient way to defrost these kinds of systems.?

[RANGER1]

ROCKY123,
Some following methods could considered.
1/ Use warmer water if available, not to hot though, might need mixing thermostatic valve, have seen 60 deg C but do not want to shock or over pressures system.
2/ Install suction auto shut off valve at evaporator outlet like Danfoss ICLX, or motorised K lok type butterfly valve.
With that install adequately sized pressure regulator/s off bottom of evaporator/s, set at say +10 deg C (515 kpa).
Liquid line have check valve.
As water heats coil it increased evaporator pressure to above 0 deg C quickly draining ammonia out of coil, so limited pump out required.
Consider Compressor may have to cycle on if pressure was to increase in accumulator to much. (maybe).

[seanf]

What if you added a second evaporator that was outside in the outdoor ambient, to use as a heat source/load only during defrost so that you can use hot gas?

[NH3LVR]

Until you have a way to raise the pressure and force the NH3 out of the coil, defrosting is a slow process as Ranger1 suggested. I have never used water warmer then about 25C. Seanf suggested a second Evaporator. We call that a "False Load Coil" here. It has to be rather large however to to provide enough Hot Gas when the the Coil is heavily iced. I have ran systems that used Water and Hot Gas both. You cannot use water from a Plant supply at 10C. It will not work. If you have a ample supply of Hot Water you can employ a mixing valve to temper it. Another possibility is to use Condenser water from another system. This could present a Sanitation issue however. We did not worry about that years ago.

[ROCKY123]

Hi Ranger,

Using hot water is a good choice, specially if i can recover some heat from condenser to make this hot water.

I was trying to see if there's any way i can drain at least half of the coil volume during defrost, but these coils usually sit close to the floor and there's always around 8 meters vertical piping to the headers and when external defrost like water defrost is used, it takes some time for coil volume to start evaporating because of ice formations on the coil.

Another grey area is dimensioning the pressure regulating valve/defrost set valve in defrost drain line, usually we dimension it by considering 50-50 gas liquid which is the case for normal evaporators but if return is full gas, the valve size will be significantly bigger.

[ROCKY123]

when you are using hot gas and water for defrost, what's the usual sequence. hot gas for draining coil and the water for removing ice..?

[ROCKY123]

this is a good idea, did you try how this compares to water only defrost energy efficiency wise..?

[RANGER1]

Hi Ranger,

Using hot water is a good choice, specially if i can recover some heat from condenser to make this hot water.

I was trying to see if there's any way i can drain at least half of the coil volume during defrost, but these coils usually sit close to the floor and there's always around 8 meters vertical piping to the headers and when external defrost like water defrost is used, it takes some time for coil volume to start evaporating because of ice formations on the coil.

Another grey area is dimensioning the pressure regulating valve/defrost set valve in defrost drain line, usually we dimension it by considering 50-50 gas liquid which is the case for normal evaporators but if return is full gas, the valve size will be significantly bigger.

ROCKY123 not sure how many evaporator coils you have, but liquid drain line off bottom of coils, defrost relief valve on this lowest connection would mainly be for liquid.
If suction & liquid isolated, water put on coil, will force liquid up into roof wet return line.
A pressure gauge on unit/s would help to monitor how quickly pressure rises once water defrost turned on.
Is liquid feed top or bottom feed into evaporator?
A 20 or 25 mm line normally adequate for defrost drain line on evaporator, but you may need several to cope.
8 metre riser sounds like it needs double riser if not on already, as big head of liquid could cause it to be inefficient.

Depending on availability & cost of water, also product type, water can be recycled in some cases.
In current scenario hot gas defrost not really viable, as false load, or heat sink would have to be pretty big, running compressor even longer.
Without knowing how your system is piped up, isolation valve positions etc hard to say.
In some plants 60 deg C water used to also sanitise equipment, but has to be designed accordingly.

[mbc]

Hi

Sounds for me it is flooded system with large evaporator .

Best and Economic way for defrosting this type of systems is hot water and in UAE with environment temperature .

To use hot gas in this type of evaporators to remove ice from evaporator, also you need large separator to hold liquid refrigerant going back from Evap. and it takes more time than hot water

defrosting water should pour as much as you can not spray

Normally it takes 15 to 20 minutes.
plus 5 minutes for dripping water.

[RANGER1]

when you are using hot gas and water for defrost, what's the usual sequence. hot gas for draining coil and the water for removing ice..?

Rocky123,
If you use hot gas, it is usually hot gas only.
I was suggesting the water will raise pressure in coil if isolated, which will help for quicker defrost, as well as warm water.
As MBC suggests hot water only to pump down coil like you are already doing, at some point turn off fans & hit with hot water.
This is cheapest option if you can get the hot water.