Calculation for working out the duty of a piece of copper tube

[Andy W]

Hi there,

Does anyone know the formula / calculation to work out the duty of 50 linear metres of 5/8" copper tube submersed in a tank of water to build an ice bank, evaporating at -10oC, i need to double check a calculation from a manufacturer.

[Sandro Baptista]

Maybe around 5...10 kW. However this value can be very different since it depends basically:

- Refrigerant and feeding system
- Ice tickness (the heat transfer will continuosly depend on this value)
- Water temperature. At initial phase, if the water on the tank is higher than 0ºC the heat exchange will be higher
- If the water is agitated or not. This will have a stong efect on the Uglobal (W/m2K) specially if the water is above 0ºC.

I hope I have done a good estimate.

[Andy W]

The refrigerant is currently on R22, and was designed to be used with Alco TIE HW expansion valves with No 3 orifices which will at -10oc give a duty of 7.5kw, due to heavy frosting back the orifices were changed to No 2 which will give a duty of 4.71 oC, now due to previous problems i am trying to find out what the duty of the coil would be, basically 3 coils are piped in parralell with 3 expansion valves and a copeland MCV V9 ZB75KETWD scroll condensing unit sits on top of the ice builder with no suction accumulator, the duty of the unit at -10oC is 19.2 kw = 6.4 kw per circuit, but the manufacturer is also saying that a MCV V9 ZB56KETWD rated at 11.5 kw at -10oC = 3.8kw per circuit is also ok to fit, but my question is the copper tube will have a nominal duty but what?

The water is not agitated and relies on building up an ice bank off peak but they apparently have always had problems with the compressors completely frosting over and after 3 failures recently on 2 systems i have been bought in to shed some light on problems.

[mad fridgie]

You need to start at the beginning, how much ice do you want over what time period.
This will give total duty.
You can then work out hom much ice is going to be around each pipe.
As the ice gets thicker (increased insulation effect) the lower the evap temp (SST), this will reduce comp duty (presumed some form of head pressure control fans cycling, etc).
TXV do not have a fixed KW rating it changes with the conditions. Do NOT just look at nominal rating.
Ice banks are normally stable, setting of superheat wants to be completed towards the end of the ice forming process.
My gut feeling is that you have your ice thickness control set to large. More refrigeration capacity than load.
Your TXV bulb is it insulated, if not it maybe being influenced by an external heat source "ambient etc"
Are the coils top feed or bottom feed.
(from TXV does it go into the bottom of the evap coil or the top)
Is the refrig unit above or below the evap coils.
You may need to install ariser of each coil before commonning each suction.

[monkey spanners]

Txv bulbs on wrong evap outlets?

Superheat pixie been at the adjusters?

I've had trouble with mop valves on ice builders and fit non mop valves to them now.

I'm not keen on alco valves and would most likely change them for danfoss if i thought they were suspect.

Where are the bulbs sited? Often they are below the water on icebuilers.

Are you sure it has R22 still in it?

Whats the superheat when its frosting back?

[Andy W]

Thanks for the reply Mad Fridgie, i do understand what you are saying and have already considered the points you mention, my feeling is the system should of had some kind of capacity control as the compressors are oversized once the ice banks have started been made, also no head pressure control devices are fitted which i have already pointed out, basically i have inheritted a system with problems which need to be rectified including poor design of the liquid line to the expansion valves, undersized and liquid starved due to T piece arrangements, there are many problems with this system lol.

[Andy W]

I have only been involved as a third party up to yet monkey spanners, i have already thought about the coils been crossed and wrong bulbs fitted to valves, if i do the pipe work mods i intended to prove whic circuit is to which, the bulbs are above the water and outside of the tank insulated of course, this was how the machine was built, it is still on R22 and the other side is on R404a and taht isnt working efficiently.

I havent seen it frosting back myself but have been told the compressor becomes a ball of frost.

I may just walk away from this one.

[monkey spanners]

You would need to see what the icebank looked like when it was frosting back, i've seen them freeze solid and they will then run with frost all over the compressor. May be as simple as a fault in the ice bank control!

[mad fridgie]

Thanks for the reply Mad Fridgie, i do understand what you are saying and have already considered the points you mention, my feeling is the system should of had some kind of capacity control as the compressors are oversized once the ice banks have started been made, also no head pressure control devices are fitted which i have already pointed out, basically i have inheritted a system with problems which need to be rectified including poor design of the liquid line to the expansion valves, undersized and liquid starved due to T piece arrangements, there are many problems with this system lol.

OK, the liquid lines themselves, should reduce flood back,(actually should increase head pressure and suction superheat) but indirectly may have caused your problem.
if there is significant pressure drop through the liquid lines (as indicated) you could be getting flash gas entering the TXV inlet. Over time this could be causing the TXV seat to be "Drawn" (lots of scratches). A drawn seat will not seal to its normal potential (approx 25% in case of TXVs) The valve may be closed to its physical limits, but with the scratching more refrigerant can pass! Wow we have flood back.
No head pressure control (not saying this is all bad), means liquid pressure could be very low, thus it follows that the evap pressure will be very low. Even with a few degrees of superheat, the suction at the compressor will be very cold causing the compressor to frost "this is not a problem"
The manufacturer stated a smaller unit, then to ensure warranties you should use what is recommened.
You should not need capacity control on an ice-bank.

[Andy W]

Thanks guys.

[mad fridgie]

Thanks guys.

you could give the scales a nudge

[mad fridgie]

Cheers Boys lol

[tonyelian]

hi
i think u need more length ur pipes are short
p=kxsxdt
s=p/dtxk
s=19200/7x500

s=5.5square meter
s=3.14xdx l
length=110m5/8inch or85meter7/8inch

if u have a capacity 19.2 kw

[Andy W]

That is some formula, would you mind explaining what it all is please.