Desuperheater designing

[GBRD]

Hi All,

Wondering if someone know about designing this euipment. I've NH3 system having a refrigerant mass to condensers 4.5 kg/s @ 28 deg C SCT. Ref temp is 70 dec C (screw comp system).

I need to design a coil of 50NB pipe to heat the water up, stored 10,000 lit in an insulated tank. Assume that water to be heated in 2 hrs to max possible temp. Water return would be 15 deg C after defrosting.

Would appreciate designing concept with calculation method.

regards

GS

[Tesla]

HI GBRD
I would suggest using the pressure enthalpy chart to calculate how many kWs you will be able to remove from the refrigerant. then Q=M*shc*td or Q/(10,000*4.2kJ/kG*K)=td for the water. But you might use a heat pump for improved heat transfer and efficiency. It will take a little playing with the figures and calcs results for the best method. Take into account your storage losses and heat transfer.

[mad fridgie]

If heating to 60C then you need approx 525kwhr plus losses,
A simple way to look at what is available is the draw of your electrical motor, if for example is 300kw. then you no that you are going to have 300kw of superheated vapour within your system.
You may also consider using your oil cooler (if already water cooled) just a question of arranging the water pipework.
If you need to install a de-superheater, your priority should be refrigerant pressure drop, when there is little or no heat transfer to the water. nowadays this seems to be forgotton, sizeing is based upon best heat transfer to the water. (Driving up the head pressure at the end of the heating process))

[Sandro Baptista]

Hi All,

Wondering if someone know about designing this euipment. I've NH3 system having a refrigerant mass to condensers 4.5 kg/s @ 28 deg C SCT. Ref temp is 70 dec C (screw comp system).

I need to design a coil of 50NB pipe to heat the water up, stored 10,000 lit in an insulated tank. Assume that water to be heated in 2 hrs to max possible temp. Water return would be 15 deg C after defrosting.

Would appreciate designing concept with calculation method.

regards

GS

The average heating power to accomplish +50ºC is 203 kW (in 2 hours it represents 406 kW.h of energy).

The hot capacity sensible heat we can get from +70ºC » +50ºC is 243 kW. Counting with 40 kW for heat losses through tank and pipes we are with "secure calculation".

I said average heating power because if you select the heat exchanger for a medium logaritmic difference temperature for heat exchanger purposes determination then at the beginning (at +15ºC) you will get more heating power (the water temperature will increase more at the beginning and the outlet NH3 superheated vapor will be less than the +50ºC. This happen due the high exchanger at the first period when the water is being reheated. It could even happen the NH3 enters the liquid-vapor phase (condensation).

[GBRD]

Hi Telsa, thank you for your suggestions, but I reckon its not straightforward when designing coil.

Hi Mad Fridgie, you are right with capacity requirement, but what about coil designing?

Hi Sandro, spot on mate. What's your LMTD? 26.8 deg C? Also, what about U factor of coil? I need to calculate total length of pipe & therefore need total surface area. Calculation of U factor is MOST critical in this calculation, which I'm not able to do....

thank you all of you...

[mad fridgie]

Lets get this straight, you want to insert a static coil of steel tube into a static tank of water, and you actually want 10,000 litres of hot water

[GBRD]

I'm confused with your response Mad Fridgie....what length of coil should I put into?

[mad fridgie]

I'm confused with your response Mad Fridgie....what length of coil should I put into?
Sorry, you want to install a lenth of pipe "X" meters long into a tank of water, to heat the water. The water is just sitting in the tank

[Sandro Baptista]

Hi Sandro, spot on mate. What's your LMTD? 26.8 deg C? Also, what about U factor of coil? I need to calculate total length of pipe & therefore need total surface area. Calculation of U factor is MOST critical in this calculation, which I'm not able to do....

thank you all of you...


I normally use plate heat exchanger to do the job then water recirculates between the heat exchanger and the tank in a closed loop. The plate exchanger is calculated using the software of our suppliers and/or they do the final calculation based on parameters we give to them. I always pay attention so the pressure drop don't pass on the 30 kPa but this is a criterion that depends of many factors including the extra power consumption to heat a certain amount of water.

I think with heat exchanger displaced of the tank has many advantages as we can discuss later.

Regards

[GBRD]

Water would be in circulation through a small pump of 10 lit / min. Main water pump of 33 l/s comes on only when doing defrosting.

[mad fridgie]

Water would be in circulation through a small pump of 10 lit / min. Main water pump of 33 l/s comes on only when doing defrosting.
10L/M = .166L/S * 4.2 * 50 = 35Kw, a bit different to what you require.
Purchase a designed heat exchanger, with minimal refrigerant pressure drop

[Sandro Baptista]

10L/M = .166L/S * 4.2 * 50 = 35Kw, a bit different to what you require.
Purchase a designed heat exchanger, with minimal refrigerant pressure drop

You're right mad. It's not a bit but rather a huge different. And normally we seek a water flow corresponding to of about 2 ... 4 K of temperature difference (and not 50 K) between outlet and inlet so a good transfer coefficient can be obtained.

GBRD, maybe you meant 10dm3/s or 10m3/min and not 10dm3/min?

[GBRD]

Sorry to you both, a quick clarification. The small pump of 10 lit/min works only when there is no derfrosting. This pump would ensure that the water in the pipelines & in the tank remain in circulation all the time. The capacity of large pump is however based on the requirement of the evaps required to be defrosted. Hope this makes sense.

regards

[mad fridgie]

"528Meters long 50mm, 200kw"

[Sandro Baptista]

Sorry to you both, a quick clarification. The small pump of 10 lit/min works only when there is no derfrosting. This pump would ensure that the water in the pipelines & in the tank remain in circulation all the time. The capacity of large pump is however based on the requirement of the evaps required to be defrosted. Hope this makes sense.

regards

okay, I had understood on the previous email but the problem remains. I mean: During the periods out of defrosting the tank will accumulate heat. Like you have asked 200 kW of heat capacity during the accumulating is impossible to do with 10 lit/min...maybe after all you don't need so much heat power during the accumulation period but rather on the defrost period where a high rate of energy is needed.

[GBRD]

Hi mf,

How you arrived at this fig pl? Can you guide me through pl?

regards

[mad fridgie]

It would take to long write.
But i will point you in the right direction.
Firstly this not the right method for this application.
Ok you water in the tank is not goin to move with the little pump, especially when it is full of "baffles, your pipes"
so you have very poor heat transfer, but when you first start (cold water) you will get natural movement over the pipe because of the temperature difference, breaking the boundry layers, this increases the heat transfer rate. (and reduces over time)
Your LMTD also reduces over time, again this made worse to natural stratification in the tank (effectively makeing part of the heat exchanger useless)
So knowing that the system is totally unstable, you then have to pick some moving averages across your TD, Time and heat transfer co-efficient.
Bobs your uncle. Done
For the above reasons, is why you would pick a flowing heatexhanger, you have constants, and normally the only thing to worry about is TD

[RANGER1]

Why would someone want 50 C water for defrost ?
BAC have coils for this application , why not ask them .