desuperheater design

[mehta]

Hi
I plan to install a desuperheater in my existing refrigeration system. System consists of two reciprocating compressors with ammonia as the refrigent. TR for the compressors combined is 90 TR. the suction pressure into the compressor is 20 PSI and discharge pressure to the condenser is 180 PSI. About 5000 L of water is to be heated at 20 C to maximum temperature possible. Please help me calculate the area of heat exchanger. How much heat do i actually have????

[Magoo]

Contact your local AlfaLaval agent and explain your requirements. With desuperheating you could expect to recover approximately 30% of total heat of rejection for heating water

[mehta]

Contact your local AlfaLaval agent and explain your requirements. With desuperheating you could expect to recover approximately 30% of total heat of rejection for heating water

Thanks Magoo for the reply....Alpha laval heat exchangers are too expensive and not within my budget...I plan to construct it by myself and have the facility to do so..If you could help me with the calculations???

[Magoo]

If building a vessel yourself for refrigerant, you will have to submit a fully engineered unfired pressure vessel design for approval before construction. A long and expensive process. You would be off to select an off the shelf vessel already approved and certified.

[mad fridgie]

ON THE CHEAP!
Just get 50meters of 3/8 copper pipe, and wrap it around your discharge pipe insulated, counter flow.

[Magoo]

Good idea MF, that would work.

[mehta]

MF and magoo..thanks for the reply...i have a SS-304 shell and tube heat exchanger with the following dimensions:

tube length-6 feet
no of tubes 160
tube dia - 0.75 inch
shell diameter- 20 inches
shell length - 9 ft
single pass with no baffles in shell

the heat exchanger was previously used to heat water with saturated steam.

Will this work???

[Magoo]

I will not recommend anything that is not specifically designed for the process.
That HX is huge and way too big if applied as a desuperheater.

[Rob White]

the heat exchanger was previously used to heat water with saturated steam.

Will this work???

I will not recommend anything that is not specifically designed for the process.
That HX is huge and way too big if applied as a desuperheater.

Also if it was once used for saturated steam, what pressure is it rated / tested to?
And to throw another complication into the mix is it wet inside?

Regards

Rob

.

[Kherson]

God day gentlemen. I have just joined, this is my first post and I apologize before hand if my reply is out to lunch.
First of all, how are your recips cooled? Second, what can be the actual flow rate of your water that you want heated, 5000 litres an hour or something else? The greater the water flow, the smaller the H/Xer required. I am assuming that you have a tank that holds 5000 l of water. If your heat recovery system ends up doing more than just desuperating, do you run the risk of liquid refrigerant flowing back into your compressor? Theoretically, you should be able to heat your water up by 15 to 18 C. Finally, there is no need to heat water above the actual temperature needed for the process required...
Kherson

[mehta]

Thank you all for your time..

@magoo - what complications could i face? I feel the least that can happen is ammonia might condense. I can send the liquid ammonia back to the receiver. what is your say? Please correct me if i am wrong.

@rob- the heat ex changer is rated to a pressure of 300 PSI so that would not be a problem. yes it is wet from inside. the water flows through the tubes.

@Kherson- Presently i have an atmospheric type water condenser to cool of the gas. the actual flow rate is 5000 l/hour. I feel i do not risk the liquid refrigant flowing back to the compressor because i can send it to the ammonia receiver tank. correct me if i am wrong. i need to heat water to a max rise in temp because i want to send the preheated water to boiler for making saturated steam used in other applications. The requirement for boiler is 5000l/hour.

[Kherson]

The type of unit that I would be using is one that Doucette Industries Inc out of Pennsylvania builds. I have first hand experience with these and they work great...
Kherson

[sandybapat]

Mr. Mehta
It seems you are having evap / cond temp of -15/35C with Ammonia. For 90 TR around 100 kW power consumption using 2 Nos. Kirloskar KC3 comp. I.e. you have superheat of approx 100 kW out of which you may be able to recover around 75 to 80 kW with out risking condensation. Therefore you can expect heating of 5000 L of water from 20 to 33 deg C max. As suggested by Magoo Alfa Laval phe would be an economical solution.
Have you measured discharge temperature?

Sandybapat

[mehta]

Mr. Sandybapat
thank you for your time
I have frick compressors and am not too sure about the evaporating condenser temperature. The discharge gas temperature is about 70 C.

[sandybapat]

Dear Mehta

Have you measured thetemperatrue any time? It cant be so low for Ammonia compressor with pressure ratio of 9. What is the power consumption of each compressor? What is the duty of these compressors? Brine chilling or ice plant or cold room?

Sandybapat

[RANGER1]

mehta,
5000 l/hr = 83 l/sec, thats a lot of water to heat up or in your case very slightly warm up.
I don't think you could justify the pay back on such a small refrigeration plant.
Maybe heat recovery off the boiler in some way would be more beneficial.

What type of boiler is it? gas, coal or other

[mehta]

@sandybapat - the discharge pressure is 140 PSI these days because of winters here in north india. The discharge temperature corresponding to 140 PSI is 70 C. this discharge pressure goes up to 200 PSI during summers and i am not sure about the temperature. Power consumption is 60 HP each. The duty is cold rooms.

@Ranger1 - 5000l/hr=1.38L/sec you have to divide by 3600. the boiler is wood fired.

[sandybapat]

Dear Mr. Mehta

The discharge temperature in summer may go up to 120 deg C and power consumption also will be higher around 70 to 73 HP.
Your boiler feed temperature in winter may be much lower than 20 deg C.

Anyway you can expect a temperature rise of 10 to 13 deg C in boiler feed water temperature for 5000 lph flowrate. I expect heat exchanger cost around INR 250K.

Wood stock has calorific value of 3.5 to 4 kWh/kg. For 70 kW heat recovery you will save 20 kg of wood per hour.

You may be able to work out payback on this data. Please work out and let me know.

Sandybapat

[mehta]

Thank you Sandybapat for your time

Please can you explain how you reached the figure 10-13 C.

payback period according to your calculation is 1 year.Do you know any manufacturer in north india?

[RANGER1]

@sandybapat - the discharge pressure is 140 PSI these days because of winters here in north india. The discharge temperature corresponding to 140 PSI is 70 C. this discharge pressure goes up to 200 PSI during summers and i am not sure about the temperature. Power consumption is 60 HP each. The duty is cold rooms.

@Ranger1 - 5000l/hr=1.38L/sec you have to divide by 3600. the boiler is wood fired.

Sorry my mistake.

Would it not still be more viable to instal some type of heat exchanger in the stack of the boiler?

[sandybapat]

Mr. Mehta

Assuming you can recover 75 kW of superheat from discharge gas. 75 kW=64500 kcal/h. 5000 lph water low rate . use m.cp.delta T equation to arive at 12.9 deg C temperature rise.

Sandybapat

[mehta]

hi guys can anyone help me calculate convective heat transfer coefficient for super heated ammonia?
I need to calculate the overall heat transfer coefficient for heat exchanger. I calculated convective coefficient for water in tubes to be 2913 W/m2 C and for SS304 it is 16 W/m2 C. For ammonia i used the following eq
Nu = 0.3*Re^0.6*Pr^0.33
I am not too sure about the mass flow rate ammonia and took it to be 0.2 Kg/s. and got a velocity of 0.11 m/s. with this data i found convective heat transfer coefficient for ammonia to be 60 W/m2 C and overall heat transfer coeff = 12 W/m2C.

i fell this value is incorrect and too small for calculating the area required????

[sandybapat]

Mr. Mehta

You can use the methods given in book (Title: Process Heat transfer by Kern). Under chapter two phase condensation, you can use first part of desuperheating from this chapter. This chapter gives calculations from first principle. Consider Ammonia vapours on shell side and water inside the tubes.

Sandybapat