Evaporator Coil

[haniyah]

Dear all,

I am going to design a DX evaporator coil to be immersed inside the domestic water storage tank of 1000 gallons to reduce the temperature of water from 50 degree centigrade to 28 degree centigrade.

Please tell me the way of calculating compressor capacity, evaporator coil length, dia and suitable condensor size for making this system.

I want to put a thermostat in the tank and make a connection of that thermostat with the compressor motor that can turn off the compressor when water temperature reaches 28 degree centigrade and start it again when it reaches 30 degree centigrade and keep the compressor on untill water temperature reaches 28 degree centigrade.

The capacity of water storage can vary and it will depend on the occupany of the building, so i want a general guideline for designing this system.

Looking forward for your replies......

Regards

Muhammad Abdul Samad

 

[MechEngNCPE]

Go take a Thermodynamics class...I hope its not a IED.

 

[marcoh]

Not that hard to design, but you need to time you want to take to cool the water down. It would probably be cheaper and easier to just buy a small chiller with an integrated hydronic module (Carrier, Trane, York etc all make them)

 

[haniyah]

Thanks alot marcoh for your reply.

Actually its the first time i am going to design and manufacture this system on my own and my calculations for the system are as follows:

q = 500 * gpm * (T1 - T2)

if i take a compressor having capacity of 5 ton then the equation will be 60000 btu/hr = 500*flow(gpm)*(50-28)degree centigrade

Flow = 5.45 gpm
if the capacity of water tank is 500 gallons

then time = 500/5.45
time = 91.74 mins

That means 5 ton capacity compressor takes 1.5 hrs to cool down 500 gallon water from 50 to 28.

The water will be used as domestic water and once it starts draws from the tank the same amount of 50 degree centigrade water starts coming inside the tank and the temperature of water starts rising and as this temperature reaches 30 degree centigrade the compressor starts again and keep on running untill the temperature of water reaches 28 degree centigrade.

This is what in my mind but my question is that the compressor take 1.5 hrs only first time and after that it will be running on and off depending on the demand of domestic water.......
So upto what limit of the capacity of water can i take 5 ton capacity compressor or what equation of compressor and water capacity should i make???????

Thanks in advance

 

[willard3]

You will find that this is a violation of plumbing codes to use this air-to-air heat exchanger in potable water.

 

[DRWeig]

Willard caught it before I did, but he's right on the money.

A failed air-to-air dx coil won't put any oil or other nasty things in anyone's drinking water if it's mounted in air. It'll be a problem if mounted in potable water.

Your coil needs to be designed with fail-safe protection and probably submitted to a testing agency for approval.

Good on ya,

Goober Dave

 

[ChasBean1]

Regarding the last two posts, not sure I’d share that concern. Water coolers are everywhere. Find a refrigerant with a low side pressure lower than the pressure in the cooling water storage tank — any failure would have water leak in to the refrigerant loop and not vice versa. I didn’t check the math on your calculations but your logic appears to be sound. -CB

 

[haniyah]

@ willard3 and DRWeig: What is so amazing in this design??? if any, then plz tell me how the evaporator of screw, scroll and piston type chiller works???

@ ChasBean1: please check and reply to my answer........

@ marcoh: please share your comments regarding my calculations....

I am looking forward for your replies........

 

[imok2]

I believe your centergrade should be farenhight because 50* C = 122*F and 28*C = 82.4*F that's not very cool drinking water?

 

[DRWeig]

ChasBean and Haniyah,

I didn't take time to look up the UL standard, but I've taken two water coolers apart in my day -- one had a double-wall-tube evaporator in the water, with the interstitial space piped to the drain. The other had a copper evaporator coil wound around the outside of the stainless water tank (with a hefty dose of insulation around both).

With a single pipe, I think that even with refrigerant pressure lower than the water pressure, after a leak happens, everything eventually comes to equilibrium pressure. Then, since there's a hole in the copper pipe, oil and other nasty stuff can migrate into the water if you haven't taken pains to prevent it. There's nothing to stop people from drinking the contaminated water.

I'll see if I can find cutaway views. I could be wrong, there may be a direct immersion coil out there for potable water, but I've always been extra careful about cross-connections and contamination.

Good on ya,

DRWeig

 

[willard3]

Pretty surely this will require a double-wall heat exchanger in the potable water to meet the code. Don' want to mix what is in refrigerant system with drinking water....ever......

 

[DRWeig]

Hi folks,

I did some looking last night late, verified that all the major refrigerated water cooler manufacturers (Oasis, Elkay, Haws) have the evaporator coil wrapped around a stainless steel tank and brazed together. Then the insulate the heck out of the combination coil/drum deal. (see attached pic). For larger systems that serve several fountains, they typically have a tub full of a secondary heat transfer liquid that is stirred. The copper evaporator tubing coils through the fluid, not far from a stainless steel potable water coil. It keeps them separate.

Refrigerant piping never comes in contact with the potable water.

I advise strongly against putting an evaporator directly in the drinking water. You might consider putting a sealed containment around the evaporator and any tubing that enters or leaves it.

Best to all,

Goober Dave

 

[DRWeig]

And here's the larger chiller.

Note: chiller pic courtesy of Alpine Chillers, Australia. Combination tube-tank pic courtesy of an unnamed supplier on alibaba.com.

Safety first!

Goober Dave

 

[haniyah]

ooohhhh....... that water will not be use as drinking water....
actually in Sharjah-UAE... its very hot during summer specially in day times its impossible even to touch the water coming out of the tap and for this purpose my company located in sharjah wants me to design this system.......

 

[317069]

You have to consult an older engineer in your company first.

 

[willard3]

Simply put, this is a dangerous idea and bad Engineering.

Plumbing codes, even if you are in the UAE, are written because they have data on how what you propose has failed.