NH3 Desuperheater & General Education

[r551n]

I have been asked to retrofit an existing ice skating rink ammonia refrigeration plant with heat recovery desuperheaters. The intent is to take hot ammonia gas at about 150 psi into two domestic hot water recovery tanks to produce hot water for both a snow melt pit and a Zamboni ice resurfacer. The hot gas would then continue onto two existing evaporative condensers.

I have used the ASHRAE Refrigeration Handbook, but I cannot find enough information regarding piping design and layout as needed. I am assuming I can pipe the suction into these desuperheat tanks as I would a parallel condenser, and pipe the discharge as compressor discharge, minus the oil separator.

I also need to control the water temperature in these tanks at appx. 160F. I can place an aquastat in the tanks, and use it to control a solenoid valve. I could either use a three-way diverting valve or a two-way solenoid valve.

Any comments or experience with these types of systems?

Also, any recommendations on good textbooks for ammonia piping design?

[RANGER1]

The first thing that comes to mind would be shell + tube heat exchanger in or parallel with discharge line .

Maybe a liquid drain would have to be installed on bottom of h/e piped to liquid reciever for any condesate that occurs .

Use a water pump to circulate water through tubes of h/e and turn on/off with t/stat .

This would be same system as freezer floor heating except they use glycol .

Pipework on frig side simple

[Magoo]

Hi r551n
Do you know the total heat of rejection of system. Generally de-superheating will recover between 15>20 % of that figure, to get 70'c / 160 F water, you may have to regulate water flow.

[davidlleida]

I would contact Vahterus dealer in your region. They have long experience and the y can help you to sizing the system.

[US Iceman]

As mentioned, the amount of high quality heat you get is very small. With a system running 150 psig discharge pressure it will be even smaller. And, if you don't have enough mass flow of refrigerant it will become even less.

Everyone looks at de-superheaters as a miracle cure for heat recovery. Magoo & Ranger1 gave some good basic hints at what you have to do or be aware of.

It's also important to keep the heat exchanger drained of liquid refrigerant. Improper piping can cause it to quite working.

[mad fridgie]

If you are using a screw comp and not after heaps of hot water, may be easier to recover the heat from the oil

[US Iceman]

Screws are another problem area. The oil cooling load is the only place where good temperatures exist. Think what happens when liquid injection is used. No discharge temperature to speak of...

[RANGER1]

If recip is used it has been found that its probably not worth doing in most cases as penalties outweigh benefits .

If heat recovery is in discharge line any pressure drop through PHE or shell + tube H/E will use more power as well as a circulating pump on top of that .

Could a small H/E be used to preheat make up water to hot water system ?
I don't know if there would be that much benefit .

How much water are we talking about anyway ?

There seems to be a big push for heat pump hot water systems at the moment , but they are purpose built .

[r551n]

The maximum load on this system would be the 100 gals/hr of water heated from 50F to 160F, so roughly 92k BTU/hr. The Zamboni is usually filled once an hour, but for high use time, we'll have back-up DHW from the arena system.

The total refrigeration load is nominally 90 TR, coming from three recip compressors. The discharge can be as high as 180 psi depending on outside air temperature, and the load on the condensers.

The unit I am looking at using is a Thermastor heat recovery tank (bit.ly/1to3ER) . It is a 100 Gal DHW tank with an integral refrigerant heat exchanger.

I'll also be connecting the recovered DHW to a 5 gpm loop to a plate heat exchanger for the snow melt pit. This is a roughly 50k BTU/hr load that will be disabled on a call for Zamboni water.

Thanks for all the heads up.

[tejashree]

Have you finished and is it working?

[charlie n]

This application is as common as dirt in Canada. Payback is about 2 1/2 years when installed during construction, a little longer if it's a retrofit. This heat is way too high grade for snow melt. Use it only for the ice resurfacer & showers. A liquid subcooling heat exchanger or a small condenser or screw compressor oil cooling heat works better for snow melt & sub floor heating
For 2 standard equipment suppliers see http://www.thermastor.com/ and http://www.doucetteindustries.com/products_vented.htm.