desuperheating not condensing

[CCLPaul]

We have a customer with approximately 30 condensing sets running R22 ranging from 3 to 30hp. We are about to change them to 50kW packs using R404a. Our customer uses a lot of hot water and we will be using rejected heat to increase the temperature as high as we can before it enters the boilers. My design was to use shell and tube condensers, but I have recently been advised that I require de-superheaters not condensers. The hot gas is to run through the de-superheater and then through an air cooled condenser with fan speed control. This means that instead of getting all of the THR I am only going to get 70% or so.
Can anyone help? Surely if I have 65kW THR per pack I should be able to use it.

[Pooh]

Paul
it depends on what temperature you want the water to leave the condenser to go to the boiler, thinking about it logically from an air cooled condenser the liquid temp is only about 25 degC so the water leaving the is not going to be much more than 50 deg C. If you are just preheating the return water to the boiler then the return temp will be in the area of 55 to 70 degC so you wont get any subcooling in the liquid line.

Ian

[CCLPaul]

Ian

I didnt explain myself properly, at the moment about 75 cubic metres of water is supplied straight from the mains and heated to 82C only to be blasted out of the pressure washers to waste. If we can use the rejected heat to warm this water we can save a small fortune on the boiler gas bill, so the warmer we get the water the better, obviously without compromising our refrigerating effect.

[The MG Pony]

Sounds like to me you'd want to be taking from the discharge gas

have the mains water come in through a sub-cooler then have it flow through a discharge side heat exchanger then to the boilers would yield a nice double effect.

I'd allow some of the other guys to comment on that first how ever!

What is the flow rate to the boiler from the mains?

[CCLPaul]

The maximum is 5.5 litres per second, we will be running a water/glycol loop around all heat recovery exchangers then using plate heat exchanger to transfer heat to incoming water. The calcs show we have more than enough heat for the job

[Peter_1]

If it was me, I should install a PHE sufficient to condense the whole capacity.

The available desuperheating capacity is only a very small fraction of what you have as a total condensing capacity.
You will add perhaps 15 kW in the water where you have the possibility to add perhaps 65 kW.
You then blow off 50 kW to the outside air which costs you additional electricity to run the condenser fans.
The desuperheating capacity is not 70% of the THR but perhaps 10% (Coolpack has a special option top calculate this)

You additionally must take then advantage to reduce the high pressure to increase the COP of your compressor.

As soon the water becomes warmer, you will more and more use the condensor (not a desuperheater) as a desuperheater and you will finally reach discharge temperatures (if you have enough compressor capacity and not too much water capacity).

The biggest saving is not the water heating but the reduced headpressure of your compressor. See then the heating of the water as a further saving on the total electricity bill.

Then use this preheated water to feed a second small reservoir (standard electric boiler) with an electrical heater. Don't forget that for certain applications you have to heat the whole system to 70°C on a regular base. In public places in Belgium, this has to be done several times/week to avoid legionella.

[CCLPaul]

Peter

Many thanks for your response, I couldnt agree more, I am still not sure why we have been advised otherwise.

Paul

[Peter_1]

In http://www.refrigeration-engineer.co...t=10895&page=2
I made in my post (n°35) a rough calculation for a freezer floor. The desuperheating capacity is even less then I suggested.
Who suggested this? A wholesaler?
Why? I think pure ignorance.

[US Iceman]

I have not read all of the posts here, but do agree with what Peter is saying. The amount of heat energy available by desuperheating is very small (even with ammonia systems).

Desuperheaters are almost always recommended by the people who manufacture them or sell them.

Even with the small benefit you can get with them, a minor incorrect piping arrangement can cause you to loose that if the desuperheater fills with liquid refrigerant.

[rudyreg]

Peters' suggestion of using the condenser to heat the water is valid if you only pass the water through the condenser once, but then it doesnt heat up much and once you have filled your 8000 litre tank then the rest of the water will go to waste. Water is expensive also! To save on bolier gas bill you need to heat the water as high as possible using only wasted heat from your refrigeration plant. For this you need to put it into a loop through your desuperheater where it increases in temperature with every pass. You cant do this with a condenser or your condensing temperature will rise and rise with negative effects on your fridge plant.

Does this make sense or am I missing something?

[Peter_1]

I think you’re missing something or I told not the whole story: if the heat exchanger is located outside the water (not submersed in the water), then you must circulate the water of course.
Then use a pump with a stainless steel inner housing or otherwise it will rust (oxygen rich water).

As you can find in another old thread where I posted some pictures, we mostly insert the heat exchanger in the water so that we don’t need an additional pump, so again saving energy.

We’ve installed reservoirs from 200 l (52 US Gal) to 15.000 l. (3.965 US Gal) on 70°C (158°F) without increasing the high pressure

[The MG Pony]

What Peter is saying (Or at least what I am reading it as) is use a condenser sized plate HX for the water heating in cojunction With the Air cooled condencer giving the maximum heat scavenging rather then use the de-superheater with a much smaller ability

[CCLPaul]

Many thanks for all your comments, the heated loop will run through the condensers, with this loop heating the incoming water. This site uses more hot water than we have the capability to heat, so our responsibility is to use that heat, all of it. The only time we will be rejecting more heat than they can use is on a sunday, we may use a buffer tank so that Monday is an even better energy day. I am now sure that our advisers did not listen, or maybe understand exactly what we are working to achieve. We will also fit an air cooled condenser, so that the refrigerant either passes through the water or the air cooled and then to a sub cooler.

[SteinarN]

Hi This is my first post in here. This topic about heat recovery to heat water is a very interesting one. I have some experience with it.

There is a number of considerations to be done. To start with is the total heat rejected compared with the wanted input capacity to the water. It requires 23 kW to raise the tempereture of a waterflow of 5,5 l/s with only 1 degre C! (5,5x4,18x1) As you have concluded you can easily inject the total condensercapacity, as long as it is in the kW range and not in the MW range, into the waterflow of 5,5 l/s, and still the watertemperature only increases a few degres.

Howewer it is possibly to reach impressing water temperatures when there is more corelation betwen the wanted capacity and the heat rejected. One property of R404A is that it has actually a high density in gasseous state and corespondingly a high energy content compared to the volume of the gas. This property make R404A to an ideal refrigerant to be used in a high temperature water recovery system.

At a condensingtemperature of 40 degres C and a discharge temperature of 70 degres C the desuperheat capacity is 20% of total heat rejected, at 90 degres C it is 28%. At 30 degres condensing and the same discharge temperatures the percentage is 24% and 31%. Refrigerant such as R22 and even more so ammonia has considerably less desuperheat capacity at the same temperatures.

In order to acheive a high water outlet temperature and still maintain a moderat condensing temperature it is absolutely necesary to have a high discarge gas temperature. The installation of a suctiongas PHE is an elegant way to acheive this. In addition this increases the capacity and COP of the compressor in the range of 8 and up to 20 % dependend on the aplication.

One such project i designed was for a large restaurant which produces all their food from the bottom. I designed a centraliced direct evaporation refrigeration system for the cooling rooms and other cooling equipment. -10 degres C evaporation and 40 degres C condensing, inverterdriven compressor. R404A, suctiongas PHE, 25-30 degres C suctiongas temperature and roughly 95 degres C discharge. I installed an PHE desuperheater/partially condenser betwen the compressor and an outside aircooled condenser. The outside condenser has inverterdriven 3-phase fans controlled acurately by the condensing pressure in order to maintain exactly 40 degres C condensingtemperature. Inlet watertemperature to the desuperheater is roughly 5 degres C. Water outlet temperature is regulated to 63 degres C by an electronically controlled valve in the waterflow. If the water outlet is below 63 degres C the valve closes in a bit, and oposite if the water outlet is more than 63 degres C.

This configuration gives a total heat recovery of 70% with a water outlet of 63 degres C and 40 degres C condensing. The desuperheater reclaims all the superheat in addition to roughly 50% of the condensing. Only 30% of the total heat is rejected to the outside condenser. The refrigerant flow from the desuperheater to the outside condenser is a mix of liquid and gas. A higer water outlet temperature require reduced waterflow and subsecuently reduced heat reclaim. But still the heat reclaim vill be roughly 50% at 80 degres C water outlet. This assume a properly calculated desuperheater.

It became a lenghty post, but i hope it can give some ideas to future projects

[IceMan_4000]

Howdy RE Crew

Interesting thread fits right into my crazy heat recycling brain. First off why not desuperheat then condense? Desuperheating is all about the work of our compressors which superheats the discharge stream. This can be applied directly to the water and is one of the biggest savings we can offer our customers.

I personally utilize 3 types of heat capture. Sometimes in conjunction and some time as solo methods.

1. Doucette

Before you totally Poo-Poo desuper heat check out this equipment. even with ***** you can pull off a 140F storage http://www.doucetteindustries.com/ac_desuperheater.pdf

I posted the ***** because this is a 404a debate if you are looking for Nh3 they have a unit for that side as well. I call it the free flood water system

From my experience the Doucette is amazing, I have installed in arena application and now that facility has next to free flood water.

Standard arena in Alberta is flooding with 140F/60C - 160F/70C water. Our City Cold Water (CCW) is entering most buildings around 50F/10C. That is a big lift for a commercial tank and a massive thermal shock is suspect for causing these to prematurely failure.

eg. I have one facility that in the morning when he arrives to do his first flood his Doucette has heated 800Gallons to 160°F water waiting for him
The Doucette will give you 50°F rate of rise at stated GPM on the ammonia units so weather you think this is us do not over look this wonder


2. Condensing heat removal
There is a lot more energy here but it is at a very low caloric value other wise the temp you will achieve is only the temp you make liquid at. In my ammonia applications I am getting about 90°F/33°C

eg. I have a twin arena with 6 sheets of curling ice. At this facility I have installed 20,000 Liters of glycol to condense all that waste heat and not use my evap condenser. This glycol is pumped to several different applications. Snow melt, Heat floor, pre-heat of outside air for air handling unit

3. Head, Jacket and oil cooling
This one is probably my favorite this one is extremely low risk installation, no refrigerant tie in. and the amount of heat that can be produced is pretty amazing.

eg. I have another arena this one is an air cooled ammonia application that was having difficulties with some of the heat the Mycom 4B produced. These difficulties were translating into premature bearing failure do to excessive oil heat.
So I installed a advanced metal press tanks in the head cooling loop and proposed to preheat the water before the commercial tank set up. Now even on slow days we are keeping this pre-heat tank at 80-90°F giving my customer 30-40°F of free lift in his water temp. That translates to about 30% on his KW for his commercial tanks.
Recycling heat is all about timing and sizing the recovery effort with the capacity to use the recycled heat

Ice_Man 4000

[Electrocoolman]

Interesting! lets keep the ideas coming

[IceMan_4000]

Hello Electrocoolman I will use your statement for use of a hyjack.

I have project I am working on and would love to sound board of the great minds of the RE boards

My question is one of time I am about to quote out a job that I pitched a contact I have not turned into a customer yet.

This is for a twin arena each with a seperate plant. This one is a twin in every aspect 2 N6A Mycom compressors, a chiller and condenser for each side.

On the water side they have 2 500,000 BTU comercial tanks and two massive storage tanks. around 400 gallons each. They get complants of cold showers all the time.

So here is the plan

Each Mycom compressor can produce 26,600BTUH so a total of 106,400BTUH @ 115F.

I will install one 120 gallon water storage tank with two glycol coils in series with the current head cooling loops. Therfore when the Water is at full temp I can still dump the heat outside. this tank will be added to one of the big 400Gallon tanks the other big tank will be direct conected to the comercial tanks foucused on the flood water production.

this will split the water into two ranges.

Storage #1 520 Gallons @ 50-115F

Storage #2 600 (including comercial tanks) 160F

Next we need the max storage capacity of our head cooling loop to get the time required to take the storage from 50F to 115F.

Q = MC/DELTA\T

_________M______________C____/delta\T
Q= (560galX8.35lbs/gal)(1btu/lbs)(115F-50F)

That equals a maxium input of 303,940BTU

Jaket + Head + oil cooler = Mycom N6A cooling requirments = 26,6000

That means that storage one will reach full temp of 115F in 5.7 hr based on 2 compressors.

There will be some over lap in run times were all 4 compressors are running I fell this will probabbly shave the time to 4 hours. This is with no water demands adding cold to the system.

I do not have a confirmed size on these large tanks yet so I was just using 400 gallons as a nice round number. when I confirm this size will add or remove time to total temp.


Any coments?

Besides howly cow IceMan_4000 spelling is uber bad.

[PoodleHeadMikey]

With 300 GPM going to waste - why not reclaim the waste water heat from the drains as well?

PHM
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