Freezer floor heating and sub cooling

Hi All,

I am hoping to get some practical advise on the issue of hydronic heating of a freezer floor.

Firstly I am interested in the benefits of subcooling my liquid line on a low temperature rack. I am about to quote a new freezer room which is an addition to existing equipment, hence the oportunity to sub cool. I have never used anything other than elactric floor heating in the past.

The freezer has a floor area of about 200 sq M. The rack capacity is approx 45kW (-25 deg C SST and 48 deg C SDT for those that need to know) at full load. If I allow about 10 kW as the heat I would like to be removed from the liquid line (well in excess of what the freezer floor requires i know). My problem becomes the amount of PEX pipe required to dissipate that heat to the ground/base slab at approx 0.35W/M/K. If I don't get rid of the heat then the glycol or water temperature would rise to a point that the subcooling would become minimal I am guessing.

So along with peoples thoughts my question is:

If I ran a copper loop under the base slab rather than in the slab can anyone see issues with this. I am suggesting copper because of the higher heat transfer (28W/M/K). I am also suggesting under the slab so the copper can move a little if it needs to. Copper water pipes have been run to peoples houses for years. If I run PEX pipe I will need a massive run of pipe as well as a large Td which will almost negate the subcooling potential. I am hoping for 20K or better.

Looking forward to the input.

Regards

Rob

If you run copper pipes below slab, instead in the slab, than your heat transmission to slab will be minimal (radiation and not transmission heat transfer). You want concrete screed all over pipe for best heat transfer. You can calculate with more than 100 W/m2 of heat emission with PEX pipes in slab (10-20cm raster) and 5K water temperature difference. You only need good insulation below the slab! At 200m2 with 100W/m2 that is 20000W of possible floor heating in that freezer room. No need for copper!

What you don't want is more than 100m pipe in loop. Therefore you need multiple loops intelligently wounded! Best is to subcontract that to experienced company.

I made some floor heatings in my life and these numbers are real!

I don't think copper in slab is a great idea for refrigerant or glycol as might be alright for a few years, but has potential for failure.
After failure, you cannot fix.
As you say you cannot give off enough heat for total floor area anyway & cannot incorporate into existing equipment.
No expert on it, but thought poly pipe for glycol or PVC for air.
If glycol you have to run a pump continuously, safety controls for flow, leaks etc.
Why not just drop discharge pressure for efficiency &/or incorporate subcooling in condenser?

Thanks for the replies.

Ranger1 - My condenser TD is already at a designed 5kTd so with a 43 deg ambient I do not have many options to reduce the head pressure without going to alternative condenser styles. An additional heat exchanger to cool just the liquid line would at most give me maybe 3 or 4 degrees better subcooling - plus the extra hassle of additional refrigerant lines to pipe the liquid line back outside.

Nike123 - I can believe that in a typical floor heating senario that 100 W/M2 is possible but that would mean a water temperature much higher than I will hopefully have. You say that 100W is possible with 5K water temperature difference. I assume you mean difference between water in and water out temperatures.

In 1 sq M I could see no more than 5 metres of PEX pipe. The rating I have for the PEX pipe is 0.35W/M/K so if I have a slab temperature of 15 deg C and 5 metres of pipe I will need a TD (water to slab) of approx 57 deg C so the supply water temperature will need to be about 70 deg C.

I want the ground to absorb the heat so the insulation is above the slab not below. I will have a water temperature I hope of 25-30 deg C, lower would be even better. This will give me much lower td to work with, my rough and ready calculations from memory put it at about 1.4kM of PEX pipe - not going to happen!

I have to heat the slab anyway so I was looking at trying to massively improve the COP of the equipment at the same time. I can no doubt improve it but the cost of the PEX pipe required is looking like electric mat will be the go. Running a pump continuously will be more than made up for by the savings in compressor power so that is not an issue. I don't see the heating of the slab as the big issue as long as the heat is put under the freezer. The slab will still absorb heat from its surrounds.

Hi
dear Robearoz
I think Ranger is quite right
if for any reason have a leak under floor you can not fix it ( expansion of pipe during off and on system and changing temp. and also earthquake )
also you have not enough heat from sub cooling of liquid line to heat floor

On a few industrial jobs either glycol is used with heat exchanger on discharge line or in one place air cooler cooling plant room.
On others natural ventilation through PVC pipes under slab.
Never heard of electric heaters on larger jobs.
43deg C ambient would that be for very long, or just a few weeks of the year?

I do understand the risks of a leak in copper, being more "brittle' than PEX pipe but it is also recommended to run a second circuit for Pex pipe as back up, why not the same for copper?

I would argue that there is more than enough heat in a liquid line to heat the floor well in excess what is needed, however, it has just now become obvious to me that the temperatue is the issue not the quantity of heat.

The poor transfer of heat from PEX pipe causes a need for a very large TD to be able to get enough heat to transfer through the PEX pipe. That is obviously why it is the discharge pipe that is used to heat the glycol.

The benefits of using the liquid line for heating far outweigh de-superheating the discharge line but because the temperature is so low PEX pipe is not the answer.

The freezer does not have any clearance on 3 sides so there is no opportunity for natural ventilation, even forced ventilation would be difficult but i guess possible.

The high ambient is only couple of weeks max so the condenser is to many peoples standards oversized but that is how I like it. I never like to see a freezer hit 50 deg SDT.

Thanks for the input

I don't think that it is good idea. This approach has many minuses. I would install heat exchanger. Liquid refrigerant at condensing pressure on one side and boiling refrigerant at suction pressure on another side. This is similar economizer in industrial refrigeration. An economizer is a type of sub-cooler that uses part of the total refrigerant flow from the condenser to cool the rest of the refrigerant flow. Size of heat exchanger will determine subcooling but it will be significant.

Quote:

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Originally Posted by Robearoz

Nike123 - I can believe that in a typical floor heating senario that 100 W/M2 is possible but that would mean a water temperature much higher than I will hopefully have. You say that 100W is possible with 5K water temperature difference. I assume you mean difference between water in and water out temperatures.

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Standard household floor heating water entering temperature is 30-40°C. Yes, temperature difference refereed here is difference between water in in pipe loop and water out from pipe loop.

With (for example) 50mm polystyrene insulation below cold-room floor your transmission losses to slab will be around 10W per square meter!

In your case these temperatures could be lower than that since you only need to maintain slab above 0°C.
How many WATS per square meter you need depend on insulation above slab and heat conductivity of slab and ground below slab!
Ground will be always positive temperature if it is not Siberian permafrost:)

That is regarding slab heating!
I did not considered subcooling issue in this answer.

Hi Segei, thanks for the input. I understand an economizer, but I am using single stage refrigeration so no intermediate stage. In my case I am looking for "auxillary" or "relatively free" subcooling from the ground under the freezer. Without an intermediate stage then the subcooling is of no nett benefit in power consumption.

nik123, Hi. Your 30-40 degree water in temp suprises me but I have never worked on this type of system so I will accept that as correct. I have only seen ratings for a couple of PEX pipe brands and they were 0.35 and 0.43W/M/K as their thermal conductivity rating.

i will have 150 mm of insulation above the slab and can easily calculate my losses there but I have no references for conductivity from the slab to the earth.

I think I will proceed with this project using as much pipe as will fit and a liquid line heat exchanger anyway. My thoughts now are that, I am really after the "free" subcooling. if it ultimately is unable to maintain temperatures above zero under the insulation then it can easily be modified with an alternative heat source such as a de-superheater for the discharge off one or all the compressors involved (rack running more than this room). This will probable take a couple of years to show up as a problem anyway, but with temperature alarms set then should be caught well before a problem.

I was just totally shocked at the extremely poor heat transfer of these pipes, i guess the slow rate of heat transfer keeps the slab temperature fairly even across its entire mass. If it transfers too fast then the slab would heat up unevenly.

I did some rough and ready calculations in the beginning without looking at all the variables and was thinking about the massive jump in COP if i could drop the liquid line to about 20 degrees from 48 deg SDT. This would be roughly 10kW of sub cooling but i now realise that this will not happen at the low td temperatures I will have. Then again as/if the slab temperature drops the td will increase, so the balance will be interesting.

In any case I will learn something new!

So you want to save energy. In your previous posts you didn't mentioned about energy savings.
To achieve energy savings we should answer 2 questions. How much energy will be saved? How much will it cost? Additional subcooling will give you limited energy savings but it will require significant investments and additional risk will be taken. Why don't reduce minimum allowable condensing pressure? Thi is not easy but can be done and a lot of energy can be saved.

Quote:

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Originally Posted by RANGER1

I don't think copper in slab is a great idea for refrigerant or glycol as might be alright for a few years, but has potential for failure.
After failure, you cannot fix.
As you say you cannot give off enough heat for total floor area anyway & cannot incorporate into existing equipment.
No expert on it, but thought poly pipe for glycol or PVC for air.
If glycol you have to run a pump continuously, safety controls for flow, leaks etc.
Why not just drop discharge pressure for efficiency &/or incorporate subcooling in condenser?

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Nothing wrong with evaporative condenser, just got to pay for ongoing water treatment.
More capital cost initially, VSD on fan for constant head pressure.
With only 45kw freezer load, may not be viable anyway.
Could use PHE condenser with cooling tower, but costs more to run with extra water pump.
Subcooling is important for TX valve operation, so if "lift" in liquid line & hot roof spaces will effect it.

Segei may be able to confirm if running bare liquid line in freezer would help that, or something else!

If you had a higher temp unit running load out dock or chillers, it could be used for subcooling circuit, but sometimes to much subcooling is not desirable if evaporator circuit not designed for it.

I prefer a heater mat and solar panels and an inverter would make the system very economical

Hi Sergi and Ranger1, I appreciate your interest and input but I have to disagree strongly with some of both your comments.

I would argue that sub cooling the liquid line will generate much bigger energy savings than trying to reduce my discharge pressure more than I already have. Sure water cooling or evaporative condensers will have a big impact but at what on going cost. Large initial capital expense plus higher maintenance.

My high Ambient of 43C is relatively minor consideration as the average is probably around 32 deg with an average relative humidity of 30% so yes these condensers will reduce power requirements and compressor sizes but I then still have to heat the floor on top of these additional costs.

Running the liquid line inside the freezer with bare pipe will subcool the liquid but for every kW gain in the liquid line I add that saving straight back into the suction so I have no nett gain to power input.

This whole excercise began because I need to heat the floor so I am looking at killing two birds with one stone, raise the COP of the freezer compressors and heat the floor.

I do have access to medium temperature equipment so it would be possible to subcool the freezer's liquid line from the cool room equipment, however it would require larger compressors to do so. Yes there will be a nett gain in system performance and reduced power consumption but then I still have to heat the floor. Now if I then use the coolroom compressor liquid line to heat the floor! Hmmm

I will put some figures to my arguments.

-25CSST, 6K usable superheat and 20K total suction superheat @ 48SDT and 2 C subcooling from condenser

10.85kW refrigeration power, 8.8kW input power, COP 1.23, 16.6 A

Same compressor with a liquid line temperature of 20C after giving up its heat to the freezer floor.

15.95kW refrigeration power, 8.8kW input power, COP 1.81, 16.6
Note: the input power remains constant because the compression ratio has not changed but the compressor is now roughly 50% bigger.

Same compressor with water cooled condenser, don't have figures for that on hand so will guess - not really fair.
will use 35 deg as the SDT and 5K subcooling which I am guessing is pretty optimitic and favouring the watercooled condenser on a 43 degree day.

15.55kW refrigeration power, 8.13 input kW, COP 1.91, 15.71 A

based on these figures it is a slight win to the water cooled condenser but at what cost? Larger initial capital cost, larger ongoing maintenence, larger pump so higher electrical cost. probably bigger fans than an aircooled condenser also.

In my mind the sub cooling of the liquid line wins hands down.

Am I wrong?

Deleted because posted twice

Oooops looks like I posted twice!

Hi Cadwaladr, to be honest solar electric had never even crossed my mind, certainly worth a look.

just found this on the Forums

http://www.refrigeration-engineer.co...r-construction

Interesting reading.

Dear Robearoz,


as visible from your profile you are here on RE forums for almost 18 years ....

Quote:

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Originally Posted by Robearoz

just found this on the Forums

http://www.refrigeration-engineer.co...r-construction

Interesting reading.

---

... so many times before I used to suggest to posters to read old posts regarding matter they would like to find out more ...

I am sorry I miss to give you the same suggestion ... but fortunately you discovered that by yourself ...

hope you'll find answers ...


Best regards, Josip

When you subcool liquid refrigerant, you reduce amount of flash gas and this will improve COP. When you reduce condensing pressure, you reduce amount of flash gas and energy use by compressor. This is double improve of efficiency. Definitely, the question is how much subcooling you can achieve and how much you can reduce condensing pressure. It will be good if you tell us what refrigerant do you use and what is minimum condensing pressure you have and why it is minimum. Regarding 48C SDT and 20C subcooling. Do you have 48 C SDT all year around? Probably not. How long will you run plant at 48C SDT? Assume that at 48C SDT you will achieve 20C subcooling. With reduction of SDT subcooling will be reduced as well. Less heat will be release under the floor. So minimum condensing pressure is the most important because majority of time a plant will run at this pressure.
A refrigeration plant never run at design condition it merely cross them at times.

No denying about subcooling increasing efficiency.
I realize running liquid line through freezer does not give you more efficiency, but may help Tx valve with flash gas.
With your proposed system how much extra refrigerant is used.
how long before subcooling starts to take effect.
If you get a leak under floor, then what.
If you want to pump it down is receiver bigger to handle it.

Guys!
Are we talking TXV Evaps, or Flooded Coolers?
Also if Hot gas defrosts apply then lowering the Condensing HP has a greater effect upon the system efficiency.
Does it not?
Just thoughts
Grizzly

Hi Josip,

Wow 18 yrs...

I have often just browsed the forum but I, for some reason have difficulty narrowing any search I do. i have recently found google as being better at narrowing searches on the forums.

Ranger1,

I think you miss understand, I plan to use a PHE and glycol so little extra refrigerant. I imagine the subcooling would be relatively quick. I do not plan to have a large storage vessel of glycol. Just a header tank above everything and a circulation pump through the PHE, into the floor manifold through the floor and back to the pump. I don't see a large reservoir as being necessary. in any case, once the piping is in the floor, plenty of time to get the heating of the glycol right. It will take a few years before the floor is in danger, not that I would let it go that long to sort it out.

Hi Sergei

R404a will be the refrigerant, the plant does not exist at present. My head pressure will be as low as the valves will handle, have not looked too close at that yet but my compressor limit is 5 deg C SDT according to the data. Obviously I would expect mechanical valves will require substantually more than this and my ambient temperatures would rarely (if ever) allow this low a temperature anyway. It is me pushing the energy efficientcy side of things, I do not think EEV's will be in the budget as I am trying to get other items included (for other equipment) before going there, but I will try. I have had wonderful results with them and low condensing pressures. My customer wants "cheap and simple". Total cost of ownership is not always easy to sell but on machinery of this size savings can be substabtial.

I also understand that my liquid temperature will probably not change too much between summer and winter
by having a PHE cooled by the ground under the freezer. This means that the subcooling will drop away substantually during winter months when the head pressure drops considerably.

It may even mean that the liquid line is providing little to no heat to the floor through winter, but should more than make up for it in Summer. My heat load through the floor is only about 1500W. i have not tried this before and am interested to see the results, If it is ultimately insufficient heat then it is no big deal to swap over to a de-superheater to up the glycol temperature or even use an air to liquid fan forced heat exchanger. I will not be running high discharge pressures just to keep the liqiud temperature up to heat the floor, The saving grace for me in an "experiment" like this is that the freezer will take a long time to hit a danger point and I will gain some practical knowledge, rather than theoretical. I built a freezer half this size that has not energized the heaters after 10 years. I know a bigger freezer will have a bigger need for heating but it is truely amazing how long it takes to get to that point. I guess soil conditions play a big part here.

By constantly providing heat from the liquid line rather than waiting till heat is needed to prevent freezing than sub soil temperatures may not drop much at all.

Hi Grizzly,

TXV fan coils with electric defrost, I have never actually looked at designing hot gas defrost, now you got me started, but I would imagine I am already pushing the budget so the extra costs there may be too much. Too many evaporators for it to be easy to install.

But I will now look at it for sure, worked on a few, even cool gas defrost but have not seen one for many years except on small selfcontained equipment.

You can install liquid refrigerant pump to increase liquid pressure. This will give you a lot of energy savings. Probably it will be cheaper than EEVs.