underfloor heating of freezer room

[knighty]

hi

I've just had a quote back for my new planned freezer - approx 8m long, 5m wide, 4m high to run at -20'C

the compnay quoted for a heated floor, I'd never heard of this before.. a quick google and it looks like I'll need one (north east uk)

the floor added £5,500 to the quote....

I still need to have some concreat poured where the freezer will go to level it out a bit better....

could I have pipes as used for underfloor heating in this layer and then pump water (with anti-freeze) through them to stop the ground freezing ?

I could take the chill off the water either useing air with a simple heat exchanger (maybe an old evaporator?) or could use a flat plate heat exchanger on the liquid line of the compressor ?


does this sound ok to everyone ?
(the guy who gave me the quote wasn't sure)

the advantage is, I could do this myself and save a lot of money, when money is tight... the £5,500 extra I hadn't planned for means I won't be able to go ahead with the freezer built yet

thanks

Alan.


(p.s I'm dislexic and my spell cheker is broken, I've read over this 3 times but have probably missed some errors.... don't take offence, I'm doing doing it because I'm lazy / don't respect you!)

[mad fridgie]

Yes, but it is more common to use a de-superheater as the heat source

[knighty]

cool, thanks :-)


I assume I just need to keep it above freezing ? not actually warm ?


colder = better right ? (for the freezer anyway)

[savoc]

i may have miss understood your question. In austraila some freezers use coolroom panel underneath and the concreate is poured on top, the freezer wall also start under the finshed floor level to give a seal. some use natural air flow from out side, pvc pipe use is placed under the slab. some even have fans pulling air from the outside blow air under neath. but it all depends on your out side temp. with out underfloor heating your slab will crack. i have never seen the method your talking about may be its the differant climates,

[brian_chapin]

All you are trying to accomplish with an underfloor heater in this case is to keep the ground underneath your freezer from "heaving". When water freezes it expands, this can cause the ground to "heave" and break your floor. As it heaves more liquid gets under it, freezes, and heaves again. I've seen buildings nearly completely destroyed by this but those are rare occurrences. You can use any heat source you have handy since you are only looking for about 35f or 2c. I've seen people use outside air blown through PVC pipes. I've seen glycol and water loops. Whatever keeps the ground underneath the floor from freezing.

[Peter_1]

A PHE on the liquid and subcool this way the liquid with the cold glycol (till +/- 10°C) from the underfloor will improve your overall COP a lot. This give you 2 benefits at once: a safety and free heating and less running costs for your compressor.
I've seen PVC tubes with dramatic results ...See other thread here somewhere.
The cheapest way: just under the isolation one mat with electrical resistances they use everywhere for underfloor heating.
And install 2 probes in a close tube so that you can follow/control underfloor temperature. You can go as low as 5°C but your freezer dimensions doesn't actually need a underfloor heater but it's of course safer to prevent problems.
Circulating just water/glycol through PE (teh cheap black ones) with a common circulation pump (brass or stainless steel housing) is also an option.
5.500£ is in my opinion too much.

[mad fridgie]

Peter, I am a massive fan of liquid sub cooling, and normally I would agree with you, but practically in this case (Size, Use, location and ambient temps) you are unlikely to have enough run time in winter for the liquid sub cooling to give enough energy to the underfloor to ENSURE that frost heave will not occur. Maybe the if an additional controlled inline heater was installed, then you could have the best of both worlds.

[Peter_1]

MF, I have several freezers of that size installed and none of them has 'SC heating' and some have electrical heating. The heating is postponed (thermostat set at 4°C) till the night (low electricity rates here from 10 PM till 6AM)

In fact, we only have 2 running with a SC PHE and we never took really hard figures to prove the savings.

I have the following numbers from PC Koelet - a world renown Belgium manager of what's now Johnson Controls - and in his book he wrote after decades of experience in industrial cooling worldwide that you have enough with +/- 7-10 W/m² to protect a freezer from frost heave. To have a certain safety margin, he advises +/- 15 W, max 20 W/m²

In the case here, 20 W/m² x 40 m² gives me 800 W heating capacity needed, safely.

Now some calculations: we will have here - a rough guess - an installed cooling capacity of +/- 10 to 12 kW, so a mass flow of +/- 0.09 kg/s (-30°C/35°C/5SH/R404a) where we can subcool +/- 20K (from 35 tot 15°C) and a Dh of +/- 33 kJ/kg (+/- 254 kJ/kg - 221 kJ/kg) gives me +/- 2.8 kW available heating capacity.
COP of 1.78 with 2 K SC and COP of 2.25 with 20 K SC or a power reduction of +/- 25%

My experience is that freezers with a diagonal smaller then 10 m to 12 m doesn't need any underfloor heating. The energy and the renewal of the energy in the ground is enough to protect if from frost heave.

I have a client who runs a fan on a PVC tube net from +/- May till September and then shuts down the fan the rest of the year. The freezer is at least 30 years old.

On all those with electrical heating, there's a running hourmeter installed on the heaters and as far as I can remember all have 0 hours on the meter.
+/- 5 of the thermostats are connected to the supervising thermostats (Carel, Dixell, Eliwell...) and we notice a stabilizing temperature of 5°C (winter) to 7 °C (summer) without heaters.

But as said, all this is my experience and I'm never to old to learn something.

[mad fridgie]

MF, I have several freezers of that size installed and none of them has 'SC heating' and some have electrical heating. The heating is postponed (thermostat set at 4°C) till the night (low electricity rates here from 10 PM till 6AM)

In fact, we only have 2 running with a SC PHE and we never took really hard figures to prove the savings.

I have the following numbers from PC Koelet - a world renown Belgium manager of what's now Johnson Controls - and in his book he wrote after decades of experience in industrial cooling worldwide that you have enough with +/- 7-10 W/m² to protect a freezer from frost heave. To have a certain safety margin, he advises +/- 15 W, max 20 W/m²

In the case here, 20 W/m² x 40 m² gives me 800 W heating capacity needed, safely.

Now some calculations: we will have here - a rough guess - an installed cooling capacity of +/- 10 to 12 kW, so a mass flow of +/- 0.09 kg/s (-30°C/35°C/5SH/R404a) where we can subcool +/- 20K (from 35 tot 15°C) and a Dh of +/- 33 kJ/kg (+/- 254 kJ/kg - 221 kJ/kg) gives me +/- 2.8 kW available heating capacity.
COP of 1.78 with 2 K SC and COP of 2.25 with 20 K SC or a power reduction of +/- 25%

My experience is that freezers with a diagonal smaller then 10 m to 12 m doesn't need any underfloor heating. The energy and the renewal of the energy in the ground is enough to protect if from frost heave.

I have a client who runs a fan on a PVC tube net from +/- May till September and then shuts down the fan the rest of the year. The freezer is at least 30 years old.

On all those with electrical heating, there's a running hourmeter installed on the heaters and as far as I can remember all have 0 hours on the meter.
+/- 5 of the thermostats are connected to the supervising thermostats (Carel, Dixell, Eliwell...) and we notice a stabilizing temperature of 5°C (winter) to 7 °C (summer) without heaters.

But as said, all this is my experience and I'm never to old to learn something.

Yes your figures are right, this presumes that the refrigeration unit is running.
The installed capacity/duty will be based upon peak load conditions. lets say a nominal 18hrs run time.
So using your figures 800 watt floor duty or we could say 33.3kw/hrs per 24 hour day.
So based upon your potential heating capacity of 2.8Kw, we would need a minimium run time 11.9Hrs.
If the load was based upon 18hrs (peak load in summer), the run time would reduce dramatically in winter, less heat ingress through the structure, reduced loading to air change factor and greater improvement in net refrigeration capacity, due to lower ambient conditions.
We also need to take into consideration that this northern england where air temp drop to low levels and natural ground freezing occurs, reducing the heat ingress into the floor slab by the ground itself.
So even though i do agree with you in just about all, there is still a potential risk??? So you either do nothing, or you install a heat device which will guarantee to ensure that frost heave will not occur, do not see the point in doing half measure, no measure or full measure

[Peter_1]

We're busy with one no of 11 x 4.2 m floor space and we will install one electrical heater mat of 80 cm wide in the center just for safety. We will install also 2 probes for control and safety.

[mad fridgie]

I can say for small rooms, we used to buy pre-manufactured floors (similar construction to the walls, but with tread plate on one side) and we sat the floor on array of bearers (400mm centres) no need for heating at all.

[knighty]

thanks for all the extra info guys

the freezer is to be used for storage and as a blast freezer - we already have 2 x 30hp compressors etc.. ready to use... they're much bigger than needed but they came up cheap (reconditioned units)

the rough plan is to only run them overnight... but we'll run them and see if that's enough...

might install another systemr (7hp ish) from our existing freezer to run during the day and keep the room cold (door opening etc)


I'll have the slab laid with the pipes in it... and price up the heat exchanges to heat the water... maybe I could install one for each compressor ?

I'll install a thermostat/alarm in the water loop to warn me if the water tempriture drops too low - I can always heat electrically if I have to :-)

[Magoo]

Hi Knightly.
recommend installing flexible plastic type piping, rigid piping will fracture with movement in the concrete slab. Add accessible headers as well. If one circuit fails it can be isolated/ plugged.

[Peter_1]

Theromstat in the waterloop is good but one in the undergound is better. If your pump fails, your thermostat will still sense warm temperatures - depending on its location - but underground will not be warmed any longer. From the scrap of the PE-tubes, bend over one end 180° and tie wrap it so that you get a closed end. You then can insert a retractable Eliwell-like probe in it. We always insert two probes in two separate PE-pipes and never had a failed probe. The center of the freezer will be most prone to freeze up.

60 HP total in it? I should do it the same way as you proposed, let it run during the day on a smaller unit. Is it to keep cold or to keep -20°C during the day? I guess the freezer will be filled up the whole day production day and once the production day is ended in the evening, the freezer will start to freeze down the products. If so, this is exactly the same way we work in a chicken slaughtery.

Additional benefit with a smaller unit which keeps the good during the day at 0°C is that a lot of humid will already be removed till the freezer starts.

Some pitfalls and perhaps useful information: pay special attention to a proper air circulation (right place and enough ventilation), defrost cycles which perhaps can be postponed till the freezer gets 0°C in the morning and then can be done via forced ventilation.
Give instructions to the user how they must load the freezer according to your instructions (no big gaps between charriots or goods) and not how it fits them the best.
If used as a cooler during the day and a freezer during night, floor might get wet and freeze at night and will be slippery in the morning.

[knighty]

hi peter and thanks for the reply (and everyone else too... magoon I'll make sure the pipe is flexible!)

I'll install a couple of pipes with probes in them as a backup

yes 60hp in total... it should be a lot more than needed we're running about 12hp right now in a 8foot x 8foot x 26foot room (back of a lorry)... but it's not always enough to freeze everything properly if we've had a busy day

the smaller unit is just to stop the meat in there defrosting when we're opening the door a lot... I could always run one of the larger systems through a thermostat to come on if the smaller unit can't keep up ?


I was thinking of running a false ceiling/wall so the cold air will blow out of the evaporates across the ceiling then down inside the wall and out through the racks containing the meat.... my fridge engineer thinks this will be overkill but I'm keen to future proof... we could always install the false wall now and then add the ceiling later on
(easy to add ceiling but would be hard to add a wall later on when the freezer is full!)


thanks again to everyone for all there help/input/ideas.... I know a good engineer and trust him a lot.... but it's nice to have extra info / a second opinion when I'm spending such a large chunk of money (to me anyway)
(and maybe I have a bit of OCD and like to know what's going on and why!)

[chillerman2006]

Hi Knighty

If you have your floor laid by a proffessional screeder you will not need under floor heating

They will the concrete with a good waterproofer in the mix

Once fully set this will not absorb any water and thus will not crack

The only downside is if you get water spilt on the floor this will freeze and could be dangerous

[Peter_1]

Chillerman, the problem is not the upper floor -this one will always freeze - but the floor under the isolation which may never be lower then freezing temperatures. There are some good practices to start up the freezer floor to freezing conditions but never do it very quickly.