Converting a Freezer to a Curing Box

Hello, I've been looking over "Refrigeration Engineer" for a couple of weeks and it's a great site, so I decided to join.

This is going to raise a few eyebrows no doubt :eek: , and I wanted to see what the experienced view is on this. Here goes:

I am interested in trying to mod a freezer (or fridge) to turn it into a meat curer / salami incubator / drying box.

The major conversion would be the fridge circuit to operate like a heat pump. (Yeah, I know.)

I've been thinking about some operating figures:

The box will live in the garage so:

Outdoor Temps:
Winter: -3 deg C
Summer: 30 deg C

Box Interior Temps:
Salami Incubation: 30 deg C
Meat Curing: 2 deg C

So based on this:
Max cooling delta T = 28 deg C
Max heating delta T = 33 deg C

I'm reckoning that an old fridge might have a wall U-value of about 0.75 W/m^2 . deg C

And an internal surface area of about 4.5 square metres.

So in cooling (assuming dry air - poor assumption) duty required = 94.5W

In heating duty required = 111.4W

So I think the cooling duty is low because I haven't taken into account any latent heat yet.

That's as far as I am with the calcs.

Controls and reversing the cycle direction are going to be the two big conversions right?

Can a control module be bought off the shelf to switch a small valve box - also off the shelf?

Then I'm thinking " why heat pump? why not just turn the fridge off and turn on a 100W light bulb in the box when the heating is required?"

But I think it would be good to use the heat pump idea for two reasons:
Light and meat don't go well together (bulb could be shielded)
I want to avoid runnning an electric cable into the box.

Any guidance anyone can offer with this would be great. Is this utterly unfeasible? Or is it workable with a bit of effort?

Thanks alot,

Robbo.

well i dont know much about heat pumps etc but another alternative from a bulb would be a ceramic heater that you would use in a reptile vivarium wich would get rid of the light issue (same fitting as a bulb iirc)

Hi Robbo and a big welcome to the forum.:)

With your bio of being a whelk peeler and your desire to modify a fridge as you do certainly brightens up the evening ;)

I would be dubious of converting a fridge into a heat pump using the existing parts because they are designed for one purpose only and that is cooling down the box. They tend to run on the limits so getting them to do something else could be playing with fire, or dead compressors.

The evaporators are not designed for emitting heat and the condensers aren't that good at extracting heat from the surrounding air.

I tend to favour Spenny's suggestion of a ceramic heater if you are worried about light.

Quote:

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Originally Posted by robbo

Any guidance anyone can offer with this would be great. Is this utterly unfeasible? Or is it workable with a bit of effort?

Thanks alot,

Robbo.

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Hi ya mate.

Like all things in life you could do what you required if you lob enough money at it.

Your control issue could easily be sorted with the use of one or two cheap electronic fridge controlers. One set to work the cooling and one set to do heating.

Your sizeing would easily be achieved by a fridge system. Your figures of 1kw and 1.1kw if correct are quite small in fridge terms. A fridge on heat pump would atchieve the extra duty on top of the cooling side because the compressor energy would be put into heat form.

Now the $65000 question can it be done with normal fridge equipment. I would be tempted to say yes but a lot of modification would be required and therfore make it unfeasable.

It might be worth considering converting a 1.5kw air conditioner to work at the temps you require. The controls would need over-riding but maybe if done right it would work.

Cheers taz.

Thanks Brian & Spenny. I like the ceramic heater idea, this may go into the light fitting in the freezer itself. So i'd disable the doorswitch and rewire the light to a thermostat.

So if I go for the cooling only option, what will I need to do to get the freezer to operate in the range i'm after?

Robbo.

If you get hold of an electronic controller you could set it up to switch the heat or the cool.

Just had a thought, you say that you want to 'dry' also. Are you really meaning dry as in less moisture or something else?

If you heat up the box the moisture will stay in it because the box is sealed.

well if you go to a local good pet shop they should be able to supplie you with a ceramic heater and a seperate thermastat for that job if ya need more heat tho have a look at RS componants or someone similar as these types of heaters are used all over (obviously) and not knoing too much about fridge/freezer construction i cant imagine it being too hard to mount a heater and thrmastat probe into the fridge/freezer body

P.S sorry bout the spelling had a few :eek:

Quote:

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Originally Posted by spennyy2k

P.S sorry bout the spelling had a few :eek:

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Wrong forum then you need to be on the real ale discussion.;)

Cheers taz

Quote:

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Originally Posted by robbo

I am interested in trying to mod a freezer (or fridge) to turn it into a meat curer / salami incubator / drying box.

The box will live in the garage so:

Outdoor Temps:
Winter: -3 deg C @ ___ RH?
Summer: 30 deg C @ ___ RH?

Box Interior Temps:
Salami Incubation: 30 deg C @ ___ RH?
Meat Curing: 2 deg C @ ___ RH?

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In curing/incubating/drying, RH is important.

Is the freezer evaporator fan/coil or static coil?

Thanks Brian & Gary,

RH is important - salami incubation RH 86-88%
curing - temperature is more important

It would be ideal to be able to set the temperature from one control. I'll take a look on Maplin.

When I say "dry" I mean dry food out. At present i'm in the foothills of difficulty I think. When it gets to the drying part this sort of adds a new dimension.

I'm thinking about fitting 4" spigots to opposite walls of the freezer. I could cap these off normally with insulated lids. But at other times I could fit a 4" fan and speed controller on one, and a damper on the other. This would let me get an air flow through the box to help drying. I think i'd be after about 1-2 litre of air a second.

So if i wanted to keep the box temp at about 24-26 deg C, with an air flow of 1-2 l/s that shouldnt put too much of a stress on either a heater in winter, or the fridge plant in summer should it?

The heater duty I can work out: Q.=m. x Cp x dT for the air flow plus the heat loss from the box. So in the region of 72+111=183W

The cooling load takes me to the edge of my ken, bit rusty here. Is the freezer plant likely to be capable of holding a temp of 22-26 deg C inside with outdoor temp of 30 deg C and 2 l/s air flow?

I think of the fridge plant operating between a couple of set points. The temperature in the box will be, say, 25 deg C. The compressor will run and drag the temp down to 2 deg C, and then shut off. The temp will slowly rise to, say, 3 deg C. The compressor will run again and drag temp down to 2 deg C.

Is this how the control regime works? Or is there more to it than this?

Thanks for all your ideas and comment so far guys, this is a great help.

For controller have a look at CAREL.....some have a digital (switch) input that will allow you to select between two separate setpoints.
One could be used for heating and the other for cooling.


Another thought....if you used a commercial freezer with air blown evaporater, you would have electric defrost element which could be used for heating cabinet

Quote:

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Originally Posted by robbo

RH is important - salami incubation RH 86-88%

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If you take air that is -3C @ 100% RH and heat it up to 30C the humidity will be right around 9%. Air that is -3C @ 50% RH will warm up to about 5% RH @ 30C.

So, if you are going to keep it out in the garage you better make sure it is absolutely airtight... and don't open it.

Quote:

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Originally Posted by robbo

I think of the fridge plant operating between a couple of set points. The temperature in the box will be, say, 25 deg C @ ___ RH?. The compressor will run and drag the temp down to 2 deg C @ ___ RH?, and then shut off. The temp will slowly rise to, say, 3 deg C @ ___ RH?. The compressor will run again and drag temp down to 2 deg C @ ___ RH?.

Is this how the control regime works? Or is there more to it than this?

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The "more to it than this" is... Humidity. You are dealing with humidity sensitive products. If you don't plan accordingly, it will bite you in the butt. You will have the driest meat in town.

Here's how I would do this, on the cheap, with minimum skills required:

I would start with an old freezer. I would get a very small window air conditioner, and move the thermostat sensing bulb to the other side of the coil, so that it senses evaporator leaving air temperature. Mount the A/C through the side of the freezer.

I would set the thermostat to shut off just above freezing (.5C) to keep the humidity up.

I would mount the heater in the A/C supply air and control it off another thermostat (in series with an on/off switch) on the other end of the box. I would set this thermostat for 30C.

I would have 4" PVC pipes at opposite ends of the box, with a small fan in one of them to feed air into the box as close as possible to the A/C return air. The other pipe would have a flap damper to relieve pressure.

In meat curing mode, set the A/C to HI-COOL, the heater on/off switch to OFF and cap the PVC pipes.

For salami incubation mode, set the A/C to LO-FAN, the on/off switch to ON and cap the PVC.

For drying, set the A/C to LO-COOL, the heater switch to ON, uncap the PVC, install the relief damper and start the small ventilation fan.

And I would keep it in the house (although I might vent the relief damper outdoors, as well as the condensate drain).

How critical is the temperature control and how much do you want to spend? My feeling is that the compressor won't be suited to heat pump work, but regarding the simpler solutions illuded to by others, I made an incubator for chicken eggs in a similar fashion. Thermostats and heater elements can be bought as kits. I bought mine for £25 about 10 years ago. I just used trays of water to get the RH up.

I have designed and built prototype refrigeration systems which run reverse cycle to rapid thaw product -dual purpose fridge if you like...but tight temperature control was very difficult. I found that an electrical heater on a proportional controller gave the best results by far, and cost less.

Ed

Forgot to mention, if you use a proportional controller for the heater, you can still use the fridge for cooling if it goes over temperature (provided you have a suitable cooling stat). I have tried deadband controllers, but it is very hard to size your cooling and heaters and position your probe suitably to get stable control (because of the lags in the heat/cooling being transferred to the air) unless you have a wide deadband on the temperature control.

Good luck,

Ed

I'm trying to keep the costs quite low, but i'd like to get to a neat solution. I think it's looking like a heater wired inside the box, and wiring in a new controller to pick up heating and cooling. Using 4" PVC tubes with a fan in one and a flap damper in the other.

If I try wiring in a new controller I need to pick up the existing relay contacts that control the compressor. These can then be switched by the new controller right?

If it's an old freezer will I need to worry about defrost cycles or anything like that?


Thanks Gary, Electrocoolman and en9ech for all your genius. I need to go away and do a bit more research and thinking now.

I shall take a look at some vivarium heaters, carel controllers and window ac units.

I see what you mean about "dryest meat in town" Gary. I shall see about keeping this indoors, and I shall put a high temp / low temp and RH thermometer inside.

Thanks for your interest in my "meaty" project.

Robbo.

Why don't you just use a freezer with aftermarket temp controller?,... And as for the heating and humidity, why not just use the freezers defrost heater? It has a built in protection and fans could be run during heater on time to circulate warm air.... Just add an ultrasonic humidifier and you're away..... Old post but I could not resist.... :)