Designing a Miniature Air Conditioning System

That's not a room. That's a refrigerator.

Dear, I need to maintain the temp 22 deg c. Outside temp is 35 to 40 deg c. Regards, Keyur Shah

Ah! Some data! That's a much better start, though there is no humidity information.

So far, a 'fridge could still be used.

Dear sir, Relative humidity is around 50%. Regards, Keyur Shah

And the design humidity within the 'refrigerator'?

design humidity is ~ 55%

The next step is to plot the outside and inside conditions on a psychrometric chart, and read the enthalpy difference between the two. Which is how much?

http://www.expressvending.co.uk/?gclid=CKzPlZKlvpkCFQMEZgod0E9I5g

Hi Keyur, I was thinking of somekind of ground cooling. My idea would be a small circulation pump, maybe 25 W, some ordinary ½" gardenhose, length depending on how much heat=watts you have to dissipate, and then dig the hose 3 feet down in a trench in the ground in your garden. A rule of thumb is 25 w/10 feet. An old radiator in the box and a simple expansiontank(maybe a plastic bottle with airhole/refill hole) and a thermostat in the box that can switch the pump on, when the temp in the box exceeds you desired temp. Fill the system up with water and enjoy you new cool box. Bevare of condensing water inside the box, you have to make drainage under the radiator. And insulate the box. Relatively cheap, and not consuming too much power, and also relatively simple. But more complicated(=mechanical engineering!) than some of the other suggestions mentioned in the thread.

Where I live, there is a constant temperature of 7 deg. C. 3 feet down the ground, but that can be different, where you have your app.

Best of luck

moe

I think it might be hard to find a refrigerator with a thermostat that goes as high as 22C.

Also it occurs to me that the 'outside' temperature and humidity may vary quite a bit...

Gordie.

One could change the thermostat for one that does....

This is often the case for electrical cabinets, and there are companies that sell small "cabinet coolers" and HVAC units.

For electrical cabinet, the steps to sizing are based on 1) the internal heat load from the electrical equipment in watts, 2) load from transmission through the skin of the box, which may or may not be insulated, and 3) heat from any outside air that leaks into the box.

A Yahoo search at http://search.yahoo.com/search?ei=utf-8&fr=slv8-sem&p=cabinet%20coolers%20hvac%20unit&type= gave me 555,000 entries, plus or minus. Google search probably would give similar results. Units are even available in explosion-proof construction, if needed.

Put a wet rag, cloth or fury animal skin over "it" and have some ventilation around. This will give you what you need. If you want it for a presentation you will have to advise us with a bit more information. Will it go on a space ship or will it be used like in the outback of Australia? Please advise and take my suggestion as serious if you have a problem that needs a quick solution. It works and has done so for thousands of years.

Stay cool and all the best, Ky.

Commercially available wine coolers fit your description well. Anyway they are very expensive. Please continue with the discussion. I also will like to build one myself.

We need someone who has AC & Ref experience.

I cannot afford to buy the commercial wine coolers! ( for that matter wine too!!)

What about a AC unit from an automobile?

Carlos

"The choices you make might be mistakes but it's never too late to turn around." Johnny Lang-musician

The original poster wanted one that uses minimum energy consumption.

After obtaining the enthalpy difference from the psychrometric chart, the next step is to ask for the U-value for the insulation around the walls and ceiling, and the U-value for the floor. Multiplying each by their respective area and the temperature difference, then dividing the sum of those into the enthalpy difference gives the energy to be pumped, and dividing that by a typical coefficient of performance gives the power needed to run the equipment.

The next step is shopping...

Dear, I need to cool a cabinet of 3ft x 3ft x 3ft made of 1" Wood. Following are my data: Out side Air Temp: 45 Deg C & 40% RH, Inside Air Temp: 25 Deg C & 57% RH, Calculations: Total Area: 6 x (0.91x0.91) = 4.96 m2 U Value of wood = 0.64, Temperature diff. = 20 Deg C Enthalpy difference = 50 KJ / Kg of Air, i.e Energy to be pumped in = 4.96*0.64*20/50 = 1.26 Watts, Now, lets sat COP = 1.5% i.e total energy = 1.26 / 1.5% = 84 Watts, Is this calculation done by me are correct? if this is correct then is it possible to get such a small compressor? Regards, Keyur Shah

I would like an expert in Refrig/AC field confirm this calculation.

Assuming it is correct what keyur needs is a 1/10hp or 1/8hp compressor which is commercially available, a condenser, capillary tube and other accessories. His cabinet will be the evaporator. With a thermocouple & temp controller he can control the temp inside the cabinet.

Does it really have to stay at exactly 57% RH all the time?

Is that a maximum?...an average? Is that what you get when you cool 45C air at 40% to 25C?

I suspect there will be some condensation on the evaporator coils.

This is just a thought, but a very old fashion ammonia system can run off heat with out an electric motor.

If energy is no problem, why not look at a Pelletier cell system (food coolers used for camping - not that expensive + no moving part, except for ventilator which you may be able to do without...)

The original poster didn't reveal that. Good point, and on-topic.

As aviator mentioned, peltier chips might be a good fit for the volume of area you're thinking of. You can find more info by googling or here's an example of costs and power capabilities:

http://customthermoelectric.com/tecs_imax.html?gclid=COT0trOpvpkCFQ8gDQodMwkc5g

When determining the amount of cooling power you need, you will have to take into consideration other factors such as "What is the R value of the walls of the box?" and "How often will it be opened and exposed to the external temperature/atmosphere you mentioned?"

I stand corrected as Peltier is the correct spelling - Thanks !

As a side note, a really innovative application of that principle (to me) was in machining intricate parts : Lets say you wanted to mill the surface of an existing gear, you would place that gear flat in a tray containing 2mm of water, turn the switch to 'cold' and within 4 seconds, get that gear frosen in thin layer of clear ice at the bottom of the tray. Tried unsuccessfully to move that gear out of the thin layer of clear ice at the bottom of the tray ! Machining could then proceed after which you would switch back to 'heat' and get that gear free of ice, again in a matter of seconds. Most impressive...

The enclosure should consist of 2 rooms.An inner, and an outer.The inner room should be of non-porus material,such as glazed porcelain.The outer covering shall be larger than the inner one,maintainig a 4 inch minimum gap between.The outer wall shall be porus unglazed clay,such as that from which flower pots are constructed.The space between shall be filled with sand.The sand shall be kept moist by mechanical or automatic means.If manual, this will consume no electrical energy, and result in a temperature differential of around 25 degrees, depending on outside humidity,air flow,etc.

This process has been patented by a fellow in Africa, although I do not recall his name.

Try it, you will see.

----------SSB-------------------------------

bull sh*t.... what is the scientific principle here. Remember you are doing posts to a community of engineers and/or like minded educated people.

May be patented by Navineethan Pillai..(fellow in Africa).

I agree with the method. I am positive, this is the cheapest and best practical method. You certainly deserve a GA.

Hi Joshi,

Rather than paying lip service if you are so positive why don't you explain the scientific principles behind it. Here in this forum we accept ideas based on scientific evidence not based on beliefs.

Until someone tell me hard facts I refuse to believe.

Sisira, I believe the cooling process would be evaporation cooling. Guest posted that the amount of cooling would be dependant on air flow and humidity. Depending on how fast the water would evaporate from the porous clay outer sheel/sand matrix would depress the liquid water temperature. Might work, except the original poster did state the ambient humidity was relatively high. He is also dealing with a wood box, which isn't water proof. If the box could be made water proof, he might be able to hold 25C based on only a 100 watt heat load.

Re-read poster outdoor conditions. 45C at 40% RH is at a 31C wet bulb temperature. I doubt evaporaative cooling can get close to 25C at this wet bulb.

GA from me. With evaporative cooling the lowest temperature he will be able to get is the wet bulb temp. That is under perfect conditions!

I was about to comment and you best me to that.

It is very clear about the room, but please confirm the heat generating sources inside the room , is there an operator in the room? The excavator cabin is approx. same size. Confirm your power source ? AC or DC.Approx. 9000 to 10000 BTU/hour should do.

Dear, This room I want to design for some medicines. So, i think there is not much heat load. Can anyone give the idea of heat load of these medicines. Regards, Keyur Shah

keyur_197

Now I see what you mean. If you really need it, go to some luxury car company and tell them you bar fridge is broken and you need a new one. Once it arrives take everything away that you don't like and ad to it as you like. At least you can see how they solved the problem and reverse engineer from there.

Your first question was not so clear about it all and you said something about cheap.

That's what I did 20 years ago any way, to build a cooled medicine cabinet and bring it to the market. I failed but good luck to you, Ky.

This data makes sense!Do you really want to build the system or just modify what is available?According to me, the most appropriate solution would be - buy a bottle cooler or a ice cream refrigerator and use it. However if power saving is the main criteria, then convert the compressor & fan to variable frequency versions such that you consume only power required to maintain temperature difference. I can give more technical data if you are game!

Unless the stored medicines are undergoing chemical reactions, they will not give off heat. It sounds like all you want is to store at an acceptable temperature to prolong the safe storage of medicines. This explains the 22C temperature requirement.

Dear, My power source is 230 v, 50 Hz, 1 phase supply. Regards, Keyur