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stuartsjg
27-08-2007, 08:25 PM
Hi,

I bought a used portable air conditioner from eBay which was rated as 7500BTU cooling.

I have peasure the power input as 600W (comp and fans)

7500BTU means 2.2kW of heat being moved.

This should mean that 2.8kW of hot air gets blown out the back.

I have ran the system and found that given a 20C condensor air inlet, exit temperature is 35C giving a 15C temp rise.

I set up an experiment to check if 2.8kW of heat is coming out the back.

I ducted the air flow leaving the AC unit through some heaters and applied power unitll i got the same temperature rise (15C) then that will tell me what the power is.

I have done this and found that only 1.5kW was needed to warm the air leaving the condensor by the same ammount the condensor does.

Given that the input power was 600W, 600W of the 1.5kW was input, therefor i was only producing 700W of cold air as opposed to 2.2kW of cold air.

Does anybody know why the system would be operating at such a poor level or what the test criteria for domestic portable AC systems are?

Thanks,
Stuart

Lowrider
27-08-2007, 09:43 PM
To much free time on your hand? I could use some help painting my house!


Part of it can be explained by using the same conditions as specified by the manufacturer. I then think of humidity and entering air! The manufacturer will specify the capacity of the unit at specific indoor and outdoor humidity and temperature. Any deviation from it and the unit will do less or more!

The other part it some, as I call it, wishfull thinking from the manufacturer or perhaps overstating, which ever you prefer!

And last but not least, when you add ducts that are normally not there, you'll change the amount of air coming of the unit and thus changing it completly!

montreal
27-08-2007, 09:51 PM
Stuart,

It is very difficult to measure exactly how many BTU's the device is actually pumping from one side to the other.

I too tried to create a formula which would equate delta T across a condensor with BTUs transferred.

Delta T is not the only measure of heat transfer. You have to measure delta RH as well, and that is very difficult without calibrated proper instruments.

Another thing is that the compressors on these units have lower power factors so that even if the amps are high, the watts drawn may be half. Less watts means less additional BTUs picked up from either compressor and ventilator motor.

And when you have a ventilator motor drawing power as well, it only contributes to your net gain if this motor is located in the air stream that is bringing recovered heat into your home.

These AC units work much better when extracting heat from a 80 degree F air stream and sending it to a 120 degree air stream. This is the case for two reasons.

First, for the pump to transfer heat, you have to feed it heat. Feeding cool exterior air at 50 degrees or below into the evaporator is not providing a lot of heat compared to 80 degree summer indoor heat travelling across the same evaporator.

Next, the less heat absorbed by the evaporator, the less heat is available to come off the condensor, and if there is less condensor heat and your device has a fixed orifice (most portable ACs do), then there will be less pressure available to spray the ***** liquid back into the evaporator. Less liquid sprayed in means less droplets available to absorb the little heat that there is available to be absorbed by the evaporator. It's a vicious circle.

Often what happens with this senario is that the evaporator cools down too much and it begins to frost up quickly, especially if your AC has a high efficency rating (SEER above 10.5). A high SEER usually means that the fins on the evaporator are quite fine and close together, compared to the density on the condensor. With little distance between the evaporator fins, the fine spaces clog up faster with frost. Once that happens your heat output drops.

Idealy you nead a two stage evaporator where the air passes through one evaporator which only cools the air down enough to condense to a liquid, but not cool enough to turn to frost. Then this dried out cooler air from the first evaporator should be fed to a second evaporator where the air temperature will be dropped even lower.

That would entail connecting two ACs back to back and having some way of controlling the air flow through each evaporator so that they each operate at the desired temperature. And that means that as the outdoor air temperature changes constantly throughout the day, the air flow has to be modulated to maintain the equilibrium. Not easy without thermostats, variable speed fans and a logic controller.

A better solution, no pun intended, would be to adapt you portable AC to feed liquid across the condensor instead of air. The liquid could be your usual automobile antifreeze. You would have to pump this liquid through a maze of copper pipes (use aluminum finned hot water baseboard heater cores) located outdoors to absorb what heat there is to be absorbed on a cool day. You will no longer have the problem of the condensor frosting up since you will be immersing it in a bath of antifreeze.

Remember you can't turn these portable units (or any other type) on their sides in order to make the unit compatible with a liquid bath. You would have to build a waterproof chamber around the existing condensor in the same plane as it is already located.

These small portable AC units are actually capable of transferring more than their rated BTUs as long as you keep the evaporator being constantly fed with as much heat as is available (superheat within limits) and you increase the air flow over the condensor as much as possible without having the condensor outlet side temperature drop below 110-120 degrees F (subheat within limits).

So the cost to transfer each BTU from the outside to the inside can be quite economical compared with a commercial heat pump as long as you keep your DIY liquid system (resevoir,variable speed pump, and plumbing) and variable speed air ventilator costs (including connected closed loop feedback thermostats) as low as possible.

montreal
27-08-2007, 10:01 PM
In the previous post, initially I mixed up the words evaporator and condensor in a number of paragraphs and I had to edit twice. I hope this description is now stable. In case you were reading within the minutes immediately following my posting.

P.S. Stuart, the 15 degree C. rise across your coil is very similar to mine which is a 5200 BTU window AC adapted with custom ducts and variable fans on all inlets and outlets. I had about 125 CFM.

And don't forget that the water collecting off your evaporator was passing to the sling fan on the condensor and fudging your measurements. When you did your dry test with the calibration heaters, you were not supplying any water.

taz24
28-08-2007, 07:33 PM
Hi,


Does anybody know why the system would be operating at such a poor level or what the test criteria for domestic portable AC systems are?

Thanks,
Stuart

I don't know the answer to your question but did you measure the volume of air flowing across the cond / heaters? They need to be equal for your figures to hold up.

taz.

montreal
28-08-2007, 11:01 PM
I don't know the answer to your question but did you measure the volume of air flowing across the cond / heaters? They need to be equal for your figures to hold up.

taz.

And the number of gram of water in the form of humidity has to be equal, because moisture contains latent heat.

In Stuart's portable AC, there is usually moisture being removed by the evaporator which drips over to the condensor where it gets slung onto the condensor coil and lowers the temperature of the air coming off that coil. Even though the dry bulb temperature from the condensor output is lower due to the water, there is actually more latent heat there than meets the eye.

Why not assume that the portable AC is working correctly and is capable of pumping the 7500 rated BTUs and start by creating a configuration whereby the evaporator is constantly fed with 7500 BTUs of heat.

To do this starting with outdoor air which is much cooler than the AC was expecting means that the evaporator air flow has to be radically increased so that the ***** sprayed into the evaporator can evaporate as quickly as if the AC was operating under normal summer conditions to cool down interior air at 80 degrees F.

To know if the evaporator is getting enough heat from the cool outdoor air stream, you have to do what I did, attach the remote probe of a digital thermometer to the suction line right next to the compressor. Cover the probe end with insulating foam material.

Measure the temperature with the portable AC running in the mode it was intended (summer conditions - indoor air at 80 degrees, exterior air at 95 degrees). Note the suction line temperature.

Then try under a fall and spring situation in heat pump mode to get the suction line temperature to be the same by increasing the air flow across the evaporator. At some point the exterior air temperature will be too low regardless of air flow to make that work. And at some point the evaporator is simply going to frost up and you will need a defrost cycle.

Defrosting a commercial heat pump is simple. You just change the ***** direction with the reversing valve. But the portable AC in question has no reversing valve so all you can do is turn off the compressor and keep the air flowing across the evaporator until the frost dries up. That may take a while if the frost has completely blocked the evaporator and ice has begun to form. Better to defrost ahead of time in anticipation.

Another problem is deciding when to defrost. I have tried different methods to measure when the air flow through the evaporator is just beginning to slow due to the first traces of frost forming. But my electronic air flow meter worked only when calibrated for a specific outdoor air temperature. Better is to have some very sensitive mechanical detector like a propellor that spins and it's speed is measured by generating pulses. The pulses per second are counted and a speed determined. If the speed drops below a threshold, then the defrost cycle of fixed duration is triggered.

I went down this road last fall and stopped the experiment once the outdoor air fell below 40 degrees F.
The time lost due to ever increasing defrost cycles as the weather got colder made the device uneconomical.


If I had it to do all over again knowing what I know now, I would have build a liquid system. But if I was determined to do an air to air system, I would have had the fixed orifice replaced with an expansion valve. But that valve would end up costing more than the $90 CDN for my 5200 BTU window air conditioner used in my experiment.

Argus
03-10-2007, 10:31 AM
what the test criteria for domestic portable AC systems are?

Thanks,
Stuart



The Test Standard for these things is EN 14511.

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