thermo

[Norman Albers]

I have a practical thermodynamic, engineering question. I run for a few hours in this season, a dehumidifier in my home and piano shop. If we ignore electrical inefficiency in the motor, are there thermodynamic exchanges heating my house with some fraction of the power (500 W)??? You might be surprised at the heat of vaporization of water, the fact that a pint (a 'pound') takes one thousand BTU's to change phase. I calculate this to equal 0.29 kilowatt-hours. My unit consumes half a kilowatt. I could measure the output in pints per hour, but what are the thermo considerations?

[swansont]

Yeah, I get 0.29 kW-hr as well. What thermodynamics do you want? The system cools with some coefficient of performance, allowing condensation of the water and pumping that energy away.

 

http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heatpump.html#c1

[Norman Albers]

I am confused about the energy exchanges here. If we were not collecting water from the cold coil, the device would simply spend its 500 W heating the room. How much of this input can be used to condense water? If there were total efficiency the output would be 5/2.9 pints per hour, 1.7+.

Edited June 24, 2008 by Norman Albers

[swansont]

It's going to send its 500W into the room, no matter what, and will send additional energy pumped from the water. The question is how much heat can it remove from the water in the room while doing so. That's what the coefficient of performance will tell you. In the ideal case, it's going to depend on the temperature of the room and the temperature of the condensing surface.

[Norman Albers]

Thanks Swansont. Thermo is one of my little dyslexias. I wasn't thinking clearly about the flow of the heat of vaporization. If the 500 W can transfer two or three times that much heat energy (typical performance, yah?) it could be several pints per hour. I shall measure! It sounds like the total heating of the room is 500W plus the heat of phase change.

 

Running my dehumidifier in my house at 55% relative humidity and low morning temperature of 20 C, there is only about 2/3 pint per hour. This seems low on a 'coeff. of performance' level. Only half the input energy spent (500 W) is yielded in water condensation. Given a price of ten cents for a typical daily run of three hours. I can still appreciate the dehumidification I must have to work on and rebuild acoustic pianos. They are very large humidity gauges. I do not want to snug about 300 screws to see the RH go down by ten or more points. Some of the hammer flanges will then click when the hammers strike. This, not to mention the soundboard and bridges. This dehumidifier I direct into the main space of my 1100 square foot home, leading out from the shop room. I have learned from this discussion that there is more heat put into the system than I realized, and this helps me figure the best habits about airing the house at night, and about achieving the control I need with a little heating and dehumidifying, and then some AC with a 500 W air conditioner upstairs, which cools but also takes out water vapor.

Edited June 26, 2008 by Norman Albers
multiple post merged

[Norman Albers]

It turns out my "lab equipment" was not adequate. I use a relative humidity gauge, circular needle dial bought for $30-40 and hanging on my living room wall. Also I have a digital temp-RH gauge that I use when I service people's pianos in their homes. I compared the two and the analog read six points RH higher than the digital. In the rear plate you could readjust the whole mechanism to recalibrate, and this was distressingly easy. This tells me that it had been knocked out of calibration. This is important for my conclusions about the energetics of dehumidifiers and house heating/cooling. Acoustic pianos are to be kept in a midrange, roughly 40-50% RH, for good stability and long-lasting fits of wood and metal together. I ran my dehumidifier in the soggy weather of a late spring and measured what was happening. Now I can say, having reset the mechanism of the analog gauge close to what the digital one reads, that I was running at 50% or a bit less in RH. This is to say I had gotten rid of the "excess moisture" in my interior environment. If you read the manual for a small wall-mount AC unit (also 500W) it says "gets rid of excess moisture" and this is well-stated, and important. I define this as RH over 54%, personally. My skin and my pianos are happy under this. One can see that with greater relative humidity, the cold coil on the compressor unit will condense more water, as indeed a technical consultant of the manufacturer dwelling in Houston, Texas, reassured me! I said, yep, I'm in relatively dry southern Oregon so I respect your statement. THE UPSHOT IS THAT MY HOUSE IS NICE AT AN RH IN THE HIGH 40'S, AFTER RUNNING BETWEEN $5-10 FOR THE MONTH OF HIGH AMBIENT HUMIDITY. A DEHUMIDIFIER DRIES AND ALSO WARMS. as was detailed earlier.