Crazy DIY evap design

jreckart

I've been an HVAC tech for about 10 years, dealing more with refrigeration and commercial HVAC, than residential. Mostly self-taught, and always wanting to learn more.
We have put up a wall in our shop, and want to put different controls, equipment, etc on this wall in a "learning lab" type scenario.
For a small refrigeration demo system, having a traditional evap in a box would take up too much room, so I was thinking maybe a 30' length of 3/8" cooper tube, coiled up, in a 5-gallon bucket, or maybe 55 gal drum.
Here's the question, how would i go about measuring the BTU capacity of this DIY evap coil?

jesmed1

As a first approximation, you could start with a coil in a 5 gallon bucket, then add a known quantity of water at room temperature, say 4 gallons. You know water weighs about 8.34 pounds per gallon, so now you have about 33.4 pounds of water.

You also know that one pound of water loses one BTU for every degree F that it loses. So you can run your evap coil for a known amount of time, say 15 minutes, measuring the water temp both before and after the test, stirring well each time to be sure you're getting an average water temperature.

Let's say your water temperature dropped 10 degrees during that time. 10 degrees x 33.4 lbs x 1 BTU/degree-lb = 334 BTU lost. That's over 15 minutes, so now multiply by 4 to get hours. 334 x 4=1336 BTU/hr.

This isn't entirely accurate because you will get some heat transfer from room ambient air into the bucket water while the evap is cooling it, but you're just trying to get ballpark numbers.

That gets you a heat transfer rate for the evap coil in water, which of course will be different for air. I'm not sure how you would make the conversion to a heat transfer rate for air, because the heat transfer rate for air will depend to some degree on the flow rate of air past the coils, and of course the different thermal properties of air vs. water. But it might give you a theoretical maximum for your evap coil, since water will be a more effective heat transfer medium than air.

jreckart

jesmed1 said:

As a first approximation, you could start with a coil in a 5 gallon bucket, then add a known quantity of water at room temperature, say 4 gallons. You know water weighs about 8.34 pounds per gallon, so now you have about 33.4 pounds of water.

You also know that one pound of water loses one BTU for every degree F that it loses. So you can run your evap coil for a known amount of time, say 15 minutes, measuring the water temp both before and after the test, stirring well each time to be sure you're getting an average water temperature.

Let's say your water temperature dropped 10 degrees during that time. 10 degrees x 33.4 lbs x 1 BTU/degree-lb = 334 BTU lost. That's over 15 minutes, so now multiply by 4 to get hours. 334 x 4=1336 BTU/hr.

This isn't entirely accurate because you will get some heat transfer from room ambient air into the bucket water while the evap is cooling it, but you're just trying to get ballpark numbers.

That gets you a heat transfer rate for the evap coil in water, which of course will be different for air. I'm not sure how you would make the conversion to a heat transfer rate for air, because the heat transfer rate for air will depend to some degree on the flow rate of air past the coils, and of course the different thermal properties of air vs. water. But it might give you a theoretical maximum for your evap coil, since water will be a more effective heat transfer medium than air.

This is what i was thinking, but was looking for others ideas. thanks

hot_rod

Fairly easy to build a BTU meter. A simple plastic water meter and a dual temperature head for your VOM.
Math can be simple or complex if you want to take into account the density and specific heat at the exact temperatures.

Some good info and formulas here.

https://www.caleffi.com/sites/default/files/media/external-file/Idronics_24_NA_Fundamentals of heat metering in hydronic systems.pdf

mattmia2

Maybe more think a refrigerator sized compressor so you have less heat to try to source and dump or transfer from condenser to evaporator. Or make the evaporator and condenser coils in buckets and have a pump that can circulate between the buckets so you can control the load.

hot_rod

Measuring a moving stream may be the most accurate. In an open pail you will get some stratification, and heat loss from the surface area, as well as loss through the wall of the bucket.

Flowing through insulated pipe will get you a good delta T to put into the formula. If you're looking to be that accurate.