Replies

1. 
   Wet_Head | Jul 05, 2002 10:02am | #1

   Why?  Aren't the EDR tables available for your radiator?  There are very few this info is not available for.

2. 
   TLRice | Jul 16, 2002 02:47pm | #2

   Only indirectly. You could measure entering water temperature, leaving water temperature and water flow. You would have a measurable change in energy, which would be the device output.

3. 
   DavidThomas | Jul 18, 2002 03:45am | #3

   Water flowrate (in gpm) x 8.3 pounds/gallon x 60 min/hour x 1 BTU/pound/degreeF x delta temp (in degrees F) = BTU/hour.

   And infrared thermometer is a quick way to get the in and out temperatures (about $85).  But two thermometers, wrapped under some insulation, will come to pipe's temperature in a few minutes.

   Flowrate can be closely estimated by looking at the pumps performance curve versus the pressure drop in the piperun.  A really rough estimate would be that the water is traveling 5 feet per second in the piperun.  Multiply 5 feet per second by the cross sectional area of the pipe (in sqaure inches).  Divide by 231 cubic inches in a gallon and multiply by 60 seconds/minute to get gpm.  Could be way off, but in properly sized piperuns, it will be pretty close.

   Or, in a house of known construction, crank the heat and see where it stabilizes.  Calculate the energy loss from that area at the observed indoor/outdoor temperature differential.  Could be difficult (and uncomfortable) to perform this test in the summer.

   If you place the radiator in a box and blow air through, the only tricky part is measuring the airflow.  (Temp in and temp out is easy).    Once you've got cubic feet per minute (a whole other discussion) multiply by 60 min/hour, by the air temperature  difference, by the density of air (0.76 pounds/cubic foot) and by air's heat capacity (0.23 BTU/pound/degree F).  That will give you BTU/hour for the particular test conditions you set up.  Cooler air temps and hotter water temps can increase that BTU rating a lot.

   Why do you want to know?  Need to order a replacement unit?  Or trying to increase its output?

   David Thomas   Overlooking Cook Inlet in Kenai, Alaska
1. 
   TLRice | Jul 19, 2002 02:51pm | #4

   "Once you've got cubic feet per minute (a whole other discussion) multiply by 60 min/hour, by the air temperature difference, by the

   density of air (0.76 pounds/cubic foot) and by air's heat capacity (0.23 BTU/pound/degree F)."

   Didn't crunch your numbers, but the easy formula for air is:

   CFM x dT x 1.085 = btu/h

1. 
   DavidThomas | Aug 25, 2002 03:30am | #5

   Yep, that works. And I had a typo in mine. Density of air is 0.076 pounds/cubic foot, NOT 0.76. So mine works out to CFM x dT x 1.05 = BTU/hour. Very close to yours.

   David Thomas   Overlooking Cook Inlet in Kenai, Alaska

1. 
   TooManyTools | Aug 25, 2002 04:14am | #6

   Better watch that air density in your formulas.  In Denver (5000 ft) the air density is only 0.063 and the per cfm constant is 0.92 Btu/cfm deg F.  I've made a few dollars off sea level engineers forgetiing that.

1. 
   DavidThomas | Aug 27, 2002 10:28pm | #8

   With 700 feet of ocean beach, I guess I do have a sea level perspective. Another altitude problem many people miss (including engineers) is the wet and dry lift of pumps. Since is really atmospheric pressure pushing the water towards the pump inlet, pumps have less lift at high elevations.David Thomas   Overlooking Cook Inlet in Kenai, Alaska

2. 
   TLRice | Aug 27, 2002 03:23pm | #7

   The difference in density of air, for the purposes of calculating the energy change in an air stream, here, there or in Denver (at 5280 ft) is negligible, when compared to the inherent error in the variables used in the calculation and the precision to which they can be measured.

1. 
   UncleDunc | Aug 27, 2002 10:52pm | #9

   >> ... the inherent error in the variables ... and the precision

   >> to which they can be measured.

   So how much uncertainty should we expect with this formula? Plus or minus 10%? 50%?

2. 
   DavidThomas | Aug 28, 2002 04:56am | #10

   There's nothing I know in engineering within 10% except how many hours I worked last week and how much I'm going to charge the client for them.

   Seriously, those numbers are good to 2 or 3 significant figures, IF you check all the conditions behind them (pressure, temperature, etc). In practice, they are good to a percent or two without checking references.

   Even the high-altitude concern often goes away. If someone doesn't think about the lower air density, they probably also didn't calibrate their air velocity meter for the lower density. They two errors largely cancel each other out.

   David Thomas   Overlooking Cook Inlet in Kenai, Alaska

3. 
   TLRice | Aug 28, 2002 02:48pm | #11

   25% give or take, my best guess. Uncertainty analysis would pin it down more closely, but I'm not that interested to do that.