heat transfer mystery
heat transfer mystery
(OP)
Can anybody explain why my numbers are not equally out?
At work, I am trying to determine the heat transfer taking place in a water cabinet. Both loops have water in them. There is a counter current flow heat exchanger where the transfer takes place.
I take the temperature measurements with a Microscanner D1001 IR thermometer. With this device I contact the surface of a pipe and take the measurement. I take about 3-5 measurements and average them out. I do this at the same spot on the pipes for the other inlet/outlet to the heat exchanger. Each pipe has the same diameter and the same material. (I know this is not a 100% correct method, but I am hoping it gets me close enough, because the thermocouple connected to the pipes are not calibrated. I have done this step about 50 times to get enough values to minimize any outliers.
Next I take the flow on the two loops.
From there I use the equation:
Heat tranferred = specific heat of water * flow * delta temperature. (I have no issues with the units).
When I compare the two loops, the numbers are off on average of 5kW (the average of the loops is (30kW and 25kW). The same loop is almost always higher. And this is the same case when I take the measurements off of duplicate machines. I got my data from over 5 water cabinets that were the same.
Why are my two loops not equally? Any guesses?
Thanks!
At work, I am trying to determine the heat transfer taking place in a water cabinet. Both loops have water in them. There is a counter current flow heat exchanger where the transfer takes place.
I take the temperature measurements with a Microscanner D1001 IR thermometer. With this device I contact the surface of a pipe and take the measurement. I take about 3-5 measurements and average them out. I do this at the same spot on the pipes for the other inlet/outlet to the heat exchanger. Each pipe has the same diameter and the same material. (I know this is not a 100% correct method, but I am hoping it gets me close enough, because the thermocouple connected to the pipes are not calibrated. I have done this step about 50 times to get enough values to minimize any outliers.
Next I take the flow on the two loops.
From there I use the equation:
Heat tranferred = specific heat of water * flow * delta temperature. (I have no issues with the units).
When I compare the two loops, the numbers are off on average of 5kW (the average of the loops is (30kW and 25kW). The same loop is almost always higher. And this is the same case when I take the measurements off of duplicate machines. I got my data from over 5 water cabinets that were the same.
Why are my two loops not equally? Any guesses?
Thanks!





RE: heat transfer mystery
RE: heat transfer mystery
Does both loop have the same pumping rate/pump?
Energy is added for pumping the liquid round in the loop maybe theres a difference here?
Best regards
Morten
RE: heat transfer mystery
Regards
RE: heat transfer mystery
> The thickness of the pipe itself can lead to a temperature delta.
> The emissivity of the pipe may be questionable. This can easily lead to errors of 5% to 20% in the measured heat, which is what your IR sensor is measuring.
> If the temperatures are high, then there will be substantial differences in the radiated losses.
TTFN
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RE: heat transfer mystery
the temperatures are low, 70-95 degrees F.
I understand that taking temperatures from the exterior of the pipe are less accurate, but when i take the temp from the exterior of 4 pipes that are the same, then my calculations should still equal.
The pump is not adding heat at the place I am measuring because the pump is located about 2 meters away. I am taking the measurements just outside of the heat exchanger.
From hearing you all, it sounds like the best way to do this is to get a calibrated thermocouple and flowmeter and go from there, because my IR thermometer/flowmeter method will never work?
RE: heat transfer mystery
<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying " Damn that was fun!" - Unknown>>
RE: heat transfer mystery
RE: heat transfer mystery
i am going to bring the machine down, and close the valve allowing water to it.
there are already thermocouples hooked up to the system, but for whatever reason the temperatures displayed on the gauges are not accurate. if i hook up a flute meter to the thermocouple, this should provide an accurate reading, correct?
for the flow, i am going to hook up a circuit setter to get a more accurate flow.
any issues?
RE: heat transfer mystery
and i will be collecting the trend for both temperature flow.
RE: heat transfer mystery
RE: heat transfer mystery
RE: heat transfer mystery
The really hard measurement is the flow rate of the two streams. I'm not sure what a "circuit setter" is, but anything that improves flow measurement will improve the calculation.
Once you nail all of your measurements, the two loops will still be different heat loads because of: (1) unmeasured losses to ambient; and (2) heat transfer efficiency of the cabinet. You should do better than 17%, but I'd be really surprised if you do much better than 10%.
David
RE: heat transfer mystery
on average it was 17%. sometimes it is near 0%, but others it is as high as 100%.
the thing i do not like is the chilled loop is the one reporting a greater heat transfer. with heat transfer efficiency, would it seem more logical that the warmer loop should provide data that suggests greater heat transfer taking place?
thanks.
RE: heat transfer mystery
David
RE: heat transfer mystery
RE: heat transfer mystery
What are the cold side and hot side temperatures, exactly? What is the temperature of the room
The choices for error are limited, and have been repeated a few times already:
> errors in temperature measurement
> errors in flow measurement
> errors in modeling, i.e., parasitic loads, or differences in the piping itself
TTFN
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RE: heat transfer mystery
Loop 1. inlet: 77.0 F outlet: 90.8 F flow: 14.9 gpm
Loop 2. inlet: 92.0 F outlet: 86.3 F flow: 29.1 gpm
Room temperature: 78.8 F
a weird note, the Loop 1(ECW loop that provides water to the entire facility) inlet temp varies between 72-82 F dependent on machine. measurements taken off the same machine under the same conditions on different days have a precision of +/- 1 F. however, when compared against other machines (same model) the precision between machines is poor. the facility engineers have no logical explanation for this.
i believe this is an temp accuracy issue, and i will be getting some more reliable numbers before this week ends.
RE: heat transfer mystery
I ran your numbers myself and given the delta T's you report and GPM of the flow, I get about 26 and -21 BTU/sec from the low and high sides respectively. Since 1 BTU = 1.06 KJ, I don't see how you get 25 KW and 30 KW for the two sides (I used 7.5 lbm/gal for the water and 1 BTU/lbm-F for its specific heat).
But, that issue aside, You need to take into account the systematic uncertianty of the thermocouple (T/C) as well as its random uncertainty. A basic text on T/C will put the accuracy of a T/C temperature measurmement system as +/- 1.5 F or so. Since you are taking the difference between two temperatures, the total uncertainty is the "Root sum square" of the two, or SQRT(2)*1.5 F. This total uncertainty in temperature difference alone is in excess of 2 F (since your delta T for the hot side is only 5.7 F, this means a relative uncertainty in temperature alone of 37%!). You can continue this using the uncertainty in the mass flow rate (I assumed +/- 0.5 GPM) and the total uncertainty is a whopping +/- 8.2 BTU/sec (8.7 KW)! Hence, your "hot side" heat tranfer could easily be as high as 29 BTU/sec (using my numbers) or as high as 32 KW (using your numbers).
Note that all my analysis so far does not even address random uncertainty, which you state is on the order of +/- 5 F for the inlet loop. Honestly, I am surprised your heat transfer rates are as CLOSE as they are!
If you do go ahead and use a Fluke meter, be sure to use an ice bath for a reference junction if your meter does not have on-baord compensation, sicne T/C's actually read a temperature difference.
I suggest visiting www.Omega.com and getting their "Temperature Handbook" (free of charge) if you don't have it already (I have no stake in the company but they have always been very helpful). I can suggest some good books on total uncertainty analysis if you like.
Good luck!
RE: heat transfer mystery
As for the calculations, I get results similar to the OP:
(86.3 F - 92 F)(29.1 gal/min)(993.3 kg/m^3) = -24.2 kW
(90.8 F - 77 F)(14.9 gal/min)(993.3 kg/m^3) = +30.0 kW
gal US = 3.7854E-3 m^3
TTFN
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RE: heat transfer mystery
Dimensionally, I can't figure out what you are doing.
The units of your answer are in F-Kg/min, which is not a kW (and computationally the magnitude is wrong as well).
I suggest (for the first one):
(86.3 F - 92 F)(29.1 gal/min)(7.5 lbm/gal)(1 min/60 sec) = -20.7 BTU/sec
Observing that 1 BTU= 1.06 kJ, this yields about 22 kJ/sec = 22 kW.
Hope that helps.
Dave
RE: heat transfer mystery
inside of the cabinet is the following:
heat exchanger
7 thermocouples (ECW Supply, ECW Return, one for each of 5 branches that the other loop breaks into).
3 flow meters (ECW Supply, ECW Return, Other Loop Supply)
1 pump (for the hotter loop)
1 water reservoir (for the hotter loop) (located above teh pump and heat exchanger).
after leaving the heat exchanger, the hot loop branches into 5 (going to various parts of the machine, coil, generator, etc).
overall, there is about 7 meters of pipes. all piping is part of one of the loops.
the pump is located about 3 ft from the heat exchanger.
RE: heat transfer mystery
I stand corrected. My mass density was wrong (7.5 lbm/gal) should actually be 8.3 lbm/gal.
Sorry for the confusion and the tone of my post.
Dave
RE: heat transfer mystery
So, what's the power consumption of the pump? I know that smaller pumps that we've used can crank about 200 W for 1 gal/min, so 29 gal/min would get you 5800 W, which, if all of that gets sucked into the cold side, would account for the heat load discrepancy.
TTFN
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RE: heat transfer mystery
Just out of curiosity, how big is this "water cabinet?"
If the counter flow heat exchanger is itself jacketed by another volume of water, then the thermal mass of the external jacket of water should be taken into account as well.
So far I have been assuming that Q_in=Q_out, but really it is Q_in-Q_out=d/dt(E_internal)= m*c*dT/dt=UADeltaT
Is there any cyclical fluctuation of the flows or the temperatures--like if the system runs at a lower load at night? If so, there could be significant lag between the power in and the power out due to a particularly long time constant mc/UA (I won't do the math because I obviously have difficulty doing that).
Just a long-shot guess based on what this system sounds like. I could easily be mistaken since I can't see this thing. Good luck.
Dave
RE: heat transfer mystery
RE: heat transfer mystery
Best regards
Morten