Temperature Change - Insulated Metal Panels
Temperature Change - Insulated Metal Panels
(OP)
Hi Everyone,
Haven't touched anything with heat transfer in over 10 years and I don't think I'm doing this correctly. I'd appreciate it if someone take a look and guide me in the right direction. Not sure whether this is the correct forum or the HVAC/R. Thanks in advance.
We're installing about 5,000 sq ft insulated metal panels on the roof and walls of a building. We will assume a 5,500 cu ft volume. The interior will be a constant 35ºF, exterior assume 100ºF to give us mean 65. R-value of the insulated panels is 47. What will the interior temperature change be in 6 hours of the 35ºF is left unconditioned?
This is what I came up with...
R-47 * 5,000 sq ft = 235,000 BTU/hr through the panels
If 0.018 BTU/HR raises 1 cu ft air 1ºF/hr, then 0.018 BTU/hr raises 5,500 cu ft air (1/5,500)ºF/hr
1 BTU/hr * 5,500 cu ft = (55/5,500)ºF/hr = 0.01ºF/hr
235,000 BTU/hr * 5,500 cuft = (235,000 * 0.01) = 2,350ºF/hr
This seems like a really, really high temperature change. Help please? Thank you!
Haven't touched anything with heat transfer in over 10 years and I don't think I'm doing this correctly. I'd appreciate it if someone take a look and guide me in the right direction. Not sure whether this is the correct forum or the HVAC/R. Thanks in advance.
We're installing about 5,000 sq ft insulated metal panels on the roof and walls of a building. We will assume a 5,500 cu ft volume. The interior will be a constant 35ºF, exterior assume 100ºF to give us mean 65. R-value of the insulated panels is 47. What will the interior temperature change be in 6 hours of the 35ºF is left unconditioned?
This is what I came up with...
R-47 * 5,000 sq ft = 235,000 BTU/hr through the panels
If 0.018 BTU/HR raises 1 cu ft air 1ºF/hr, then 0.018 BTU/hr raises 5,500 cu ft air (1/5,500)ºF/hr
1 BTU/hr * 5,500 cu ft = (55/5,500)ºF/hr = 0.01ºF/hr
235,000 BTU/hr * 5,500 cuft = (235,000 * 0.01) = 2,350ºF/hr
This seems like a really, really high temperature change. Help please? Thank you!





RE: Temperature Change - Insulated Metal Panels
The heat transfer is 6,915 BTU/hr through the insulated panles which in turn will give us 69ºF/hr. Looks more reasonable but it still seems pretty high. Any thoughts? Thanks.
RE: Temperature Change - Insulated Metal Panels
I see you have posted the same question in more than one forum. Please do not do that. The forum policies may be read from the link below my signature. They should all be of interest to you, especially #4.
Please red-flag one of your two posts, or else post a link from one thread to the other so that two threads do not get started. Thanks!
Best to you,
Goober Dave
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RE: Temperature Change - Insulated Metal Panels
Cp = specific heat
h = convective heat transfer coefficient
ko (www.msthermal.com)
RE: Temperature Change - Insulated Metal Panels
.018 BTU/(ft^3 F) / 1.25 BTU/(hr ft^3) = .014 F/hr
RE: Temperature Change - Insulated Metal Panels
ko99: Not too familiar with heat transfer. How would I use the thermal time constant in respect to the 6,915 btu/hr?
MintJulep: I believe that would be 0.014 hr/F and 69 F/hr. I've never worked with this type of application before. Is this a normal temperature change?
Thanks everyone.
RE: Temperature Change - Insulated Metal Panels
6915 btu/hr is the heat flow rate used to estimate the heat transfer coeff: h = heat flow rate/(deltaT x area)
FYI, the concept of a time constant is not limited to heat transfer. The most common example is an R-C circuit.
ko (www.msthermal.com)
RE: Temperature Change - Insulated Metal Panels
RE: Temperature Change - Insulated Metal Panels
Is the mass I'm supposed to use to calculate the time constant, the mass in the cooler or the mass of the insulation?
Currently, I've calculated using mass of insulation 2.15 pcf density * 6in thick panel * 5,000 sq ft of panel. Cp is approximately 0.346 btu/(lb Fº) for PIR/PUR foams.
This gives me Tau = 17.46 hr? What am I supposed to the time constant now? Better yet, I'm not exactly sure what the time constant is telling me...
RE: Temperature Change - Insulated Metal Panels
The temperature rise of the building vs time is an exponential curve, starting at 35F at t=0 and essentially reaching steady state at 5 time constants. From this curve, you can estimate the building temperature rise ant any time:
At tau = 1, the mass will be 63% of the way to 100F
At tau = 3, the mass will be 95% of the way to 100F
At tau = 5, the mass will be 99.99% of the way to 100F
For example, IF tau is 24 hours, 6 hours is 0.25 time constants. I'll leave it to you to work out the mass temp rise at tau = 0.25
ko (www.msthermal.com)
RE: Temperature Change - Insulated Metal Panels
TTFN
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RE: Temperature Change - Insulated Metal Panels
Note that if there's significant solar heating it will dominate the heat flow.
ko (www.msthermal.com)