Cooling Load Calculation
Cooling Load Calculation
(OP)
I'm working in a partition wall space (small, easily calculated cooling load) within a larger building. The users will be using a process in which they heat a material to 400 degrees.
Assuming I can determine the surface area of the heated material, how do I calculate the cooling required to offset the 400 degree material?
Assuming I can determine the surface area of the heated material, how do I calculate the cooling required to offset the 400 degree material?





RE: Cooling Load Calculation
RE: Cooling Load Calculation
RE: Cooling Load Calculation
Forget the process for now. A magical, constant temperature 400 degree black box of known surface area exists in the room. How is the convective heat transferred into the space calculated? How is the radiated heat calculated?
RE: Cooling Load Calculation
Now if you want to try to model all of convective heat transfer (which will not be as accurate as the calculation that I described, unless it is a simple shape and you have the airflows etc. across the body) then you will have to calculate the Nusselt number, the Prandtl number, and the Reynolds number, find a good relationship for you shaped object and air velocity direction.
The heat transfer by radiation is a little simpler. If you assume that the heat transfer from the rest of the room to your hot material is negligible then q/A=esT4 where q is the heat flux, A is the area of your body that is radiating, e is the emissivity of the object, s is the Stefan Boltzman constant, and T4 is the temperature of the object in absolute terms to the fourth power.
If I were you I would calculate the total heat gain as I originally described and then calculate the radiative heat gain by the equation that I gave to see if radiation is a significant contributor. Depending on the color of the object at 400 deg. F, I suspect that the radiation will not be a major contributor. If it did glow cherry red (like an IR heater element (which is usually higher than 400 deg. F) then it will be significant.
If you do calculate the convective and radiative heat transfer please post your calculations and sources of equations since I would be interested in seeing them. I advocate using the energy balance method instead of using the rate (heat transfer) equations.
RE: Cooling Load Calculation
If you already know the energy in, then the cooling load is just that. Your "black box" was heated through those energy "ins". Unless you've got perpetual motion...
TTFN
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RE: Cooling Load Calculation
it depends the material is in what form! Is it spread over a large surface or in a vessel or divided into parts, all these aspects differ from each other because it will account for the heat rate you are supposed to remove from the closed space. If the heated material is in a vessel then you will do lump system analysis and this will give you the least heat rate removal of all probabilites, but if they are a large number of blocks and spread over a big space then this will give you the most heat removal rate. In alll cases most of the heat is by natural convection, except if you apply forced air to the product.
you have a lack of information in what you are posting, surface area andtemp is not enough but there are other aspects to the problem.
RE: Cooling Load Calculation
A local manufacturing company is expanding one of their processes. The current line is located in a little shanty adjacent to their factory. It has NO ventilation or exhaust. Coils of a material I'm not to divulge is heated (outside the room) to 400 degrees and wheeled in where they go through a stamping/pressing type process and then are wheeled out. I'm sure they are not exactly 400 when wheeled out but for the sake of conversation I'll assume no loss. At any given time there are 6 coils in the room.
They are relocating/expanding their process and I'm designing the HVAC system. Though it doesn't have to be an exact science, I'm trying to establish a cooling load for the new room and I don't want to oversize too much as it's DX. The existing room is really hot and the only sources of load are the lights, people, and these hot coils (middle of blizzard winter north Ohio). I can't figure how to calc the coils.
Again, I didn't mean to be glib, I'm just getting frustrated because it seems like it should be relatively easy and I should know how to do it but I don't.
RE: Cooling Load Calculation
Now if this is a coil of sheet metal that is unrolled and then stamped you will have a high surface to volume ratio which will allow it to cool off faster.
Again if you can I would suggest the energy balance approach instead of the rate (heat transfer) approach.
Determine the mass of each coil. This gives you "m". The manufacturer should know this information. Determine the heat capacity of the material. This gives you "Cp". This should be available from some type of materials handbook. You know the temperature going in (T1)is 400 deg. F. Now if as you state the temperature going out (T2) is 400 deg. F then you don't have to worry because there is no heat gain into the room which of course isn't very likely. All you have to do find the temperature going out of the room. If the manufacturer does not know this information then you can do one of the following a) try to calculate the heat loss of the material using some natural or forced convection closed solution or numerical solution which unless you know what you are doing will not be very accurate and either will take some time or b) Measure the temperature of the material as it leaves the existing process with an IR temperature probe (not that accurate), a thermocouple or RTD surface probe, or some of the sticks which melt at different temperatures, all of which are available from Omega (or other sources). Now that you have all of this information you can calculate the heat gain in the room by Q=mCp(T1-T2).
If you really want to calculate the natural and/or forced convection and radiation then I suggest that you post your question in the "Heat Transfer and Thermodynamics" forum under Mechanical Engineering.
RE: Cooling Load Calculation
So, a simple swag is to assume that they cool by, say, 100º (F or C?) and that they are in the room for some dwell time. Then the specific heat multiplied by the change in temperature divided by the dwell time is an estimate of the heat load. Since this stamping process requires mechanical contact with some sort of metal structure, this might well constitute the biggest heat transfer component of all, hence the guess of 100º. In any case, you should be able ask the company what the exit temperature of the coils is.
To that, you'd have to add the cooling load of the press operation itself.
TTFN
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RE: Cooling Load Calculation
RE: Cooling Load Calculation
RE: Cooling Load Calculation
The process exists, therefore, there should be some data that either exists or can be trivially measured with an IR thermometer.
You then only need to know the material and its specific heat, from which you can determine the total energy loss during the process. That energy loss is your cooling load, regardless of how it leaves the coil.
Doing the heat transfer from fundamental principles seems to be overly complicated and rife with assumptions that cannot be readily verified.
You already have stuff that loses heat, so that can be measured and confirmed. Likewise, your coil material should be sufficiently well known to have a specific heat that could also be verified, if need be, with a simply lab experiment.
TTFN
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RE: Cooling Load Calculation
Then you can go to this website http://www.heatershop.com/btu_calculator.htm and put in your temperature rise, room size and use tight walls and see what the BTU and wattage is calculated to, just for the metal heat.
Then download my Heat load calculator, (attached) and go to the requirement tab. Plug in your real time data and use the metal load wattage as other load.
Thgis will give you a final tonnage and some ways to calculate air flow requirements, etc
RE: Cooling Load Calculation
But if its shaped and reconditioned by machines and tools it is not going to be 400-deg, and neither is it going to be any singular surface temperature on its way out...especially if the duration is short compared to the (heat content of total volume, divided by exposed area) which is much more significant in short term processes on odd shapes, than is the Biot number...
Chances are a lot of the heat from One Part was spread around in contact with the machinery in the room to a larger volume and surface area of machine, so your Heat Transfer will get pretty ugly in a hurry....
But never mind: The most realistic way of doing it was suggested by somebody up there; I'll add my twist.
Get a Whole Bunch of ice; a little water; put both in a "sieved" container like a deep frier, such that the ice is in the basket. Lift the basket out, weigh it and the ice. Throw a hot part in for a good healthy time; if its weight compared to the weight of water is such that steam will be generated: Slap a lid on all of it. When the whole thing dies down: Fish the part out, make usre its as cold as the ice water; then weigh the ice and the basket. The ice that melted, thus is not in the basket, took on the heat of the part and cooled it all the way to 32-F. from 400 deg F.
Do the whole thing over again with a typical Outbound Part. This time you will have a lower total heat in the part; thus you will melt less ice. The difference between the wieght of the lost ice in Round 1 vs Round 2 is the heat that was lost from the typical part on the way through the Shed, in BTU or Calories; to get the cooling load for the many parts: multiply heat lost from one part times the rate of production.
RE: Workign tools and machines: Those points of contact, at 3-400 FPM velocities, become the focus of your air flow if you are looking for dimensional stability or safety in terms of cool surfaces of the machines...Otherwise: Its a matter of where the hottest pieces stay still long enough to have a local radiant effect on the opposing surfaces...Its those surfaces that need some air movement to focus the cooling and ensure stability and comfort.