BTU readings for Air Conditioners
BTU readings for Air Conditioners
(OP)
I'm a summer intern and I'm working on electrical energy conervation. One area I would like to improve electrical usage in would be the air conditioners. Here at the plant, we use chiller systems and package units for some buildings. With the little bit of thermo that i have had so far, I managed to calculate the amount of power needed to heat a certain volume of air from 45 degrees to X degree in the chiller unit. I used Air density = 1.29 kg/m3; Cp = 1000W/kg-K ; and substituted K with C b/c it would be the change in T anyways. The only part im missing is the time it will take for it to actually heat this volume to the temperature. How is that calculated? Also, does the BTU listed on AC units represent the amount of energy needed to raise the room size range 1 degree?
With the package units that operate just like your conventional home unit, does the BTU represent the same as the above question? Also, how would i go above calculating how long it will take for a volume to cool to specified temeprature?
This would be very help for my project. Thanks Alot
With the package units that operate just like your conventional home unit, does the BTU represent the same as the above question? Also, how would i go above calculating how long it will take for a volume to cool to specified temeprature?
This would be very help for my project. Thanks Alot





RE: BTU readings for Air Conditioners
The BTU rating on the A/C units you mention is BTU per hour. It's a rate of heat transfer. About 15 BTU/hr per square foot is a pretty good size for residential.
Also, if you're calculating power with respect to cooling, use enthalpy differences versus temperature differences to account for latent heat.
RE: BTU readings for Air Conditioners
AC controls automatically start/stop the compressor in respond to the load. They are sized for the maximum load. So I am not sure where you are leading with regards to time to heat the space.
RE: BTU readings for Air Conditioners
Calculating the time required to produce a given change in temperature (pull-down time) is a VERY complicated thing to do, especially as you almost never have all of the information that you need.
As ChasBean notes, most cooling calculations are done assuming steady-state operation. (Probably because the transient calculations are such a bitch.)
However, if you really want to take a stab at calculating pull down time here is a VERY VERY crude approximation:
Calculate the heat load (rate at which heat is entering) the space of interest.
Calculate the total volume of air in the space of interest.
Assume that the cooling capacity of your equipment is a constant, as noted on the nameplate.
The excess cooling capacity is avaiable to reduce the temperature of the space.
For sensible heat only (in American units) capacity = 1.1 x CFM x delta T. BTU/hr = (ft^3/minute) x degrees (The 1.1 contains lots of unit conversions to change cubic feet to pounds at standard conditions. I don't remember all of them, and am too lazy to look it up at the moment. Its not important at the moment.) So, now you have BTU/hr and cubic feet Shuffle terms around a bit and you arrive at the number of minutes it takes to cool that many cubic feet of air down 1 degree.
Of course latent heat removal will lengthen this time. Also, in real life you are not cooling only the air, you are cooling all of the equipment in the room. If you know the specific heat and mass of all the things in the room, you can throw those into the equation as well. (This is where I inevitably give up on this calculation, as it is redicuously time consuming to collate all of this data.)
RE: BTU readings for Air Conditioners
We usually take 1.08 cfm dT for granted but not using the root equation can give bad results at different temperatures or high altitudes, etc. Figured this wouldn't hurt to post.
RE: BTU readings for Air Conditioners
Thanks for the dimensional analysis. I was just too lazy to work through it or look it up last night.
You are absolutly correct that 1.08 cfm dT can get you into trouble if you are significantly away for standard temperatue and pressure. When it matters use the right values for density and specific heat.
RE: BTU readings for Air Conditioners