High Pressure Air Properties

If I had a specific CFM, we might be able to convert this into a sensible BTUH of heat to be recovered.

The total flow during mormal operation is about 2300 cfm

Assuming you mean standard cfm, I make mass flow 1.3 kg/sec. Specific heat is ~1 kJ/kg at 465 psi over that temp. range. Heat flow comes to 160 kW. Sorry this is all in metric!

Using Codemaster's estimate of 160 kW, that gives about 9100 BTU/min

I know this isn't what you asked for, but have you "stepped back" to look at the big picture? How much energy is going into producing the high pressure air? Less than that is the most energy one can expect to recapture. A WAG would be 60 - 80%.

This conversion tool may help especially with BTU, etc.

http://www.connel.net/freeware/convert.shtml

Bill

The facility makes plastic bottles. They have large recip air compressors that provide >400 psi air to blow the plastic into bottle shapes. This equipment is very efficient for what they are doing. The high pressure air comes out of the second stage at a high temperature. They must cool the air prior to applying it to the plastic. Currently this heat is being released to the atmosphere and I am looking into capturing it for spac heating.

Ahhh . . . Now things are clearer. Does all the hot air go through one heat exchanger or several? Is (are) it (they) located and/or mounted in such a way that plenums, ducts and air handlers may be installed? I would assume that a cooling tower or forced air and finned heat exchangers are used now; if not what is?

2300 CFM (300-80) 1.08(K) = 546,480 Btuh

546,480/3415 = 160KW

180 Degrees F. = 82 Degrees C.

There will be some losses, but medium or low temperature hot water is a sweet option from a heat exchanger. Your efficiency would be great at 180 degrees F for building heat, domestic hot water, or a process that could use 180 degree water.

The Btuh listed above is at 100% operation based on CFM and the temperatures stated. If compressed air isn't being used, a bypass with heat exchanger could be maintained for temperature control of the water if you wanted to add heat at times using compressed air.

Isn't 1.08 a constant for standard air? This is a function of density and will be considerably differnt with my paramters of 460 psi.

Don't want to put words in TLGEngrCo's mouth, but the 1.08 takes account of the air density (a fixed figure at standard conditions, ~ 0.075 lb/ft³) and the specific heat. As I said in post #3, specific heat per unit mass is fairly constant over the temp range considered. (It doesn't vary much with pressure either at least up to 600psi, but your pressure is constant anyway)

So assuming the 2300 is standard CFM (it would be surprising if it weren't but you haven't confirmed) the 460 psi makes no difference.

Yes, temperature is effecting the air density. If we assume an SCFM of 2300, The load is 160KW. The ACFM would be in the neighborhood of 30% more given the temps. and pressure. I indicated what would be available at 100% load. There are losses through the heat exchanger from one side to the other, was my other point earlier. To add a safety factor of 1.25 would not be unheard of for a heating application, and was an industry standard until a new national energy code was introduced around 2000. It just so happens, this is almost the difference between SCFM and ACFM, they happen to almost be the same. To use a more accurate air density constant is always desired when available.

Just to follow up on Codemaster's post, the Constant Pressure specific hear of air at 425K (305F) is 1.024 kJ/kg*K. At 300K (80F) it is 1.021 kJ/kg*K. Your temperature is 300F which is about 422K. These values are from Mark's Standard Handbook for Engineers, Ninth Edition, page 4-56 Table 4.2.20. Using the Ideal Gas Law, you should be able to calculate your available heat a bit more accurately. The values previously reported are a good first order approximation. You seem to know quite a bit about the subject already so the only thing further I can suggest is to slightly oversize your heat exchangers to account for fouling and dirt accumulation.