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siggy smalls
07-22-2004, 01:15 PM
Newbie to the forum!

So I'm working on a project for my company where I am trying to take the exhaust from ten vacuum pumps in our material room and send it into our warehouse during the winter months.

The combined exhaust flows at 3000cfm @ 200F. I then used this equation:

Btuh = cfm x 1.08 x DT

My calculations show the equivalent of 324,000BTUh being produced through the exhaust of these pumps.

We have 5 Unit Heaters in the warehouse that have an output of 240,000BTUh apiece.

In addition, we have 7 RTUs for the office (independant of the warehouse but on the same gas line) that have a total output of 500,000BTUh.

These 12 existing heating units are running on natural gas.

If I have the total gas cost for the previous fiscal year, does anybody have any ideas on how I would determine how much money will be saved (ROI) by exhausting the vacuum pumps into the warehouse?

jimgpe
07-22-2004, 05:53 PM
IF the vaccuum pumps run all the time, the calculation becomes easier.

Do a bin temperature analysis of the warehouse:

Figure the heat loss of the warehouse in BTU/Hr/F.

Get the Air Force Temperature Bin Data for your location. This table says your area spends XX hours at 0F to 5F, yy hours at 5F to 10F, etc.

Multiply the number of hours at the "bin" temperature times the delta-T between inside and outside, and you get BTU's.

Knowing the efficiency of your unit heaters and the cost of a cubic foot of gas (roughly 1000 BTU's), you can figure how many BTU's you can get from the exhaust rather than from gas, and voila, your savings.

Don't forget to figure the savings from not having to bring in outside air to make up the 3,000 CFM since you will be recirculating it.

In rough terms.

Surely there is some heat generated in your warehouse which you will not be counting on, so your savings estimate will be somewhat high. Also, you are not taking credit for solar gains, which would also make your estimate high.

Be careful: 150F air will tend to stratify and hang about in the rafters and provide virtually no comfort! You are going to need paddle fans to destratify the warehouse, or you could mix some warehouse air and the 150F air to get 100F air, which will not stratify so much.

Any toxicity issues with the exhaust air? Any filtering requirements? Keep these costs in mind as well.

Good luck.

siggy smalls
07-23-2004, 10:02 AM
Thanks for the help. You went far beyond the scope of what I'm trying to do. There are too many variables to be as accurate as your parameters could be. For example, I can't even try to quantify the effects of the 7 dock doors - i.e. how long they are open, etc. But I appreciate the advice for sure. I'm just going to try to stick to simple estimates.

As far as issues go, I'm going to be using diffuser which will help. As for toxicity issues, the vacuum pumps pull virgin polyethylene from railcars outside so I'm not too concerned. In addition, the vacuum pumps exhaust into our material room right now as it is and the air quality has been tested and passed.

jimgpe
07-23-2004, 12:34 PM
Too complex an answer, eh? Yeah, I get that a lot!

In that case, take the BTU/Hr you have available from the exhaust, multiply it times the hours you believe you need the heat, and you get a first-order estimate of the BTU's will can avoid paying for.

Divide that by 10,000 to get therms and multiply that times your cost per therm for gas, divide it by .8 as an estimate for the efficiencies of your gas burning appliances (assuming 80% efficient) and you get approximate savings.

It will get you an order-of-magnitude estimate.

Have fun!