REDUCED PAYBACK THEORY

The original values were calculated on a hand held calculator and later verified on a Texas Instrument PLUS 4 computer. Everyone who reads this concept must understand that the information contained herein is based on a technology and fuel costs that existed in 1980.

There are two concepts of application that exist in this theory. The first concept is to maintain a constant velocity and volume of counter-flowing air. The second, being the better of the two, is to vary the velocity and volume. Both concepts have individual and mutual benefits beyond the existing state of the art. Application of these concepts are dependent on the requirements of the system to accept constant or varying volumes of air flow, along with a lower fluctuation of preheated air will result in a lower payback period when compared to a constant volume system. The immediate difference is not as appreciable when compared to profits over the long term.

HISTORY: In the past, the basis of the application of a heat pipe energy recovery system to an existing exhaust flow, has been to use all of the available exhaust flow. This has been typically accomplished at a standard face velocity of 500 feet per minute or 2.5 meters per second. A heat pipe energy recovery coil, constructed off an integral finned aluminum tube, with a one inch inside diameter and having nine fins per inch, will result in an efficiency of approximately 56% to 66% for an array of 6 to 9 rows. In the majority of existing applications, those heat pipes will be operating at less than 50% of maximum transport capability.

CONCEPT BASIS: Create a design condition where the heat pipes operate at near maximum transport capability. A design condition of 95% maximum is used in this concept. This is at maximum “Delta T”, which is the differential between supply and exhaust temperatures.

APPLICATION OF CONCEPT: The benefit of this concept is to substantially increase the amount of energy recovered and proportionally reduce the “Payback Period”.

EXAMPLES: All examples are based on an installation in the New York Metropolitan area and installed for operation in December. Average design temperatures are based on ASHRE degree days. I have inserted payback data at this point for impact.

EXHAUST TEMP. PAYBACK
100 Deg. F 3 Months
200 Deg. F 5 Weeks
300 Deg F 23 Days
400 Deg F 18 Days

This payback data and saving recovery are based on the following:

Fuel Cost: $1.00/Gallon
BTU/Gal: 140,000
Gal/MMBTU: $7.142
$/MMBTU: $7.142
ELECT: $0.10/KWH
GAS: $0.67/THERM