822 Measurements

Because waste-heat streams have such variability, it is difficult to list every possible measurement that might be required for its characterization. Generally speaking, the characterization of the quantity, quality, and temporal availability of the waste energy requires that volumetric flow rate, temperature, and flow intervals be measured. Chapter 6 of the NBS Handbook 1218 is devoted to this topic. A few further generalizations are sufficient for planning the survey operation: 1. Flow continuity requires that the mass flow rate of any flow stream under steady-state conditions be constant everywhere in the stream; that is, where Q is the material density, A the cross-sectional flow area, and V the velocity of flow normal to that area. The equation can also be written where Q is the volumetric flow rate. It is safer, more convenient, and usually more accurate to measure low-temperature flows than those at higher temperatures. Thus in many cases the volumetric flow rate of the cold flow and the temperature of the inlet and outlet flows are sufficient to infer the characteristics of the waste-heat stream.

2. Fuel flows in direct-fired equipment are easily measured with volumetric rate meters. Combustion air and exhaust gas flows are at least an order of magnitude greater than the associated fuel flows. This effectively precludes the use of the volumetric meter because of the expense. However ASME orifice meters, using differential pressure cells are often used for that purpose if the associated pressure drop can be tolerated. It is even cheaper, although less convenient, to determine the volumetric proportions of the flue-gas constituents. Using this data, the air flow quantity and the flue gas flow rate can be calculated from the combustion equation and the law of conservation of mass.

3. The total energy flux of the fluid streams can be determined from the volumetric flow rate and the temperature using the equation

PinQin = PoutQout

PinQin = PoutQout

enthalpy flux, Btu/hr density, lbm/ft3

specific enthalpy, Btu/lbm

Q = volumetric flow rate, ft3/hr

The density, the specific enthalpy, and many other thermal and physical properties as a function of temperature and chemical species is given in tables and graphs found in a number of engineering handbooks910 and other specialized volumes.11-13

4. In order to complete the waste-heat survey for the plant it is not necessary to completely and permanently instrument all systems of interest. One or more gas meters can be temporarily installed and then moved to other locations. In fact, portable instruments can be used for all measurements. While equipment monitoring, should be carried out with permanently installed instruments, compromises may be necessary to keep survey costs reasonable. However, permanently installed instruments should become a part of every related capital-improvement program.

5. For steady-state operations a single temperature can be assigned to each outlet flow stream. But for a process with preprogrammed temperature profiles in time, an average over each cycle must be carefully determined. For a temperature-zoned device, averages of firing rate must be determined carefully over the several burners.

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