In the operation of combined heating and power plants, there are two serious threats to economy as measured by fuel consumption for the service rendered. These are loss or exhaust steam to the atmosphere and, less serious, exhaust or steam to a condenser. The greater part of the heat supplied to the steam in the form of latent heat by the boilers is lost in either case. Use of exhaust steam for heating makes available a maximum amount of heat to useful purpose and consequently shows decided economical advantages over systems wherein the heating and power supplies are separate. Still further economies are possible by operating units having the best possible economy characteristics at the particular load range in question at any time.

As often happens when the electrical and exhaust steam requirements vary considerably, two dissimilar types of units must be installed to maintain satisfactory economy over the complete range of load demand on the plant. When the capacities of two such units are equal or overlap in some portion or the demand range, it is evident that there should be some load condition at which either or the units could be operated with equal economy; below which condition, one unit would show the better economy, and above which, the other unit would be more suitable.

The Virginia Polytechnic Institute Heating and Power Plant has one back-pressure turbine and one condensing, bleeder-type turbines. Either of these units is capable of carrying the present electrical load. When, due to electrical load limitations, the low-pressure steam available is insufficient, the reducing valves installed enable either of the units to satisfactorily supply this low-pressure steam demand. However, at high electrical and low low-pressure demands, the back-pressure unit will discharge part of its exhaust steam to the atmosphere, while the condensing unit will discharge the excess supply, over and above the low-pressure demand, into the condenser. In the condenser, the circulating water removes the latent heat of the steam. The problem, then, is one of obtaining a quantitative comparison of the two machines throughout the range of seasonal variation of both electrical and low-pressure loads.

The authors believe that the investigation will lead to fairly accurate results which, if used as a guide in operation, will aid considerably in preventing possible future fuel waste from the operation of the wrong unit for any particular set of conditions.