The well-known expression for availability, b = h − T0s, is developed in a simple yet general manner. The changes in availability throughout several typical single and double reheat steam plants having different steam conditions have been calculated. Balances of availability are presented for each of the several plants. These balances show the net plant output as the difference between the availability added in the boiler and the various losses in availability throughout the plant.
The maximum plant thermal efficiency (or minimum heat rate) is easily obtained from the ratio of the increase in availability to the heat added in the boiler.
The availability balances present in fine detail the loss distribution throughout the plant. Some very interesting results are obtained. For example, an assumed average plant will have a calculated maximum thermal efficiency of about 75 per cent based on the ratio of the availability and the heat in the high temperature combustion gases. Due to the heat transfer through a temperature difference, this value is reduced to 50 per cent when based on the ratio of the availability and the heat added to the steam in the boiler. This average plant will lose an additional 10 points in thermal efficiency, due to internal losses in availability, resulting in a 40 per cent thermal efficiency (excluding boiler and auxiliary losses). These latter 10 points due to internal losses are roughly divided as follows:
Pressure drop: 0.9
Turbine blading: 5.0
Turbine leakage: 0.6
Turbine leaving and hood: 1.0
Mechanical and generator: 0.7
Feed and healing system: 1.3
Feed pump and motor inefficiency: 0.5
An availability balance enables one to make an accurate assessment of the losses in any power plant and therefore serves as a very accurate guide in directing development effort toward reducing the more significant losses.