Methods of checking steam condenser performance
It is desirable to get a rated load or some agreed-upon load on the turbine which will be the same for each successive check and read the load, air leakage, inlet water temperature, outlet water temperature and the absolute pressure in the condenser, and convert the absolute pressure into the corresponding saturated steam temperature; also, calculate the temperature rise, initial temperature difference and terminal difference.
The terminal difference is the difference between the steam temperature and the outlet water temperature; the temperature rise is the increase in the CW temperature. The initial temperature difference is the difference between the inlet water temperature and the steam temperature (or the saturation temperature corresponding to the absolute pressure).
If this data is recorded periodically and checked, any deviation will give the operator the best indication of what has been happening to his condenser.
During a period of high air leakage, when air blankets tube surfaces, the absolute pressure, air leakage, steam temperature and terminal difference will rise and again upon correcting the leakage, will return to normal. Also, during a period of dirty condenser tubes, the absolute pressure, steam temperature and terminal difference increases and after cleaning will return to normal. This holds true with non-condenser type heat exchanger.
Other factors, which would affect the condenser trend, are the change in water inlet temperatures and change in loads. These changes do not affect the condenser performance although they change most of the condenser temperature values and do change the condenser backpressure. As a guide to condenser performance the terminal difference gives the operator the alarm and should be watched carefully.
Data taken at a different lad may be compared to that at the desired load by the following device. Load to the condenser, either in terms of BTU per hour or kw load plotted against rise and initial temperature difference. “Rise” will be a straight line from zero at no load to maximum rise at full load. Disregarding the effects of air leakage and vacuum pump capacity and some of these matters in the very low load range, the initial temperature difference will also be a straight line.
Operators should watch pump discharge pressures and pump horsepower for clues as to tube sheets plugged by rubbish accumulation. These indications can also be obtained from delta P of CW across the condenser.
Poor heat transfer due to tube fouling will affect vacuum performance at all loads but will be most noticeable at high loads.
Circulating water pump data to approximate water flow, which with rise in circulating water temperature gives another approximation of heat load.
The condenser performance is evaluated, expressed as percentage:
% = U actual >< 100
U actual can be calculated as: U = Q/A∆Tlmtd
U actual = actual heat transfer rate, btu/hr/sq ft/deg F Lmtd
Q = duty, Btu/hr