Thermal Analysis Technique for Coolers Operating in Off and Low Load Conditions

Abstract

The most conservatively designed power plant heat exchangers are designed to meet a maximum heat load with minimum fluid temperature differences. When the input temperatures are less than design maximums, the cooler will usually be in a position of over performance. This relationship is especially true when the heat exchanger is a closed Component Cooling Water (CCW) heat exchanger with inlet fluid at ambient conditions. Maintaining a consistent cooling temperature is an important concern in the operation of a power plant. It is important that the cooling needs of the equipment such as the hydrogen coolers are maintained at a set temperature. Overcooling may not be of benefit to the equipment. The component which cools the service water with the local cooling water is a component cooling water heat exchanger (CCW). The two primary methods of controlling the heat rejection performance on these vessels is to throttling the tubeside flow to get a consistent shell outlet temperature with control valves or leave the tubeside flow constant and by-pass a portion of the shellside flow. Estimating the performance of the heat exchanger with given set of inlet conditions and a fixed design point can be accomplished using a the Number Transfer Units (NTU) method. Opening and closing the control valve is based on the estimated performance. This analysis can be used by power plant personnel to gauge the operation of these vessels over varying operating conditions. The analysis can also include the effect of different values of cleanliness and the extent of throttling. As a unit experiences fouling, additional flow is required to meet the thermal requirements. Depending upon the extent of fouling, the inlet valve will be either opened or closed. Plant personnel may observe the cooling water inlet temperature and the extent to which the inlet valve is open, and use that information to determine possible fouling and setup a maintenance schedule. The following analytical approach for evaluating low, critical, or off load conditions is important in the design and operation of these types of power plant heat exchangers, piping and control valve systems.