Heat exchangers are typically employed in the process industries as a means of providing heat transfer between two streams of fluid across a medium. The heat exchanger ensures the conservation of heat energy otherwise known as heat economic operations. The heat exchanger is designed to foster contact between materials in a conduit network, with one material exchanging heat and the other material flowing within the network either counter-currently or co-currently. Heat exchangers can be classified by mode of service or by design. Mode of service classifications include cooler, condenser, exchanger, vaporizer, reboiler, etc. Design classifications include shell and tube, finned-tube, etc.
Effects of Operating Variables
To optimize and improve heat exchanger performance, process personnel must operate the exchanger within its designed and specified limits. Also, personnel must identify those operating parameters that can affect heat exchanger performance. Key operating parameters to monitor include feed material, high degree of fouling, poor maintenance culture, climatic effects, etc. This article will focus on the main control points, including heat exchanger operating pressure, heat exchanger operating temperatures, and the nature and properties of the heat exchanger.
Effects of heat exchanger operating pressure: The pressure differential between the suction and discharge of each fluid stream is the main driving force of that stream. The pressure differential is affected by fluid flow rates, pipe surface friction, number of heat exchanger passes, bulk density and viscosity. Deposits, if present, reduce the available surface area and increase the pressure differential, thus resulting in inadequate flow. If a pressure difference is noticed, the system should undergo troubleshooting to identify the cause (Table 1).
Effects of heat exchanger operating temperature: The heat exchanger operating temperature affects heat exchange. In refineries, stream temperatures can vary due to changes in the operating procedures. Any alterations in the stream temperature will create a variation in the approaches; the exchanger duty and log mean temperature difference. A low approach difference will give a corresponding log mean temperature difference, and high load vice versa. When the operating temperature limits are exceeded, the material condenses as a result of deposits and coats the internals of heat exchangers, which produces a wall temperature that is lower than the bulk limit temperature. To maintain the operating temperature, the inlet and outlet temperature must be monitored (Table 1).
