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
|A heat exchanger undergoing maintenance.|
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).
|A De-butanizer reboiler in operation.|
Effects of nature and properties of heat exchanger: Regarding the properties and nature of the heat exchanger, process personnel must pay particular attention to the chemical relationship between the heat exchanger materials of construction and the chemical nature of the fluid stream in transit. For example, process personnel would be ill advised to use a heat exchanger designed to handle cooling water for a hydrocarbon application, as the materials of construction would likely not stand up to the conditions of the application.
To ensure a long service life, process personnel should have a firm understanding of material properties and their corresponding effects at varying conditions. Further, process personnel should take special care in the operation and maintenance of the heat exchanger. For example, steady sampling and analyzing for metals, as well as monitoring surface thickness, is recommended.
As with any piece of process equipment, efficient and effective troubleshooting is key to long-term operational success. When considering a heat exchanger in particular, the first step in the troubleshooting process should be to make sure that the operating variables are maintained and controlled at the designed point. Table 1 provides guidance on troubleshooting two common heat exchanger problems.
Nwaoha Chikezie has previously worked as an operator (student trainee) with Port Harcourt Refining Company (PHRC, www.nnpc group.com/index.php) in Nigeria, and is currently working on several research projects involving flow systems design, including an initiative with the Caribbean African Student Exchange Initiative (CASEI). As part of his research, Mr. Chikezie has authored a number of engineering articles in leading international journals. Mr. Chikezie is a member of SPE, ASME, AIChe, IMechE, ICE, IGEM and Nigerian Gas Association (NGA). He can be reached at +234-703-135-3749, or firstname.lastname@example.org.