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. They are 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. By mode of service, there is cooler, condenser, exchanger, vaporizer, reboiler, etc. By design, there is 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. 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).
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.
The first step to troubleshoot a heat exchanger is to make sure that its operating variables are maintained and controlled at the designed point. The tables provide guidance on troubleshooting two common heat exchanger problems.
1.1 DIFFERENTIAL PRESSURE DOES NOT HOLD
Are the exchanger interiors confirmed?
Shut down, and inspect tubes and baffles.
Is the stream’s pressure adequate?
Check pressure gauge.
Is the heat exchanger leak tested?
Check status. Conduct leak test.
Is the stream purity on spec?
If so, take stream sample for analysis.
1.2 TEMPERATURES ARE ABNORMAL
Is the stream composition normal?
Analyze the sample.
Is the heat exchanger fouled?
Check status. Inspect heat exchanger for fouling.
Is the stream flowrate adequate?
Check flowmeter. Ensure flowmeter is showing an adequate flowrate reading.
Are the stream inlet and outlet temperatures normal?
Check inlet and outlet data. Confirm approach conditions.
Nwaoha Chikezie has previously worked as an operator (student trainee) with Port Harcourt Refining Company (PHRC, www.nnpcgroup.com/phrc.htm) 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.