How Are Heat Exchangers Thermally Sized – a Design Guide
The design of heat exchangers is a complex process, which requires careful consideration of many factors. Heat exchangers are used to transfer thermal (heat) energy from one process fluid to another, and the size and configuration of a heat exchanger determine how well it will work. This guide will explain the importance of appropriate sizing, discuss the various factors to consider for heat exchanger maintenance, and provide guidance on designing an effective heat exchanger in Canada.
Overview of Heat Exchangers
Heat exchangers are devices used to transfer thermal energy from one fluid to another, which can be used for heating, cooling, power generation, waste heat recovery, and other processes. Heat exchangers are used everywhere, but we will focus on industrial and commercial applications.
Importance of Appropriate Sizing
Appropriate sizing and configuration of a heat exchanger is essential to ensure that it will be effective and economical. If the heat exchanger is too small it will not be able to transfer enough heat load resulting in underperformance. Further, it may create high pressure-drop and fluid velocities that cause damage via erosion and/or vibration. Conversely, if the heat exchanger is too large, it will be uneconomical, and fluid velocities may be low which encourages fouling. Heat exchangers with the wrong configuration (eg, co-current flow instead of counter-flow) may severely underperform regardless of sizing.
Factors When Sizing Heat Exchangers
There are various factors to consider when sizing heat exchangers, including calculating the heat load, considering terminal (inlet and outlet) temperatures of process streams, calculating the surface area, and assessing resulting pressure drops and velocities. Each of these methods must be considered when designing a heat exchanger.
Calculating Heat Load
The first step in sizing a heat exchanger is to calculate the heat load (duty). This is based on how much energy is required to heat or cool a specific process fluid from one temperature to another. This will help determine the size of the heat exchanger required to handle the thermal energy transfer. Heat load is usually expressed in BTU/hr or kilowatts (kW).
Calculating Required Surface Area
Once the heat load has been calculated, the surface area of the heat exchanger can be calculated. This is done by dividing the heat load (duty) by a U-Value (overall heat transfer coefficient) and the log-mean temperature difference between the two fluids. The result is the theoretical required surface area of the heat exchanger.
Assessing Pressure Drop
The pressure drop of a heat exchanger must also be assessed. Pressure drop is the difference (loss) in pressure between the inlet and outlet of a single process stream as it goes through the heat exchanger. The allowable pressure drop is typically given by the process engineer and must be carefully considered by the thermal design engineer when designing a heat exchanger in Canada. In general, a good heat exchanger design uses up the available pressure drop to maximize heat transfer and velocities.
When designing a heat exchanger in Canada, several factors must be considered. These include the type of heat exchanger (eg. shell and tube, plate and frame, spiral exchanger, hairpin exchanger, etc.), the materials of construction, flow rates, temperatures, vapour phase (eg. 100% liquid or vapour, or mixed), condensing or evaporating process, viscosities, fouling factors, and TEMA configurations, among other considerations.
TEMA Type of Heat Exchanger
The TEMA type of shell and tube type heat exchangers is selected based on the process streams, operating conditions, and anticipated maintenance requirements (eg, removable bundle or fixed). The type of heat exchanger can range from a simple BEU shell and tube heat exchanger to a more complex (and expensive) AES type.
Heat Exchanger Materials
The materials used in a heat exchanger must be selected to ensure that they are compatible with the processes and design conditions. Common materials for heat exchangers include carbon steel, stainless steel (300 and 400 series, duplex), nickel-alloys (Inconel, Hastelloy, C2000), Chrome-Moly (1-1/4Cr, 2-1/4Cr), and copper-alloys (Cupronickel, admiralty brass, etc).
Flow Rates and Configurations
The flow rates and configurations of the heat exchanger must also be considered. The flow rates will affect the size of the heat exchanger, and impact the realized velocities and pressure drops. The total configuration of the heat exchanger will determine its overall efficiency and performance.
Get Quality Heat Exchangers and Heat Exchanger Maintenance in Canada from Altex
Altex is a leading manufacturer, heat exchanger maintenance centre, and supplier of heat exchangers in Canada. We offer a wide selection of heat exchangers in a range of sizes and configurations, designed to meet the needs of any application. All of our heat exchangers are designed and manufactured to the highest quality standards, providing reliable and efficient performance. Contact us for more information about our heat exchangers, or to get a quote for your project or heat exchanger maintenance.