Shopping for a new heat exchanger? You have come to
the right place. Here you can learn about the different types of
exchangers and then order one by
calling one of the many reviewed manufacturers and retailers of heat
exchangers.
Heat Exchangers: An Introduction
Heat exchangers provide a simple purpose that is controlling a system or substance’s temperature by adding or removing thermal energy. While there are many different sizes, levels of sophistication, and types of heat exchangers, they all use a thermally conducting element generally in the form of a tube or plate—to separate two fluids, such that one can transfer thermal energy to the other. Home heating systems use a heat exchanger to transfer combustion - gas heat to water or air, which is circulated through the house. Power plants use locally available water or ambient air in quite large heat exchangers to condense steam from the turbines. Many industrial applications use small heat exchangers to create or maintain a required temperature. In industry, heat exchangers do many tasks, ranging from cooling lasers to establishing a controlled sample temperature prior to chromatography. Anyone who wants to use a heat exchanger deals with a basic challenge: fully defining the problem to be solved, which requires an understanding of the thermodynamic and transport properties of fluids. Such knowledge can be in addition to some simple calculations to define a specific heat-transfer problem and select a suitable heat exchanger.
How heat gets transferred from one fluid to
another depends mainly on the physical characteristics of the fluids
involved, particularly their density, specific heat, thermal
conductivity, and dynamic viscosity.
Density is a fluid’s mass per unit volume, measured as lb m/ft (where lb
m represents pounds of mass) or kg/m. Density can be used to convert a
measurement from a mass-flow rate, such as lb m/hr, to the more common
volumetric units, such as gallons per minute for liquids, or cubic feet
per minute for gases. All through a heat exchanger, the mass-flow rate
remains constant, however changes in temperature and pressure can change
the volumetric flow rate, mainly for a gas. Therefore a gas flow should
be stated as a mass flow, a volumetric flow at standard conditions, or
as a volumetric flow including temperature and pressure. Moreover, the
operating pressure should always be spelled out.
Specific heat (c or cp for a gas, where p
represents a constant pressure) is the amount of heat required to raise
the temperature of one unit of fluid mass by one degree. Its units are B
T U / (lb m°C) or J/(kg °C). Specific heat relates the quantity of
transferred heat to the temperature change of the fluid while passing
through the heat exchanger.
Thermal conductivity (k) represents the ability of a fluid to conduct
heat. It is measured in BTU/[ft 2hr (°F/ft)], BTU/(ft hr °F), or W/(m
°C).
Dynamic viscosity specifies a
fluid’s resistance to flow. A fluid with high dynamic viscosity produces
a high pressure loss on account of the shear resistance, mainly along
the heat exchanger surfaces. Its units are lb m/(ft hr), (lb f hr) / f t
(where lb f is pounds of force), kg/(m s), (N s)/m, Pa s, and many
others. The selection of units generally depends on the industry,
however they can be converted to one of the above forms. In most cases,
viscosity is given in centipoise [1 centipoise =1,000 Pa s= 2.42 lb
m/(ft hr)].