For any heat exchange application, there are four resistances to heat transfer encountered:
- The heat exchanger tube wall.
- The inside film resistance.
- The outside film resistances.
- An additional fouling resistance.
For metal heat exchangers, the tube wall provides very little resistance to heat transfer. The surface and fouling resistances have the greatest effect on the overall heat transfer coefficient.
Plastic heat exchangers generally are less efficient than metal ones because the tube wall is the greatest resistance to heat transfer. However, metal heat exchangers have a much greater tendency to become fouled than do plastic exchangers. Fouling greatly reduces the overall heat transfer coefficient of metal exchangers.
As an example, when comparing metal with plastic exchangers, assuming no fouling of the tube wall, a plastic exchanger will require approximately six times as much heat transfer surface as a metal exchanger. When normal fouling of the tube wall is considered, this ratio is overall heat transfer coefficient is not directly related to the thermal conductivity of the exchanger tubing. For example, the ratio of thermal conductivities for stainless steel and PVDF is 900:1, but the ratio of heat transfer coefficients is approximately 3:1.
The relatively low thermal conductivity of the plastic material will be less significant if the outer and inner film become the limiting resistances. Such conditions are encountered in the following applications:
- One of the heat exchange fluids is highly viscous and flows at low velocity (open tank applications).
- Heat is exchanged between a liquid and a gas.
- Heat is exchanged between two gases.
- Heat is exchanged between a condensable and a non-condensable.
Basic CalculationsThe integrated steady state modification of Fourier's general equation is accepted as:
Q = U x A x ΔTm
A is the heating surface (ft2)
Q is the total heat transferred (BTU/hr)
U is the overall heat transfer coefficient in(BTU/hr-ft2-oF)
ΔTm is the mean logarithmic difference in temperature between hot and cold side (oF)
Using variations of this basic equation, it is possible to readily calculate the heat exchanger surface needed, the time to accomplish a heatup operation, or the temperature of a fluid bath at the conclusion of a preset period of time for the basic heat exchanger applications such as maintaining a constant batch temperature using condensing steam; condensing steam to heat up an aqueous chemical solution; circulating hot water to heat up an aqueous chemical solution; or circulating cold water to cool an aqueous chemical solution.
To determine the heat exchanger surface area needed, use the following equation:
Q / Tm • U
t is the tubing wall thickness (ft)
k is the thermal conductivity of tubing material (BTU/ft-hr-oF)
The typical steps for heat exchanger calculations are:
- Determine heat losses from open top and tank walls.
- Determine heat loss or gain from addition of liquids or metals to tank.
- Determine heat load to heat or cool the bath liquid, if a time limit for heatup or cool down exists.
- Calculate the required and available heating or cooling capacity.
- Establish in/out temperatures for both liquids (or media) and calculate the log mean temperature difference.
- Calculate or estimate overall heat transfer coefficient (U).
- Calculate required heat transfer area.
- Size the module(s) according to tank dimensions and calculated heat transfer area.
- Check for tolerable pressure drop through module(s).
Heat losses to be considered include total heat to be given or taken away from the bath or tank; losses from the top surface of an open tank or vessel; losses through the tank wall; losses from heating/cooling any materials being treated and/or added to the bath; losses due to makeup fluids being added; and losses due to splashing of treated materials.
George Fischer Inc., Tustin, Calif., manufactures plastic heat exchangers, piping and fittings. For more information on plastic heat exchangers from George Fischer, call (714) 731-8800 or visit www.us.piping.georgefischer.com.
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