08-08-2012, 12:01 PM
Plate Heat Exchanger
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Determination of characteristic settling time
Adjust the set point temperature to a temperature around T = 60C. Set the flow rate to an intermediate
value, ˙V = 300lph. Through out the measurement make sure the flow rate is at this value. Measure the inlet
and outlet temperatures for about 15 minutes at 30 second intervals. Use a graph sheet to plot the variation
in temperature. Use this plot to obtain an estimate of the time it takes for the inlet and outlet temperatures
to settle down to a constant value or to a constant periodic oscillation. Note down if there is any time lag in
the behaviour of the outlet temperature variation with respect to that of the inlet.
Both these readings, settling time and time lag, should be used in the main experiment: The readings
should be taken down only after the settling time (usually one or two time periods of oscillation) and the
outlet temperature after the time lag.
Theory
The plate heat exchanger normally consists of corrugated plates assembled into a frame. The hot fluid
flows in one direction in alternating chambers while the cold fluid flows in true counter-current flow in
the other alternating chambers. A schematic diagram of the flow is shown in Figure 1. The fluids are
directed into their proper chambers either by a suitable gasket or a weld depending on the type of exchanger
chosen. Traditionally, plate and frame exchangers have been used almost exclusively for liquid to liquid
heat transfer. The best example is in the dairy industry. Today, many variations of the plate technology have
proven useful in applications where a phase change occurs as well. This includes condensing duties as well
as vaporization duties. Plate heat exchangers are best known for having overall heat transfer coefficients
(U-values) in excess of 3–5 times the U-value in a shell and tube designed for the same service.
Plate heat exchanger is an attractive option when more expensive materials of construction can be employed.
The significantly higher U-value results in far less area for a given application. The higher U-values are
obtained by inducing turbulence between the plate surfaces. Owing to this they are also known to minimize
the fouling.
Plots
The following data need to be plotted. A sample calculation to obtain the values of the variables is shown
below.
1. Plot of the temperature transient for the hot fluid (at the outlet and the inlet) when the heater is turned
on till the temperature attains a steady state value
2. Plot of 1/U vs 1/u1/3: In this plot, for each value of flow rate, U values corresponding to the three
independent readings should be shown, apart from the U value computed from the average of the
temperature measurement. The average should be used for fitting and computing the intercept. Since
this involves several calculations, it is suggested that programmable spreadsheets be made use of
(OpenOffice.org or MS Exel)