heating water

Figure 2-27 Figure 2-28
the mechanical room. The welded
steel piping connected to this heat
exchanger has also been insulated
and jacketed. Note the installation of
thermometers near all four ports of
the heat exchanger. Drain valves that
tee into the two lower pipes can also
be seen.
The frames used for plate & frame
heat exchangers are usually large
enough to accommodate more plates
than the initial design requires. More
plates can be added if the capacity
of the original system is increased.
Figure 2-28 shows an example of the
additional “rack space” available on a
large plate & frame heat exchanger.
The extended ends of the tension
rods are typically covered with plastic
tubes for safety.
Although plate & frame heat
exchangers can be disassembled for
cleaning or plate replacement, this is
a laborious task. It is always better
to use proper dirt separation details
in the system to prevent debris from
entering the heat exchanger.
As is true with all heat exchangers,
it is very important to pipe flat plate
heat exchangers so that the two
liquids pass through it in opposite
directions (e.g., counterflow). In most
cases, each fluid passage through
a flat plate heat exchanger leads
back to two connections, one above
the other, at one end of the heat
exchanger, as shown in Figure 2-19.
Flat plate heat exchangers have
several benefits relative to other heat
exchanger designs. They include:
• A very high ratio of surface area to
internal volume. This allows flat plate
heat exchangers to be significantly
smaller than a shell & tube or shell
& coil heat exchanger of equivalent
heat transfer capacity. It also allows
for a rapid response to temperature
changes in either fluid. Small brazed
plate heat exchangers can achieve
steady state conditions within a
few seconds after two stable fluid
streams (e.g., constant flow rate and
stable entering temperature) begin
flowing through them.
• The metal plates can be thinner
than the tubing used in other types
of heat exchangers. This reduces
the thermal resistance between the
two fluids. Later Sections will show
20