Page 351 - HVAC Pump Handbook

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Pumps for Process Cooling

348 Pumps for Open HVAC Cooling Systems

processes use heat exchangers of the shell and tube or plate and
frame types that are cooled by cooling tower water. The piping of such
heat exchangers is similar to that for heat exchangers in the HVAC
field; however, there are some conditions that may be found in these
industrial processes that require special circuiting of the heat
exchangers. There are some industrial processes that use heating or
cooling coils for gas streams. The pumping and piping connections of
these coils are much like those for HVAC heating and cooling coils.
The pressure drops across industrial heat exchangers may be
greater than those of HVAC applications because of the cost of the
heat exchangers. It is imperative that the designer determine exactly
the true pressure drop across the heat exchanger and its control
valve. The manufacturer of such process equipment may provide a
higher overall pressure to ensure that adequate water is available to
the equipment.
Some industrial processes may contaminate the cooling tower
water, and it may be necessary to provide cleaning of the water before
it is returned to the cooling tower. Figure 12.1a describes a cooling
system where the return water is collected in a trough and returned
to a hot well where it is cleaned before being pumped through the
cooling tower. The primary-secondary pumping described in this figure
allows for constant flow through the cooling tower and variable flow
to the process loop. This provides excellent cooling tower operation
throughout the year. A bypass valve is shown for bypassing the water
around the tower under very low process loads in freezing weather.
One method of regulating this valve is through the installation of a
temperature transmitter on the return water line to the tower. A pres-
sure transmitter at the end of the loop controls the system pump
speed and maintains the needed pressure in the supply header. The
set point can be varied with the load or type of cooling being carried
on in the loop.
Other industrial processes may be such that a typical closed loop
can be used on the cooling tower water (Fig. 12.1b). If there is still
some contamination of the cooling tower water, cleaning equipment
can be provided at the cooling tower pumps. With the closed loop, a
differential pressure transmitter should be used instead of a pressure
transmitter for controlling pump speed to maintain the desired differ-
ential pressure for the heat exchangers. This is due to the variable
pressure drop in the return header. On some applications, the pressure-
gradient diagram (Fig. 12.2) may be helpful in evaluating the total
installation.
The hot-well, cold-well sump of Fig. 12.1a may be required in cold
areas where the heat from the process load is a variable. The bypass
valve of Fig. 12.1b may be acceptable as long as there is no possibility

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