Page 218 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

216 Chapter 7 Industrial cooling systems

COC usually, range from three to seven in petroleum refinery cooling towers and may be much
higher in some large power plants. While a high COC reduces the makeup water requirement of
the cooling tower, it allows higher dissolved solids concentration in circulating cooling water,
which results in scaling and fouling of heat transfer surfaces.

Holding Capacity or System Volume (HC) is the amount of water held up in the cooling water
system expressed in cubic meters. This includes the holdup in the basin, additional sump if any, and all
associated equipment and circulating water piping.
Time per cycle is defined as the time taken for all the water held up in a system (HC) to make one
trip around the recirculating loop (from the discharge side of the recirculation pump back to the suction
side of the pump). Mathematically,
Time per cycle ¼ HC=C (7.11)
Holding Time Index or Half-Life index (HTI) indicates the time required to reduce the chemical
or makeup water added to a system to 50% of its original concentration. It is essentially the half-life of
a chemical added to the system and is estimated based on the assumption that the rate of decrease in
concentration of the chemical at any instant is proportional to its concentration. The expression for
calculating the holding time index usually reported in hours is e
HC
HTI ¼ 0:693 . (7.12)
ðB þ DÞ
The HTI is important for a chemical treatment program and is also used to determine the
requirement of some biocides to achieve proper control of microorganisms.

7.2.5 Codes and standards

Cooling Technology Institute (CTI), USA, established in 1950, is a body that has standardized the
cooling tower design (Industrial Cooling Tower Standard - STD-203), testing (STD-202), and several
other aspects. CTI codes are among the most popular codes used these days. To properly select a tower,
the designer should consider towers with CTI certified listing as this is the most widely accepted
standard.

7.2.6 Thermal design
Quantitative treatment of cooling tower performance by separately dealing with mass and heat transfer
is laborious. Therefore, the simplifying approximation of Merkel’s total heat theory has been almost
universally adopted for cooling tower calculations.
Briefly, Merkel’s theory states that all of the heat transfer taking place at any position in the cooling
tower (dQ) is proportional to the difference between the enthalpy of saturated air at the temperature of
0
the water (T) at that position in the tower (h ) and the enthalpy of the air at the same location (h).
Mathematically,
0
dQ ¼ Kaðh hÞdV (7.13)