Chemical Feed Equipment
The purpose of
most chemical treatment control programs (other then certain biocides) is to
maintain a constant concentration in the recirculating water at all times. The
chemical feed pumps also referred to as metering pumps are used for this
purpose.
The
chemicals are fed from a bulk storage tank by variable stroke pumps, which can
be either manually or automatically adjusted in proportion to circulating water
blowdown. Because the pump has to inject a treatment chemical into what is
frequently a high pressure stream, common chemical feed pumps are positive
displacement units which use a reciprocating (piston) action to alternately
fill a pumping chamber and discharge a fixed volume of solution.
There are two
types of cooling water systems: 1) closed-loop systems and 2) open systems both
requiring different approach.
Closed loop
systems
In a closed
system, there is very little or no loss of water into a known volume as system
water remains in the piping. Little, if any, make-up water is required to maintain
a filled system as little or none is lost through evaporation or steaming as in
an open system where the water content of the system is open, at some point, to
the atmosphere. In addition, the closed loop systems are pressurized at all
times so that excess air can be eliminated through simple automatic air venting
devices.
The
water treatment in closed system is thus not very critical. Once the initial
volume is chemically treated, the quality of the circulating fluid needs to be
monitored on a regular basis and additional chemicals added as required to
maintain recommended residual concentrations of treatment chemicals.
Open loop
systems
In an open loop system such as cooling tower, there is constant
loss of water due to evaporation and a constant addition of makeup water into
the system which constantly changes the quality of the recirculating stream.
This constant change in the system quality may require frequent water quality
monitoring along with the frequent addition of chemicals to control corrosion,
scale formation, and growth of biological agents. Some combinations of
chemicals used may also change the pH of the system significantly which will
require pH monitoring and adjustment.
Because evaporator towers scrub the air that passes through them, they are prone to
absorbing oxygen and collecting debris from the air. The oxygenated water is
primary cause of corrosion and the debris can accumulate and cause flow
restrictions; as well as aggravate corrosion. Also, after the water evaporates,
dissolved minerals are left behind and accumulate rapidly. For these reasons,
the water quality in open systems must be regularly monitored and treated to
control the following conditions:
1. Lime scale and other water mineral deposits
2. Corrosion of all types
3. Micro-biological growth, such as algae, bacteria, fungus and
molds
4. Suspended solids accumulations, such as airborne dirt and debris that is washed into the cooling tower water.
