Man L23/30DF Project Manual Download

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Description

This guideline specifies the basic demands made

on cooling water for cooling tower operation.

Should the cooling tower manufacturer make fur-

ther demands on the water quality, these require-

ments must, by all means, be observed.
Moreover, it must be taken into consideration that

additional demands will be made on the water qual-

ity depending on the material of the coolers, which

are applied with water. Additional requirements for

the cooling water made by the cooler manufacturer

must also be observed.

General

The raw water system with cooling tower re-cooling

concerns an open circulation system, which dissi-

pates the heat absorbed from the water by evapo-

ration into the cooling tower. This results at the

same time in a continuous water loss due to evapo-

ration. In order to restrict the incurring salt concen-

tration, a certain water amount must permanently

be topped as additive water.
Water losses due to evaporation and blowing down

(depending on the additive water quality) may

amount up to 3 % of the circulating water quantity.

Blowing down

An increasing evaporation loss results in a higher

concentration of the salts and the suspended sub-

stances in the water and, therefore, in an increasing

tendency to corrosion and the formation of deposits

in the system. In addition, the raw water absorbs

impurities from the ambient air. Deposits have a

negative effect on the heat dissipation in the coolers

and the control system function.
In order to avoid excessive concentration, a part of

the thickened circulating water must be removed

from the circuit and be replaced by less concentra-

ted additive water. Blowing down has a regulating

effect on the concentration constituents of the cir-

culating water. The amount of the water to be

exchanged depends on the water quality and has to

be chosen as to ensure constant compliance with

the limit values specified for the circulating water

(see Table 1).

Additive water

The system water losses caused by blowing down,

evaporation or leakages must be replaced by con-

tinuous additive water topping during operation.

The required amount of additive water depends on

the quality of the additive water and the climatic site

conditions.
Certain demands have to be made on the additive

water quality, which is based on the requirements

for circulating water taking the concentration degree

into consideration. If the required water quality can-

not be achieved, the water has to be treated chemi-

cally (e.g. softening or hardness stabilisation) or

mechanically, if necessary. Otherwise
• deposits due to precipitation of hardly soluble

salts,
• sediments of disperse solid substances,
• corrosion,
• growth of micro organisms
are to be expected.
The cooling tower should, at least, be run with a

concentration by factor 2. Higher concentrations

are, in general, more economic. In order to permit

this, the content of substances must not exceed

half of the amount of the contents permitted for cir-

culating water. For the absolute minimum require-

ments,

see Table 1.

Water treatment

Depending on the water quality, various treatment

processes come into consideration:
• Decarbonisation, acid injection
• Desalinisation
• Cooling water conditioning (chemical treatment).
By using special chemicals, so-called stabilisers

and conditioners, deposits and corrosion in the

cooling water circuit can largely be controlled.

These means permit operation at increased con-

centration and, therefore, a reduction of the

required additive water.
When using chemical additives for cooling water

conditioning, the cooling tower manufacturer is to

be contacted.

MAN Diesel & Turbo

1699250-5.0

Page 1 (2)

Quality of raw-water in cooling tower operation

(additive and circulating water)