Cooling Water Systems
What is water?
Water has some basic properties that make it ideally suited for use in cooling water applications. As a starting point water is safe enough be handled with ease, it is available almost everywhere and usually at a low cost. In addition it is very effective as a means to transfer heat.
However using water in a cooling system also produces less desirable side effects:
Corrosion
With the ability to dissolve many substances as well as gasses the cooling water can cause significant corrosion to the metal pipes.
Scale
As the cooling water circulates through the system it will inevitably reach a concentration that exceeds the solubility of the dissolved minerals in the water, which results in scale forming in the system.
Fouling and macrofouling
Foulants can come from external sources such as dust or fibres in the cooling air or from internal sources like by-products of corrosion. The build-up of suspended materials leads to deposits in the heat exchange equipment.
Macrofouling occurs in once-through systems and water intakes in lakes or rivers, where clams, mussels and other marine organisms enter the system and attach to the piping reducing water flow and increasing corrosion.
Biological contamination
Microorganisms such as bacteria, algae and fungi can form a thick layer of organic sludge in the cooling system. This layer greatly reduces the heat transmission as well as preventing the corrosion inhibitor from coming into contact with the metal thus risking further corrosion of the surfaces.
In order to avoid or minimize these detrimental effects and to ensure the cooling capabilities of the system, it is vital to perform proper water treatment and preventive maintenance.
Why use a cooling water system?
Since most of the processes involved in industrial production produce heat during operation, the ability to control the temperature of the process fluids is essential to ensure an efficient and continuous production.
Cooling water systems achieve this by transferring heat from the hot process fluids to the cooling water which is then cooled before being reused or led away and replaced by fresh makeup water.
Why is treatment important?
With modern production being heavily reliant upon efficient heat transfer from the processes involved, it is clear that when a cooling system is unable to remove the heat, the entire production suffers and as a result the costs increase.
As well as increasing the costs a poorly maintained cooling system also has a direct adverse effect on the environment as it leads to excess water consumption and wastewater.
|
Cost/kWh U.S. Dollars |
0,25 mm Thickness |
1,27 mm Thickness |
2,54 mm Thickness |
|
$ .04 |
$ 8.000/yr |
$ 42.200/yr |
$ 80.400/yr |
|
$ .06 |
$ 13.200/yr |
$ 63.300/yr |
$ 120.600/yr |
|
$ .08 |
$ 17.600/yr |
$ 84.400/yr |
$ 160.800/yr |
What are the sources of cooling water?
Fresh water
Fresh water constitutes the primary source of makeup water for cooling water systems. It can be drawn from surface water such as rivers, lakes or reservoirs, or from ground water in shallow or deep wells. As surface water is more directly affected by rain, erosion and other environmental conditions, it usually contains more suspended matters and is generally more varied in temperature and composition than ground water. Surface water however also contains much lower concentrations of iron and manganese, both of which can cause fouling in cooling systems.
Salt water and wastewater
Much less common than fresh water, salt water and wastewater are still being used as sources of cooling water because of both cost and availability but also due to environmental considerations.
Using these sources of water in a cooling system without any kind of water treatment would cause the cooling capabilities of the system to decline very rapidly.
Marine fouling – or biofouling – is an unwanted growth and development of biological organisms like mussles, algea, bacterial slimes etc. on wetted surfaces within water systems.
Bio-fouling is divided into:
- Micro-fouling – biofilm formation bacterial adhesion. Appears as layers of bacterial slimes.
- Macro-fouling – attachment of larger organisms (barnacles, mussels, seaweed, etc.).
Together, these organisms form a fouling community.
Without electrolythic antifouling the marine growth enter seawater systems and find spots where temperature, nutrients, pH factor and other environmental conditions are right for settling and breeding. Underneath barnacles is a favorite site for pits to start and the deepest corrosion pits are quite often found there.
The fouling contaminates the water system resulting in:
- Reduced heat transfer in heat exchangers or condensers
- Increased pressure drop in pipelines
- Necessary system cleaning
- Evolution of corrosive gasses
What are the important water properties in cooling water systems?
As they have a direct impact on the four main problems in cooling water systems, being corrosion, scale, fouling and bacteria.
The following properties are important when determining the treatment program of the cooling water system:
Conductivity
Conductivity is measured in µS/cm and is an indication of the water’s ability to conduct electricity. It can vary from a few µS/cm for distilled water to over 30.000 for sea water. Since a treatment programme only functions within a certain range of conductivity, it becomes vital to be able to control the level of conductivity in the system.
pH
Measuring the pH level in the cooling water gives an indication of the relative acidity or basicity of the water with 0 representing maximum acidity and 14 representing maximum basicity. At high pH levels the formation of scale will be accelerated, while low levels will increase the metal corrosion. In addition the effectiveness of many biocides is dependent on the pH.
Alkalinity
The amount of carbonate (CO3-2) and bicarbonate (HCO3-) ions in the water is a measurement of the alkalinity in the water. As with pH high or low levels of alkalinity result in scale and corrosion.
Hardness
Refers to the amount of calcium and magnesium ions present in the water. It is measured in parts per million (ppm) and can vary from a few to over 800 ppm. Water treatment programmes require a specified range of hardness to be present in the water before functioning correctly.
What are the cooling water systems and how do they differ?
In general only three basic designs are used:
- Open recirculating system
- Once-through system
- Closed recirculating system
Open recirculating system
This design is the most widely used in industrial cooling. Basically it consists of a cooling tower, heat exchanger and a pump. The pump circulates the cooling water through the heat exchanger, where the heat from the process fluids is transferred to the cooling water. The warm water is pumped through spray nozzles in the cooling tower where the heat is released from the water through evaporation.
Once-through system
Less used than recirculating systems, the cooling water is only led through the heat exchanger once. This means that while the mineral content and concentration in the water remain virtually unchanged, the water consumption is extremely large.If the outlet cooling water is led into rivers or lakes, temperature pollution can cause problems.
Closed recirculating system
In a closed recirculating system the same cooling water is used in a continuous cycle. Instead of having an evaporative cooling tower, the closed circulating system basically consists of two heat exchangers and a circulating pump.
The combination of evaporation and blowdown require a substantial volume of cooling water to be replaced by makeup water continually, thus changing the basic chemistry of the water. This means that more chemicals must be present in the system, however as the water circulates in the system and the concentration of dissolved and suspended solids increases, so does the concentration of treatment chemicals. Therefore, after the initial higher dosage, only a moderate dosage is necessary to maintain the level of chemicals.
As it is a closed system and the water consumption is negligible, the basic composition of the water remains fairly constant. Ideally the system is filled once and a minimal amount of water is lost from the system.
