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Towers Q & A

Why Is Cooling Water So Important?

Industry is second only to agriculture as a consumer of fresh water. But did you know that about 70% of the water that industry uses is for cooling purposes? Although power generating stations are among the largest users of this cooling water, almost all industry incorporates processes which require the dissipation of heat. In addition to these process cooling requirements, comfort air conditioning systems also consume large quantities of cooling water. Power cannot be generated, gasoline cannot be produced, chemical processes cannot operate, and your boss cannot be kept cool without the assured availability of cooling water.

Cooling water is the most common substance used as a heat transfer medium and the most common method of use is by indirect heat exchange.

What Is Meant By Heat Exchange?

In the case of a cooling water system, heat exchange is the method by which the unwanted heat generated by the process is removed from the system. Failure to remove this heat would result in failure of the process. In order for the heat to be dissipated, the cooling water must be of suitable temperature, quality, and quantity. Each user needs to define their cooling water applications, identify the most critical heat exchangers, and determine the quality and quantity of cooling water required for each system. This can be a formidable task and the assistance of a well qualified water consulting engineering firm is recommended. In power generation plants, the main condenser demands the greatest amount of cooling water. In this case, low pressure steam from the turbine must be condensed before being returned to the cycle. Inefficient cooling can result in reduced efficiency of the power cycle and thus be extremely costly.

Where Does The Cooling Water Go

Having removed the heat from the system, the cooling water must now either be itself cooled for recycling or it must be wasted to an outside source such as a river or lake or ocean. In the old days, fresh water was plentiful and there was no problem with taking from nature all that was needed and then just returning the warmed water to the source. But those days are, for the most part long gone. Fresh water is a scarce commodity to most of us. That means that it has a relatively high cost. As though that were not enough reason to conserve water, the chemicals we add and the thermal load make this water even less desirable to nature. And that means environmental restrictions.

And that means the added cost of waste disposal of our spent cooling water. It is little wonder then that the water conserving use of cooling towers for recirculating cooling water has come into such universal favor.

What Does A Cooling Tower Do?

Heat is removed in a cooling tower in two ways. First, but less, is by sensible heat transfer. By this we mean simple heat exchange between the air and the water. heat transfer rules that involve the relative temperatures of the two fluids and the way in which they are inter-mixed are at work here. Since the amount of such heat transfer depends on the temperature difference between the air and the water, the amount of sensible heat transferred varies by the season of the year. But on the average, only about 20% of the heat is transferred in this manner.

The bulk of the heat, about 80% is transferred by the evaporative heat transfer mechanism. In this case, the latent heat of evaporation is responsible for removing the heat from the bulk cooling water. For each pound of water evaporated, approximately 1000 But of heat is removed. Depending upon what the temperatures are, the cooling water can be reduced to below the ambient air temperature. Make-up water equal in quantity to this evaporation must be provided regardless of what other water losses might occur from the system. These other losses include drift, un-controlled leaks, and deliberate bleed from the system. The bleed, or "blowdown" is normally the only recirculating cooling water loss that can be controlled. Your consultant or water treatment supplier should be capable of performing routine heat and material balances around your cooling water systems.

How Is Water Affected by the Evaporative Process?

While all waters are potentially corrosive those with increased salt content have increased corrosion potential Just as the salts concentrate in a tea kettle when water is boiled so to do they concentrate in a recirculating cooling water system
But the salt concentration effect can have a different result. The concentration process and the resulting system pH can cause certain compounds to exceed their solubility limits. Given sufficient time, this in turn will cause scale to form. Indeed, if left untreated or if treated improperly, scale and corrosion can, and often do, occur in the same system. This is why it is not possible to successfully treat a recirculating cooling water system using the classical pH balancing method

As though scale and corrosion were not enough to cause us concern, various insoluble material invariably enters the water. Cooling towers are very good air cleaners. In addition iron oxide and various other substances can be present from internal corrosion or from entry with the make-up water. These suspended materials can cause <B>deposits</B> which will restrict cooling water flow through piping and heat exchangers

And then there are the bugs. Biological deposits thrive in the air rich, warm waters of cooling towers. From algae on the deck to slime in the heat exchangers, biological deposits can seriously hamper the efficient removal of heat. Control of this major problem can be your most expensive chemical treatment cost. It can also be the most environmentally suspect.

What is Cooling Tower Fill?

While we will not discuss here the many designs industrial cooling towers can take, some mention of fill is appropriate because of its criticality to the process. Fill is a structured surface or baffles within the tower that provide a very large surface area for contact between the water and the air. Provisions are made to assure the water is uniformly distributed as it passes through the fill.
Improving heat transfer by increasing the available surface area of this fill, also called packing has been the sought for goal of tower manufacturers in recent years. At one time, fill material was almost always redwood. More recently,

What is Cooling Tower Material Balances?

Water Conservation is one of the main benefits of using a cooling tower.
Preventing deposition or scale formation, silica for example, is imperative to making it work.

Proper chemical dosing will control corrosion and certain scales and deposits.

Biological control treatments are often based on system volumes and/or residence times.

All of the above require a knowledge of how to perform a simple material balances around the system.

What is Cycles of Concentration?

This term is used to define the  critical concentration of the cooling water based on the analytical concentration of the make-up water. For example, assuming there was no other source of chloride and that the make-up water contained 20 mg/l chloride the cycles of concentration would be ten if the cooling water contained 200 mg/l chloride. This also corresponds to the ratio of volume of make-up water divided by volume of wastage (Where wastage is defined as water lost from the system by controlled blowdown, uncontrolled leaks, and drift). For this< same cycles of concentration of 10, make-up water of 100,000 GPM would result in wastage of 10,000 GPM

The goal of your cooling water treatment program must be to reduce blowdown without increasing the tendency toward corrosion, scale, or deposits. This will result in reduced costs for make-up water, for disposal of blowdown water, and for treatment chemicals.