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Variable affecting performance of CT Heat transfer - Cooling Tower

The rate at which heat is transferred in a cooling tower depends upon four factors:

  • the area of the water surface in contact with air;
  • the relative velocity of air and water;
  • the time of contact between air and water;
  • the difference between the wet bulb temperature of the inlet air, A, and the temperature of the returned water, R.

Item (1) depends upon the construction of the fill; (2) can be controlled by regulating speed of the fans; (3) is a function of (2) and the height of the tower; (4) is fixed by climate.

Wet bulb temperature can be measured with a sling psychrometer.

Under ideal conditions, when a stream of unsaturated air passes over a wetted surface water evaporates saturating the air and lowering the temperature of the remaining water. When the water becomes cooler than the air, sensible heat flows from the air to the water, eventually reaching equilibrium at the wet-bulb temperature, where the loss of heat from the water by evaporation is equal to the sensible heat. Thus, as water falls through a cooling tower, the latent heat of vaporization and the sensible heat approach each other so that in an infinitely high structure the temperature of the bulk water would be equal to the wet-bulb temperature of the entering air. In a finite tower, however, it is impossible to achieve zero approach (approach = Supply temperature – wet bulb temperature) because not all the water falling through the structure can contact fresh cool air.

One measure of the efficiency of a cooling tower is its approach, which is the difference between the temperature of the cooled water in the basin of the tower and wet-bulb temperature of the atmosphere.
The second measure of performance is the cooling range, which is the difference between the supply temperature and return temperature.

The amount o heat rejected by a cooling tower can be calculated from the cooling range and the recirculation rate. This is also known as heat duty.

1 BTU is the amount of heat required to raise the temperature of one pound of water one degree F. Therefore:
Heat duty, BTU/hr = gpm (circulation rate) >< 8.34 lb/gal >< Delta T deg F

Another important characteristic of a cooling towers performance is L/G, the liquid-gas mass transfer ratio: L/G = (water, kg/hr)/(air, kg/hr)

Heat Transfer in Cooling Tower

Two kind of heat transfer occur within the tower between warm water and air.

Latent heat of vaporization: Some of the liquid changes to vapor with the absorption of heat. This energy, called the latent heat of vaporization, is that necessary to overcome the attractive forces between molecules in the liquid state. Absorption of latent heat accounts for 75-80 percent of the heat transferred in cooling towers.

As long as the wet-bulb temperature, which is a measure of the heat content of the atmospheric air, is lower than the water temperature heat is transferred from the water to the air, raising its temperature and lowering that of the water. This is called sensible heat; it accounts for the remaining 20-25 percent of heat transferred.

Introduction

Heat is the thermal energy transit from one system to another. The thermal energy can originate from any kind of energy according to the first law of thermodynamics. Transfer of heat is due to unique property of matter, temperature, and is governed by second law of thermodynamics, which dictates that free flow of heat is possible only from a body of higher temperature to that at a lower temperature.

All heat transfer processes, therefore, involve the transfer of energy and obey the first as well as the second law of thermodynamics. The energy in transit cannot be measured or observed directly, but the effects it produces can be observed and measured. From our viewpoint, the determination of the rate of heat transfer needs special consideration.

The transport of heat energy from one region to another occurs by any (or a combination) of similar methods. In literature such three methods of heat transmission are recognized by the terms conduction, convection and radiation respectively.

If the flow of heat is a result of transfer of internal energy from one molecule to other, the process is called conduction. Through solids, this is the only possible mode of heat transmission. In liquid and gases, however, the molecules are no longer confined to a certain point but constantly change their positions even if the substance is at rest. The heat energy is transported along with the motion of these molecules from one region to another. This process is called convection. All solid bodies as well as liquids and gases have a tendency of radiating thermal energy in the form of electromagnetic waves and of absorbing similar energy emerging from the neighboring bodies. This type of heat transport is known as thermal radiation.

In industrial processes, heat transfer may occur due to one or due to a combination of more than one of these three modes of transport.

Combined Heat transfer process

In most of the engineering applications, however, heat is transferred in successive steps by similar or different mechanisms.

For instance, let us consider the case of heating of water in a tube laid in a heat exchanger (or cooling of hot fluid in shell by CW). The water will receive heat from the products of combustion that emit and absorb radiation. The heat will flow by combination of different modes through successive steps as indicated below (fig 1).

It may be clarified here that the tube wall surface temperatures are different than the fluid temperatures on the respective sides. This can be explained by assuming that a thin layer of fluid adheres to the wall on both sides. The temperature gradient exits only within this thin layer.

Considering the temperatures at each step and resistance to heat flow through each step (R) where R1, R2 and R3 are resistances through step 1, 2 & 3 respectively. Since the same quantity of heat is flowing through each step we obtain:

This equation represents the heat flow from the hot fluid to water. Here U is known as the Overall unit conductance or the Overall coefficient of heat transfer.

Boiler Type

CARRYOVER CONTROL

TYPES OF CARRYOVER

Carryover is caused by two mechanisms, priming and foaming.

Priming is the sudden violent eruption of boiler water, which is carried along with steam out of the Boiler, usually caused by mechanical conditions.

One of the major causes is a sudden increase in the steam demand. This creates a rapid lowering of pressure on the water surface.

The surface erupts, much like the water in a bottle of soda. Nothing can be done about this chemically. It is controlled mechanically, either by using slow-opening valves, or by moderating charges in steam load.

PRIMING

Priming can cause deposits in the main steam head, and around main steam header valve in a short period of time, since it usually involves the deposition of relatively large amounts of solids when it happens.

FOAMING

Boiler water foaming causes carryover by forming a stable froth on the boiler water. This is easily carried out of the boiler along with the steam. Foaming increases with the amount and nature of the impurities in the boiler water.

Higher alkalinity, higher conductivity and impurities, which affect the surface tension of the water, all increase foaming. Over a period of time, foaming can completely plug a steam or condensate line.

FIRETUBE BOILERS

In a fire-tube boiler, the fire or hot gases are directed through the inside of a large number of tubes within the boiler shell. The modern fire-tube boiler is a very efficient unit, with maximum heat transfer per square foot in mind.

These high efficiency boilers are not as tolerant of waterside deposits as less efficient units. Therefore, more attention than ever before must be given to the water treatment given for these units.

WATER TUBE BOILERS

In this tube of boiler, the hot gases are directed around the outside of boiler tubes, while the boiler water is inside the tubes. The tubes in water tube boilers are generally connected to two drums, the steam drum, where steam escapes and the mud drum. The mud drum is at the bottom of the boiler and collects boiler sludge and other materials, which are removed through blow-down. Water tube boilers are commonly larger than fire-tube boilers, and operate at pressure up to and above 1000 psig, depending on their service. Normally larger boiler installations are water tube boilers.

PROBLEMS AFFECTING STEAM GENERATING SYSTEMS

The problems associated with water in steam generating systems are classified under these categories.

1. Corrosion.
2. Scaling and deposition.
3. Carryover.

Any competent water treatment program can control these three problems.

SCALE CONTROL

As water heats and is converted into steam, contaminants brought into the system with makeup water are left behind in the boiler. The boiler acts like a distillation unit, taking pure water out as steam and leaving behind the minerals and other contaminants in the boiler. Scale forms as a result of the precipitation of normally soluble solids from solution because of heat that makes them become insoluble. Some examples of scale are calcium carbonate, calcium sulphate, and calcium silicate.

OXYGEN PITTING CORROSION

Oxygen gets into the boiler system through a variety of sources, but primarily as a contaminant from the makeup water. Under the conditions of pressure and high temperature seen in boiler systems, very small amounts of oxygen getting into the boiler can cause corrosion. Oxygen pitting corrosion will cause isolated, deep craters in the metal surface. The deep pitting caused by oxygen corrosion can lead to sudden and early failure of boilers. Dissolved oxygen is removed either by mechanical deaeration, or by chemical oxygen scavenging.

Even with a deaearator working perfectly, oxygen scavenger is necessary to ensure that corrosion is minimized. Without a deaerator, generous amount of an oxygen scavenger may be necessary to assure complete oxygen removal. Sodium sulphite is by far the most common oxygen scavenger used. In a boiler system that operates without a deaerator, it is the oxygen scavenger that is of choice. In high-pressure systems, over 450 psig, alternative oxygen deaerator may be desirable.

STEAM AND CONDENSATE SYSTEM CORROSION

Corrosion in the steam and condensate system can be due to either oxygen attack or low pH corrosion. Low pH corrosion is due to the presence of carbon dioxide in the steam. This reacts with the Condensate to form carbonic acid, which attacks the piping.

The pH of untreated condensate can be as low as 3.5-4.0 which is extremely acidic. This acidic condensate attacks the metal in the piping, particularly along the bottom of horizontal runs, or in areas where the piping is thin, such as at threaded joints.

Corrosion is increased at weld locations, where the metal is heat stressed from the process of being welded.

Low pH corrosion is controlled by the use of neutralizing amines.

The neutralizing amine functions by reacting and neutralizing the acid in the condensate water. It elevates the pH above the corrosive range. Maintaining pH levels between 7.5 and 8.5 in the returned condensate controls neutralizing amines. If the pH is allowed to fall bellow these limits, corrosion is likely to occur. If condensate pH reaches 9.0 or above, there is a possibility of the formation of amine carbonates, which can cause temporary deposit problems.

CARRYOVER CONTROL

TYPES OF CARRYOVER

Carryover is caused by two mechanisms, priming and foaming.

Priming is the sudden violent eruption of boiler water, which is carried along with steam out of the Boiler, usually caused by mechanical conditions.

One of the major causes is a sudden increase in the steam demand. This creates a rapid lowering of pressure on the water surface.

The surface erupts, much like the water in a bottle of soda. Nothing can be done about this chemically. It is controlled mechanically, either by using slow-opening valves, or by moderating charges in steam load.

PRIMING

Priming can cause deposits in the main steam head, and around main steam header valve in a short period of time, since it usually involves the deposition of relatively large amounts of solids when it happens.

FOAMING

Boiler water foaming causes carryover by forming a stable froth on the boiler water. This is easily carried out of the boiler along with the steam. Foaming increases with the amount and nature of the impurities in the boiler water.

Higher alkalinity, higher conductivity and impurities, which affect the surface tension of the water, all increase foaming. Over a period of time, foaming can completely plug a steam or condensate line.

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