Performance assessment of Cooling tower | Energy Conservation Opportunities in cooling tower
Cooling towers are a very important part of utility system of many process plants. Basic function of cooling tower is remove heat from the system to atmosphere. In the cooling tower make up water is used as a refill water lost to evaporation. Hot water from chiller/condenser is sent to the cooling tower. After reducing the temperature of water in cooling tower and is sent back to the chiller/condenser for further process.
source-(Pacific Northwest National Library, 2001)
Cooled water is needed for, for example, air conditioners, manufacturing processes or power generation. A cooling tower is an equipment used to reduce the temperature of a water stream by extracting heat from water and emitting it to the atmosphere.
Cooling towers make use of evaporation whereby some of the water is evaporated into a moving air stream and subsequently discharged into the atmosphere.As a result, the remainder of the water is cooled down significantly. Cooling towers are able to lower the water temperatures more than devices that use only air to reject heat, like the radiator in a car, and are therefore more cost-effective and energy efficient.
Components of a cooling tower
Frame and casing
Most towers have structural frames that support the exterior enclosures (casings), motors, fans, and other components. With some smaller designs, such as some glass fiber units, the casing may essentially be the frame.
Most towers employ fills (made of plastic or wood) to facilitate heat transfer by maximizing water and air contact. There are two types of fill:
Splash fill: water falls over successive layers of horizontal splash bars, continuously breaking into smaller droplets, while also wetting the fill surface. Plastic splash fills promote better heat transfer than wood splash fills.
Film fill: consists of thin, closely spaced plastic surfaces over which the water spreads, forming a thin film in contact with the air. These surfaces may be flat, corrugated, honeycombed, or other patterns. The film type of fill is the more efficient and provides same heat transfer in a smaller volume than the splash fill.
The cold-water basin is located at or near the bottom of the tower, and it receives the cooled water that flows down through the tower and fill. The basin usually has a sump or low point for the cold-water discharge connection. In many tower designs, the cold-water basin is beneath the entire fill. In some forced draft counter flow design, however, the water at the bottom of the fill is channeled to a perimeter trough that functions as the cold-water basin. Propeller fans are mounted beneath the fill to blow the air up through the tower. With this design, the tower is mounted on legs, providing easy access to the fans and their motors.
These capture water droplets entrapped in the air stream that otherwise would be lost to the atmosphere.
This is the point of entry for the air entering a tower. The inlet may take up an entire side of a tower (cross-flow design) or be located low on the side or the bottom of the tower (counter-flow design).
Generally, cross-flow towers have inlet louvers. The purpose of louvers is to equalize air flow into the fill and retain the water within the tower. Many counter flow tower designs do not require louvers.
These spray water to wet the fill. Uniform water distribution at the top of the fill is essential to achieve proper wetting of the entire fill surface. Nozzles can either be fixed and spray in a round or square patterns, or they can be part of a rotating assembly as found in some circular cross-section towers.
Both axial (propeller type) and centrifugal fans are used in towers. Generally, propeller fans are used in induced draft towers and both propeller and centrifugal fans are found in forced draft towers. Depending upon their size, the type of propeller fans used is either fixed or variable pitch. A fan with non-automatic adjustable pitch blades can be used over a wide kW range because the fan can be adjusted to deliver the desired air flow at the lowest power consumption. Automatic variable pitch blades can vary air flow in response to changing load conditions.
Types of Cooling Towers
Natural Draft Cooling Towers
The natural draft or hyperbolic cooling tower makes use of the difference in temperature between the ambient air and the hotter air inside the tower. It works as follows:
Hot air moves upwards through the tower (because hot air rises)
Fresh cool air is drawn into the tower through an air inlet at the bottom.
Due to the layout of the tower, no fan is required and there is almost no circulation of hot air that could affect the performance. Concrete is used for the tower shell with a height of up to 200 m. These cooling towers are mostly only for large heat duties because large concrete structures are expensive.
There are two main types of natural draft towers:
Cross flow tower (left figure): air is drawn across the falling water and the fill is located outside the tower
Counter flow tower (right figure): air is drawn up through the falling water and the fill is therefore located inside the tower, although design depends on specific site conditions
Mechanical Draft Cooling Towers
Mechanical draft towers have large fans to force or draw air through circulated water. The water falls downwards over fill surfaces, which help increase the contact time between the water and the air - this helps maximize heat transfer between the two. Cooling rates of mechanical draft towers depend upon various parameters such as fan diameter and speed of operation, fills for system resistance etc.
Mechanical draft towers are available in a large range of capacities. Towers can be either factory built or field erected – for example concrete towers are only field erected. Many towers are constructed so that they can be grouped together to achieve the desired capacity. Thus, many cooling towers are assemblies of two or more individual cooling towers or “cells.” The number of cells they have, e.g., a eight-cell tower, often refers to such towers.
Three types of mechanical draft cooling towers, and the advantages and disadvantages of each.
Forced draft cooling tower
How it works: air is blown through the tower by a fan located in the air inlet
Suited for high air resistance due to centrifugal blower fans
Fans are relatively quiet
Re-circulation due to high air-entry and low air-exit velocities, which can be solved by locating towers in plant rooms combined with discharge ducts
Induced Draft Cooling Towers
Two types of induced draft cooling towers: cross flow and counter flow
Less re-circulation than forced draft towers because the speed of exit air is 3-4 times higher than entering air
Fans and the motor drive mechanism require weather-proofing against moisture and corrosion because they are in the path of humid exit air
Induced draft counter flow cooling tower
hot water enters at the top
air enters bottom and exits at the top
uses forced and induced draft fans
Induced draft cross flow cooling tower:
water enters at top and passes over fill
air enters on one side (single-flow tower) or opposite sides (double-flow tower)
an induced draft fan draws air across fill towards exit at top of tower
Assessment of Cooling Towers
The performance of cooling towers is evaluated to assess present levels of approach and range against their design values, identify areas of energy wastage and to suggest improvements. During the performance evaluation, portable monitoring instruments are used to measure the following parameters:
Wet bulb temperature of air
Dry bulb temperature of air
Cooling tower inlet water temperature
Cooling tower outlet water temperature
Exhaust air temperature
Electrical readings of pump and fan motors
Water flow rate
Air flow rate
These measured parameters and then used to determine the cooling tower performance in several ways. These are:
6.Cycles of concentration
7.Blow down losses
8.Liquid / Gas ratio
1. Cooling Tower Range
Cooling tower range is the difference between the cooling tower water inlet/Hot inlet water temperature and cooling tower outlet/colt water outlet temperature.(See above Figure)
Range = cooling tower inlet water temp. – cooling tower outlet Water Temp.
Range °C = Heat Load in kcals/hour / Water Circulation Rate in LPH
Inversely with the range: The size of the cooling tower varies inversely with the temperature range.
2. Cooling Tower Approach
Cooling tower approach is the difference between the cooling tower outlet cold water temperature and ambient wet bulb temperature.(See above Figure) Cooling Tower approach is the better indicator for the performance.
Approach = Cold Water Temperature – Ambient Wet Bulb Temperature
A cooling tower is said to be performing well when approach is close to zero.
3. Make up Water
Cooling tower water mass balance gives an indication about make up water requirement in system. Due to Drift loss, Evaporation loss and blowdown, in the Cooling Tower Makeup water is required. To calculate the makeup water requirement use the below standard formula:-
Make up water Requirement in m3/hr(M)=Evaporation Loss in m3/hr(E) + Drift Loss in m3/hr (D) + Blow Down in m3/hr (B)
M = E + D + B
make sure Lower the make up water requirement lead to decrease the evaporation loss, drift loss and blowdown.
4. Cycle of Concentration (CoC)
Cycles of concentration (C.O.C) is the ratio of dissolved solids in circulating water to the dissolved solids in make up water.
Consider COC improvement measures for water savings.
5. Evaporation loss
Evaporation loss is the water quantity evaporated during the heat rejection process. To calculate the evaporation loss use the below standard formula.
Evaporation Loss (m3/hr) = 0.00085 x 1.8 x circulation rate (m3/hr) x (T1-T2)
T1-T2= Temp. difference between inlet and outlet water.
Lower the evaporation loss reduce the make up water requirement.
6. Blow down loss
Blow down losses depend upon cycles of concentration and the evaporation losses and is given by relation:-
Blow Down=Evaporation Loss / (C.O.C. – 1)
Periodically done the blow down process increase the efficiency and life of cooling tower
7. Liquid to Gas ratio
Liquid/Gas (L/G) ratio, of a cooling tower is the ratio between the water and the air mass flow rates.
L/G= h2– h1/T1-T2
Monitor L/G ratio, CW flow rates w.r.t. design as well as seasonal variations. It would help to increase water load during summer and times when approach is high and increase air flow during monsoon times and when approach is narrow.
8. Cooling Tower Efficiency/Effectiveness
The calculation of cooling tower efficiency involves the Range and approach of the cooling Tower. Cooling tower efficiency is limited by the ambient wet bulb temperature. In the ideal case, the cold water temperature will be equal to the wet-bulb temperature. This is practically not possible to achieve. This requires very large tower and results in huge evaporation and drift loss resulting in a practically not viable solution. In practice, the cooling tower efficiency will be in between 70 to 75%.
Cooling Tower Efficiency = (Hot Water Temperature – Cold water Temperature) x 100/ (Hot Water Temperature – Wet bulb temperature)
In Simply term
Cooling Tower Efficiency = Range/ (Range + Approach) x 100
In summer the ambient air wet bulb temperature raises when compared to winter thus limiting the cooling tower efficiency.
Energy Efficiency Opportunities
The main areas for improving the energy efficiency of cooling towers are:
1.Selecting the right cooling tower (because the structural aspects of the cooling tower cannot be changed after it is installed)
3.Pumps and water distribution system
4.Fans and motors
1. Selecting a cooling tower
Once a cooling tower is in place it is very difficult to significantly improve its energy performance. A number of factors are of influence on the cooling tower’s performance and should be considered when choosing a cooling tower: capacity, range, approach, heat load, wet bulb temperature, and the relationship between these factors. We will start with capacity.
Capacity, in terms of heat dissipation (in kCal/hour) and circulated flow rate (m3/hr), are an indication of the capacity of cooling towers. However, these design parameters are not sufficient to understand the cooling tower performance.
For example, a cooling tower sized to cool 4540 m3/hr through a 13.9 0C range might be larger than a cooling tower to cool 4540 m3/hr through 19.5 0C range. Therefore other design parameters are also needed.
Cooling towers are usually specified to cool a certain flow rate from one temperature to another temperature at a certain wet bulb temperature. For example, the cooling tower might be specified to cool 4540 m3/hr from 48.9oC to 32.2oC at 26.7oC wet bulb temperature. When the size of the tower has to be chosen, then the approach is most important, closely followed by the flow rate, and the range and wet bulb would be of lesser importance.
The range is difference between the cooling tower water inlet and outlet temperature, in this case range 16.7 oC = 48.9 oC - 32.2 oC. The range is thus determined not by the cooling tower, but by the process it is serving.
The approach is also important. This is the cold water temp minus the wet bulb temp, in this case Approach (5.50C) = Cold-water temperature 32.2 0C – Wet bulb temperature (26.7 0C). As a general rule, the closer the approach to the wet bulb, the more expensive the cooling tower due to increased size.
Usually a 2.8oC approach to the design wet bulb is the coldest water temperature that cooling tower manufacturers will guarantee. The heat load imposed on a cooling tower is determined by the process being served. Process heat loads may vary considerably depending upon the process involved and are therefore difficult to determine accurately.
On the other hand, air conditioning and refrigeration heat loads can be determined with greater accuracy. The degree of cooling required is controlled by the desired operating temperature of the process. In most cases, a low operating temperature is desirable to increase process efficiency or to improve the quality or quantity of the product. However, in some applications (e.g. internal combustion engines) high operating temperatures are desirable.
The size and cost of the cooling tower is increases with increasing heat load. Purchasing undersized equipment (if the calculated heat load is too low) and oversized equipment (if the calculated heat load is too high) is something to be aware of. Wet bulb temperature is an important factor in performance of evaporative water cooling equipment, because it is the lowest temperature to which water can be cooled. For this reason, the wet bulb temperature of the air entering the cooling tower determines the minimum operating temperature level throughout the plant, process, or system.
The following should be considered when pre-selecting a cooling tower based on the wet bulb temperature:
Theoretically, a cooling tower will cool water to the entering wet bulb temperature. In practice, however, water is cooled to a temperature higher than the wet bulb temperature because heat needs to be rejected from the cooling tower.
A pre-selection of towers based on the design wet bulb temperature must consider conditions at the tower site. The design wet bulb temperature also should not be exceeded for more than 5 percent of the time. In general, the design temperature selected is close to the average maximum wet bulb temperature in summer.
Confirm whether the wet bulb temperature is specified as ambient (the temperature in the cooling tower area) or inlet (the temperature of the air entering the tower, which is often affected by discharge vapors recirculated into the tower). As the impact of recirculation cannot be known in advance, the ambient wet bulb temperature is preferred.
Confirm with the supplier if the cooling tower is able to deal with the effects of increased wet bulb temperatures.
The cold-water temperature must be low enough to exchange heat or to condense vapors at the optimum temperature level. The quantity and temperature of heat exchanged can be considered when choosing the right size cooling tower and heat exchangers at the lowest costs. .
The range increases when the quantity of circulated water and heat load increase. This means that increasing the range as a result of added heat load requires a larger tower. There are two possible causes for the increased range:
The inlet water temperature is increased (and the cold-water temperature at the exit remains the same). In this case it is economical to invest in removing the additional heat.
The exit water temperature is decreased (and the hot water temperature at the inlet remains the same). In this case the tower size would have to be increased considerably because the approach is also reduced, and this is not always economical.
The design wet bulb temperature is determined by the geographical location. For a certain approach value (and at a constant range and flow range), the higher the wet bulb temperature, the smaller the tower required. For example, a 4540 m3/hr cooling tower selected for a 16.67oC range and a 4.45oC approach to 21.11oC wet bulb would be larger than the same tower to a 26.67oC wet bulb. The reason is that air at the higher wet bulb temperature is capable of picking up more heat.
This is explained for the two different wet bulb temperatures:
Each kg of air entering the tower at a wet bulb temperature of 21.1oC contains 18.86 kCal. If the air leaves the tower at 32.2oC wet bulb temperature, each kg of air contains 24.17 kCal. At an increase of 11.1oC, the air picks up 12.1 kCal per kg of air. Each kg of air entering the tower at a wet bulb temperature of 26.67oC contains 24.17 kCals. If the air leaves at 37.8oC wet bulb temperature, each kg of air contains 39.67 kCal. At an increase of 11.1oC, the air picks up 15.5 kCal per kg of air, which is much more than the first scenario.
2. Fill media
In a cooling tower, hot water is distributed above fill media and is cooled down through evaporation as it flows down the tower and gets in contact with air. The fill media impacts energy consumption in two ways:
Electricity is used for pumping above the fill and for fans that create the air draft. An efficiently designed fill media with appropriate water distribution, drift eliminator, fan, gearbox and motor with therefore lead to lower electricity consumption.
Heat exchange between air and water is influenced by surface area of heat exchange, duration of heat exchange (interaction) and turbulence in water effecting thoroughness of intermixing. The fill media determines all of these and therefore influences the heat exchange. The greater the heat exchange, the more effective the cooling tower becomes.
There are three types of fills:
Splash fill media: splashing water over the fill media into smaller water droplets.
Film fill media: water forms a thin film on either side of fill sheets.
Low-clog film fills: higher flute sizes were recently developed to handle high turbid waters (e.g. sea water).
These influence the liquid to gas ratio, heat exchange, pumping costs, which all impact on energy use.
3. Pumps and water distribution
There are three areas for energy conservation with pumps and water distribution
Pumps: Areas for energy efficiency improvements are discussed in details in the Pumps and Pumping Systems chapter.
Optimize cooling water treatment: Cooling water treatment (e.g. to control suspended solids, algae growth) is mandatory for any cooling tower independent of what fill media is used. With increasing costs of water, efforts to increase Cycles of Concentration (COC), by cooling water treatment would help to reduce make up water requirements significantly. In large industries and power plants improving the COC is often considered a key area for water conservation.
Install drift eliminators: It is very difficult to ignore drift problems in cooling towers. Nowadays most of the end user specifications assume a 0.02% drift loss. But thanks to technological developments and the production of PVC, manufacturers have improved drift eliminator designs. As a result drift losses can now be as low as 0.003 – 0.001%.
4. Cooling Tower Fans
The purpose of a cooling tower fan is to move a specified quantity of air through the system. The fan has to overcome the system resistance, which is defined as the pressure loss, to move the air. The fan output or work done by the fan is the product of air flow and the pressure loss. The fan output and kW input determines the fan efficiency. The fan efficiency in turn is greatly dependent on the profile of the blade. Blades include:
Metallic blades, which are manufactured by extrusion or casting processes and therefore it is difficult to produce ideal aerodynamic profiles
Fiber reinforced plastic (FRP) blades are normally hand molded which makes it easier to produce an optimum aerodynamic profile tailored to specific duty conditions. Because FRP fans are light, they need a low starting torque requiring a lower HP motor, the lives of the gear box, motor and bearing is increased, and maintenance is easier.
A 85-92% efficiency can be achieved with blades with an aerodynamic profile, optimum twist, taper and a high coefficient of lift to coefficient of drop ratio. However, this efficiency is drastically affected by factors such as tip clearance, obstacles to airflow and inlet shape, etc.
Cases reported where metallic or glass fiber reinforced plastic fan blades have been replaced by efficient hollow FRP blades. The resulting fan energy savings were in the order of 20-30% and with simple pay back period of 6 to 7 months.