Knowledge for the Sulphuric Acid Industry
Sulphuric Acid on the Web
Acid Plant Database
Boiler Feed Water
Materials of Construction
DKL Engineering, Inc.
Cooling Water Systems - Cooling Towers
February 5, 2003
Types of Cooling Towers
Counter-Current Cooling Towers
Crossflow Cooling Towers
Approach to Wet-bulb
Cooling Tower Fans
Cooling Tower Bypasses
Materials of Construction
Cooling Tower Standards - CTI
The function of a cooling water system and cooling tower in an acid plant is to reject excess process heat to the environment. This heat comes from a number of different sources: weak acid cooling, gas condensers, acid coolers, oil coolers, refrigeration equipment, etc.
Cooling towers use evaporation as the means of cooling the circulating water. They are designed to expose the maximum transient water surface to the maximum flow of air, for the largest period of time.
A wet-bulb temperature that is not exceed in no more than 5% of the total hours during a normal summer is generally used as the design wet-bulb. A value of 5% is selected because the hours in which the wet-bulb temperature exceeds this temperature are seldom consecutive and usually occur for only short periods of time.
The wet-bulb temperature that is specified is generally an ambient wet-bulb rather than an entering wet-bulb. The entering wet-bulb temperature is the value used to design the tower and may or may not be the same as the ambient wet-bulb. If the ambient wet-bulb is specified, the vendor may adjust the wet-bulb temperature he uses for sizing the tower upwards to account for any potential recirculation.
There are basically two types of cooling tower in general use in an acid plant:
In counter-current towers, air moves vertically upward through the fill, counter to the downward fall of water. Water is distributed across the fill using a system of headers, feed pipes and sprays. In acid plants these towers are generally induced draft.
In crossflow towers, air moves horizontally through the fill across the downward fall of water. Hot water is delivered to the hot water basin where it is distributed by gravity across the fill through metering orifices in the floor of the basin. In acid plants the towers are generally induced draft.
The size of a cooling tower is affected by four basic parameters:
2. Temperature Range
3. Approach to Wet-bulb
4. Wet-bulb Temperature
Tower size is directly proportional to the heatload. If heatload is doubled, the tower size doubles.
The temperature range is the difference between the cooling water return temperature (hot) and cooling water supply temperature (cold). The range should be maximized to minimize the size of the cooling tower and the total circulating water flow. However, this is offset by increases in the size of heat exchange equipment in the plant due to lower LMTD's.
The size of the cooling tower varies inversely with the temperature range. Increasing the range, increases the driving force between the hot water temperature and the wet-bulb temperature similar to a standard heat exchanger.
Tower size varies inversely with the approach to the wet-bulb temperature. At a 5oF approach, the size of the cooling tower begins to increase asymptotically. In essence, it would take a tower of infinite size to cool the water to the wet-bulb temperature.
At constant heatload, temperature range and approach to the wet-bulb temperature, the tower size varies inversely with the actual wet-bulb temperature. The reason for this is that most of the heat transfer occurs by evaporation and the air's ability to absorb moisture reduces with temperature.
Two speed fans should be specified for all cooling towers. The high and low speeds allows more flexibility in the control of the cooling water temperature. In climates with severe winters, the fans should be reversible allowing the towers to be de-iced.
Fans are either belt driven or shaft driven through a gear box. Belt driven units have motors located inside the tower in a high humidity environment. Shaft driven units generally have the motors located outside the tower.
Cooling tower bypasses are generally specified for towers installed in cold climates. The bypass is used to prevent overcooling of the water when there is little or no heatload in the system. The bypass should discharge into the tower basin as far as possible from the cooling water pump suctions. This reduces the chance of cavitation due to disturbances in the flow of water to the pump suctions.
Cooling tower structures are fabricated using primarily 3 materials: wood, concrete and steel. In acid plant cooling towers the predominant building material is wood. Steel towers are used occasionally while concrete towers are rarely used.
In wooden towers, Californian Redwood and Douglas Fir are the two most extensively used woods. All wood used in a cooling tower must be treated with a preservative to prevent decay. Chromated Copper Arsenate (CCA) was initially used as a preservative but because of its arsenic content it has been replaced by Acid Copper Chromate (ACC).
Most of the smaller, factory built towers are constructed of galvanized steel. Some local building codes and/or fire codes require large towers to be constructed of steel. Stainless steel has been used on occasion but its increased cost make it extremely rare
The two most common materials for tower fill is wood and plastic. Splash type fill can be made of wood or plastic. Film type fill is generally made of polyvinyl chloride (PVC). The advantage of PVC as a material is that it is naturally fire resistant whereas other plastic must be specially treated.
Within the industry, standards for cooling towers is set up by the Cooling Tower Institute (CTI). The CTI is a self-governing, non-profit technical association dedicated to the improvement of technology, design, performance and maintenance of cooling towers. When a tower is specified as a CTI code tower, the following standards become part of the specification (if applicable):
STD-103 Redwood Lumber Specification
ATC-105 Acceptance Test Code
STD-111 Gear Speed Reducers
STD-114 Douglas Fir Lumber Specification
STD-115 Southern Pine Lumber Specification
STD-118 Inquiry and Bid Form
STD-119 Timber Fastener Specification
STD-127 Asbestos Cement Materials for Application on Industrial Water Cooling Towers
STD-201 Certification Standard for Commercial Water Cooling Towers