In an Air Cooled Heat Exchanger (ACHE), hot process fluids or gases flow through tubes while the cooling air flows across the outside of the tubes to remove heat. The cooling air is simply outside air that is propelled across the tubes by inducing or forcing outside air across the tubes. The ability of the ACHE to achieve the desired cooling is directly impacted by the ambient air temperature.
As air temperatures increase in the summer months the ACHE’s ability to cool or condense product is reduced. Under normal summer operating conditions it is common to see inlet air temperatures in excess of 100 degrees F.
When Aggreko’s rental pre-coolers are in place the ambient air is cooled to a much lower temperature. In many cases Aggreko’s pre-cooling method can reduce the inlet air temperature by 20 degrees or more.
How our rental pre-cooling systems for ACHE’S work
The hot ambient air is brought into the pre-cooler where it comes on direct contact with very cool water. The cooling water is recirculated through the cellular cooling media within the pre-cooler. Through evaporative heat transfer, the circulating water temperatures are maintained very near the wet bulb temperature. As the hot ambient air is brought into contact with the cool water it is cooled within 1.5 degrees of wet bulb before being discharged into the inlet of the air cooled heat exchanger.
Why Precooling Works
Air-cooled exchangers are used for the purpose of removing excess heat from fluid. Many industrial process plants create large quantities of heat on their processes. In order for the plant to operate properly, the excess heat must be removed. Since air-cooled exchangers are dependent on ambient air temperatures to dissipate the heat, many process plants become limited by their cooling systems during the peak temperatures of summer. A pre-cooler could be just what it takes to overcome the challenges brought on by the peak temperatures of summer.
Air-cooled heat exchangers are used for the purpose of removing heat from a system when it has no other available uses. An example of an air cooled heat exchanger is a car radiator. As the car engine runs, the engine heats up, and to avoid overheating, fluid is circulated throughout the engine absorbing heat and back through the radiator where the fluid is cooled and circulated back to the engine. When the fluid, typically water or glycol, enters the radiator the fluid is dispersed through the radiators many small tubes. The exterior of the small tubes are equipped with small fins for the purpose of dissipating the heat. A fan is used to blow air across the exterior of the finned tubes transferring the heat from the fluid into the air. This process works well for cooling a car engine as long as the radiator is in good working order and the ambient air is relatively cool.
Many industries utilize air-cooled exchangers for the purpose of removing excess heat from a process. Industrial air-cooled exchangers are similar to a car radiator in function, but are usually much larger in size. Some air-cooled exchangers are larger than a house, can weigh several tons, and be capable of removing millions of BTU’s of heat per hour. The production capacity of many process plants is limited by their ability to remove heat from the processes. For example power plants utilize air-cooled exchangers to condense steam into water. The amount of electricity that a power plant produces is limited to the air-cooled exchanger’s ability to remove the excess heat. Refineries and petrochemical plants also utilize air-cooled exchangers to remove excess heat from their processes. Again, plant production can often be limited when an air cooled exchanger is unable to remove the excess heat. When a process plants production is limited, their ability fulfill orders, and their ability to be profitable is compromised.
Under normal circumstances air-cooled exchangers, when properly designed and sized will adequately and efficiently remove the necessary heat from a process. However, in the summer when ambient air temperatures are elevated, the air-cooled exchangers can become limited. During these conditions, plant productivity can suffer. To overcome this problem, some process plants have installed wet coolers, commonly referred to as cooling towers to avoid seasonal limitations, but for those plants that have already been constructed, may find it difficult and expensive to replace their air-cooled exchangers with a cooling tower. In addition, to the expense and mechanical challenges associated with replacing the air-cooled exchangers, the limitation of the air-cooled exchanger is limited to a few months each year.
A viable solution to process plants that suffer from seasonal cooling limitations can be the use of portable cooling towers. Portable cooling towers can be placed below or adjacent to the large air-cooled exchangers for the purpose of cooling the ambient air before it reaches the air-cooled exchanger. By pre-cooling the air, the air cooled exchanger will become more efficient and its ability to provide adequate cooling for the process will be restored.
A cooling tower, similar to an air-cooled exchanger is used to remove excess heat from a process. The difference is that an air-cooled exchanger is limited by the ambient air temperature. In other words if the ambient air temperature (dry bulb) is 100 degrees F. an air cooled exchanger would not be able to cool the liquid below 100 degrees F. Conversely, a cooling tower utilizes evaporative cooling. Evaporative cooling is limited by the ambient wet bulb temperature. Wet bulb temperatures are typically much cooler than dry bulb temperatures. For example with a dry bulb temperature of 100 degrees and at a humidity of 50% the ambient wet bulb temperature would be 82 degrees (Mountain View Technologies 2009). In this example, the portable cooling tower could be utilized to reduce the air temperature going to the air-cooled condenser from 100 degrees to 82 degrees. With a net reduction of 18 degrees in cooling capacity, the plants cooing capacity would be completely restored.
The offering of a portable cooling tower as a pre-cooler to an air-cooled exchanger is unique in two ways. The first way this offering is unique is that portable cooling towers are available on a rental basis and can therefore be used only as needed avoiding costs during periods where additional cooling is not needed. The second way this is unique is that cooling towers are typically used for directly cooling hot water from a process. In this application, the cooling tower is not connected to the process; rather water is simply circulated across the tower for the purpose of cooling the ambient air prior to the air reaching the air-cooled exchanger.
Aggreko Cooling Tower Services (ACTS) is a specialty group within Aggreko that is dedicated to providing creative, safe, reliable and cost-effective solutions to our customer’s process cooling limitations.