Specifying an Air Compressor Cooling System
There are several pieces of information that your cooling system specialist will need in order to properly engineer and build a cooling system for your new air compressor. There are many types of air compressors and each has different requirements of the cooling system in order to operate correctly. This article will take the mystery out of some of the terms and specifications for your cooling system.
In the previous article titled, “The Six Basic Types of Liquid Cooling Systems” (July 2013 Issue of Compressed Air Best Practices® Magazine), we gave a basic description of the different types of cooling systems and how they operated. This article will outline the basics of how one of those systems is employed to operate as an air compressor cooling system. We cannot possibly cover all of the variations of the different types of compressors nor how they are employed or how the location climate affects the operation of that compressor.
One Sizing Scenario
Therefore, we will choose one air compressor at a single location to give the reader an idea of what to provide to your cooling system specialist. For example: One 175 nameplate horsepower air compressor located in Scranton, PA. The building has a central open-cooling tower with an abundance of tower water available at a maximum temperature of 85 ⁰F. Power is available at 460Volts, 3 phase, 60Hz. There is a compressed air dryer in the system that will work properly with 100 ⁰F inlet air from the air compressor.
At this point we need an air compressor data sheet to continue with the specification of the cooling system. In this example, the compressor data sheet gives us the actual maximum Brake Horse Power of the compressor to be 200 BHP. We use this information to determine the maximum heat load. Converting the brake horsepower to Btu/hr equals 509,000 Btu/hr. This is our heat load for the air compressor, if everything is operating perfectly. Normally, a margin is added to the system to reduce the effects of fouling, piping heat gain, inefficiencies in the system and condensation. For this example the customer wants us to use 30%. We simply multiply the load by 1.3 and we now have a design heat load of approximately 662,000 Btu/hr.
The compressor data sheet tells us that the cooling system inlet temperature approach to the outlet air temperature is 10 ⁰F. This means that the outlet air temperature is regulated by the inlet cooling fluid temperature. Therefore, we need 90 ⁰F coolant temperature to provide an outlet air temperature of 100 ⁰F, a requirement for proper operation of the compressed air dryer. All of this can be achieved with a PCX (liquid to liquid) cooling system as discussed in our previous article.
A Closed-Loop System
The customer wants a closed-loop cooling system to prevent fouling of the heat exchangers in the air compressor from the dirty tower water. The PCX system is going to be located in a building that will remain heated and above freezing. This will allow us to use treated water on the closed loop side of the cooling system. From the data sheet we learn that the compressor can use 90 ⁰F water at a flow rate of 34 gpm, with a leaving water temperature of 120 ⁰F, and a total pressure drop across the compressor cooling circuit of 5 psid, at this flow rate. This flow rate was provided by the air compressor manufacturer based on the maximum heat load from the compressor. However, we have changed the heat load by adding a 30% margin to the load. We must now use the heat balance equation to determine the new flow for the compressor.
The heat balance equation tells us that the load equals the product of the change in temperature times the flow rate times the constant of the cooling fluid. (662,000 Btu/hr = (Delta Temperature in ⁰F) X (Flow Rate in gpm) X (Constant for 100% water)
In this case the constant for water is 500, our delta temp is 30 ⁰F and our load is 662,000 Btu/hr. From this information we can easily derive the new flow to be a little over 44 gpm. Since we have changed the flow we will need to adjust the pressure drop across the compressor. The new pressure drop is the ratio of the new flow to the old flow squared and multiplied by the old pressure drop. Your cooling system specialist will use the new compressor pressure drop, the pressure drop across the customer engineered proposed piping, and the pressure loss in the newly proposed cooling system to determine the pump “head” required to provide the closed loop flow for the system.
Selecting the correct pump is critical to successful operation of the cooling system. Special controls may be necessary to avoid water hammer effects for elevation changes over 25 feet, varying loads and long pipe runs. Your cooling specialist will need to know the volume of the proposed engineered piping to properly engineer the expansion tank volume for expansion and contraction of the cooling fluid. In this example, treated water must contain lime scale and corrosion buffering agents and biocides to prevent rust and biological growth in the closed loop system.
“Selecting the correct pump is critical to successful operation of the cooling system. Special controls may be necessary to avoid water hammer effects for elevation changes over 25 feet, varying loads and long pipe runs.”
— Bruce Williams, Hydrothrift Corporation
The process of specifying a cooling system to correctly cool your air compressor is not difficult if the correct information exists to engineer the system. If the information is not available, then instruments need to be installed to measure flows, temperatures, and pressures to correctly asess the need. This is just one example of a cooling system selection process. There are other factors such as explosion-proof areas, coastal area corrosion, special power requirements and other items that will require additional engineering. If you are struggling with an existing application or have a new one, ask for help from your cooling system specialist. They have seen many applications and are familiar with many different styles of air compressors. In the end, the best cooling system is one that operates reliably and meets your needs.
For more Cooling System articles, visit www.airbestpractices.com/technology/cooling-systems