Industrial Utility Efficiency

Air Compressor Sizing for High Ambient Temperatures


Like the old game “telephone” when you were a kid, pieces of the puzzle can be lost between multiple parties, different vendors, engineering firms, and the contractors. Price often dictates what equipment gets installed as well.  

When specifying a new facility, flow, pressure, and air quality are the most important inputs to design a system. In my experience, we—equipment manufacturers—spend a lot of time talking about these three items with customers.

However, room temperature, or ambient temperature, has a significant effect on plant operation. The amount of space, or lack thereof, for compressed air equipment and the cooling flow all have an important impact on plant operation; because if the air compressors are down; so are you!

 

Volumetric Flow and Altitude

When sizing compressed air equipment, it is important to remember that most compressors are rated in volumetric flow (CFM-FAD). If the end use applications are rated in mass flow (SCFM), make sure that demands are converted back to volumetric flow so there is enough air at the point of use.

Additionally, any change in altitude around the world will affect the available mass flow. The higher the altitude, the more volumetric flow will be required to achieve the same mass flow at sea level. Therefore, the facility’s elevation will affect the mass flow. For example, if the facility is in Denver, the location will have a big effect on the mass flow. Denver is a mile high, so the air is thinner. At 5280 feet above sea level the atmospheric pressure is derated to 12.1 psia. If the system requires 1250 scfm, and the compressor is rated for 1250 cfm-fad, in Denver this compressor will be too small. Using a 1500 cfm-fad (volumetric flow) compressor, the compressor would be capable of providing about 1250 scfm (mass flow) at this altitude, whereas it would be 1500 scfm at sea level (see calculation in Figure 2). That’s greater than a 15% drop in mass flow from sea level to Denver. So be aware! If the system in this project was sized by volumetric flow instead of mass flow, this application would be short by at least 50 hp worth of air compressor.

Figure 1: Ambient air pressure at various elevations.

Volumetric flow formula

Figure 2: Calculating volumetric flow from mass flow requirement.

The typical operating range of an air compressor is between 40°F (4.4°C) and a little over 100°F (38°C). The low point is based on the freezing point of water, and the limitation of many controllers that have been integrated to compressors over the years. The maximum temperature is limited by the coolers, either aftercoolers or intercoolers, depending on the compressor. Be aware of what the compressed air equipment needs to operate properly and design the compressor room around those parameters. 

The cooling method for the air compressor is a significant design decision. Air cooled compressors and dryers need thousands if not tens of thousands of cubic feet per minute (cfm) of cooling air to maintain operating temperatures. It all depends on the size of the system. Here’s a simple test: try opening the door to the compressor room, if it slams open or requires Herculean strength to open the door, this is a good indication there is a problem in the compressor room. In these cases the compressor room is starved for air, and there is a severe negative pressure in the compressor room. This effectively creates a Denver-like atmosphere in the compressor room, causing the compressors to work harder, and for that mass flow to drop as well. 

TIP: If your compressor room doesn’t have the ability to bring in fresh air, or if the location will continuously exceed the maximum rated operating temperature of your equipment you may want to consider water-cooled equipment. More on that point later.

 

Approach Temperature

It’s hard to condense, no pun intended, all the details about high ambient sizing into one article, but one thing to be aware of is the air compressor’s approach temperature. This is how effectively the coolers will cool the compressed air before discharging to the main plant piping. The approach temperature is a reference point on how close the compressed air discharge temperature is to the ambient temperature. Most air-cooled compressors have an approach to ambient temperature listed as 10°F (-12°C), 15°F (-9.4°C), or higher; it all depends on the size of the compressor and the rated design conditions. Keep in mind that the stated approach temperature is most likely at the cooler design conditions, not the actual conditions. 

TIP: Always check with the air compressor manufacturer for the worst case conditions, and note that most manufacturers can supply either air-cooled or water-cooled compressors.  

Let’s look at an example. At design conditions, that approach temperature is about 15°F F (-9.4°C). Taking a closer look at the design conditions, that’s 68°F (20°C), 30% relative humidity, and sea level. If the ambient temperature was 68°F, the air would come out of the aftercooler to the dryer at 83°F (68°F + 15°F approach = 83°F). In a climate-controlled room, or a mild climate, such operating temperatures can be expected. However, in extreme environments with a high ambient temperature, or during the hot summer months, temperatures in a compressor room can climb significantly if the space is not climate controlled. If the ambient temperature in the room was instead 100°F (38°C), the outlet temperature would then be 115°F (100°F + 15°F approach = 115°F). This warmer discharge temperature will be passed on to the dryer(s) in the system. When designing the system, we need to ensure that the dryer is sized appropriately to handle the hotter discharge temperature (more on that later).

If the relative humidity in the air is higher than 30%, this can significantly increase that approach temperature. Note that as air heats up, it’s affinity for moisture also increases. Therefore, talk to the equipment manufacturer to make sure an accurate approach temperature is taken into account.

TIP: Make sure that your compressor discharge temperature is below 120°F (49°C) as this is generally the maximum inlet temperature a dryer can handle; and even then anything over 100°F (38°C), can seriously derate the dryer (meaning it won’t be able to dry the same amount of air as advertised). This could cause moisture in your plant production piping or cause your compressed air system to shut down.

 

High Ambient Temperatures and Refrigerated Dryers

Of all dryer types refrigerated dryers are the most ubiquitous, and they experience a significant deration in capacity due to high inlet temperatures and high ambient temperatures. Refrigerated dryers are typically rated at 100 / 100 / 100; 100 psig inlet pressure, 100°F (38°C) ambient temperature, and 100°F inlet temperature. Generally speaking, an increase in pressure over 100 psig increases the dryer capacity but increases in temperature decreases the capacity. The opposite is also true: decreasing pressure below its’ rating decreases dryer capacity and decreasing temperature below its’ rating increases capacity. Figure 3 shows sample correction factor for a refrigerated dryer. For our example, with a 300 hp compressor, capable of 1500 cfm-fad, with a 15°F approach temperature at 100°F ambient temperature, that’s 115°F (46°C) inlet to the dryer. Assuming 100 psig inlet to the dryer, the correction factor comes out to 0.69. Meaning that a refrigerated dryer of 1550 scfm (sized most closely to the compressor output), would only be capable of drying 69% of that capacity, or about 1070 scfm. Even at the altitude condition in Denver, this dryer would be undersized for the worst-case high temperature conditions. Therefore, the next larger sized refrigerated dryer would need to be considered to ensure a consistent pressure dew point during the worst-case ambient conditions. Capacity correction tables are specific to each dryer manufacturer and are published for each dryer type (refrigerated and desiccant dryers). 

TIP: Consult with your equipment manufacturer for dryer capacity correction factors. 

Capacity correction factors

Figure 3: Capacity correction factors for sample refrigerated dryer.

 

Water-cooled Air Compressors

Water-cooled air compressors are a good option when it comes to high ambient temperature. Water-cooling can ensure that the compressor discharge temperatures are maintained in the system’s comfort zone, or Goldilocks Zone. There are multiple water-cooler types, such as plate type and shell and tube, to decide from. Additionally, the required flow, pressure, and quality of the water are other aspects to consider.

One of the biggest detractions of water-cooled compressors is the overall cost. Capital cost between an air-cooled and water-cooled compressor is not huge, but the operating costs can be. For a 300 hp air-cooled air compressor, there is about a 7.5 hp ventilation fan. Running all year (8760 hours) at $0.10/kWh, it’s about $5400 per year to operate. The cost of cooling water for each facility is typically difficult to determine and needs to be confirmed for each site. However, $3.00/1000 gallons is a good place to start (https://coolingbestpractices.com/technology/chillers/calculating-water-costs-water-cooled-air-compressors). With a 300 hp, water-cooled compressor, it would need roughly 70 gallons of water per minute at $3.00/1000 gallons of cooling water (8760 hours) that’s well over $100,000 per year and may not include the maintenance of the cooling water system, filtration, etc., and any electrical devices associated with this compressor’s cooling circuit. Another consideration: is there water on site to pull from or does a cooling tower or chiller system have to be added? Any cooling tower or chiller is a big capital expense as well. Plus, many companies are reducing their carbon footprint and reducing their water consumption, so adding a cooling tower or chiller may be less desirable. 

Air Cooled Compressor

Figure 4: Example of air-cooled air compressor.

Water cooled air compressor

Figure 5: Example of water-cooled air compressor (with plate type heat exchangers).

 

Dirty, Dusty Environments

High ambient temperatures are often accompanied by additional severe duty conditions, such as dusty, dirty environments. If the compressed air system is in a dusty or dirty environment, and/or high ambient temperatures, the service intervals will need to be shortened to ensure no unplanned outages. Motor greasing or rather lack thereof can be devastating to the compressed air system. Remember that with an oil-flooded rotary screw compressor, the lubricant is keeping the temperatures down in the compression circuit as well as filtering the ambient air and sealing the system. The hotter the ambient temperature, the more moisture in the compressor circuit. High moisture content or particulates can have a severe effect on operation. 

TIP: It’s always best to sample your oil to ensure it is within manufacturer recommendations and adjust your service schedule based on this data.

Air-cooled air compressors and dryers rely heavily on ventilation and exhaust to ensure proper operation. Insufficient or lack of proper ventilation and exhaust can be devastating and an on-going problem for a compressed air system. Location of the compressed air equipment is a huge factor in determining the equipment’s lifetime. 

TIP: Putting your compressed air system in a closet with no windows and no doors is a recipe for disaster; it’s an exaggeration for sure, but think of it this way: Your system may not be in a high ambient climate but you’re simulating one for your compressor room with improper ventilation. Moving the compressed air system to a more hospitable location will have the most significant benefit.

 

Summary 

Don’t forget heat recovery, whether it is an air-cooled or a water-cooled air compressor. Using space heating during winter months can offset the cost of operation and is highly recommended. Using the waste heat to heat a process is also a great opportunity and production cost offset, and therefore highly recommended. 

The best tips for ventilation are to make sure there is enough inlet air, and that the room is set up to remove the hot air. Recirculation is great during the winter months, but the system should be designed to ensure there is not too much. Inadvertent recirculation can wreak havoc on a compressed air system and production. Remember that refrigerated dryers also require cooling air and have hot air exhaust, so this should be accounted for as well.

Exhaust ducting

Figure 6: Sample exhaust ducting with recirculating louvers for space heating and exhaust fan for dryer ventilation.

Figure 7: Sample inlet louvers for an indoor compressor room.

Keep the air compressor operating in the Goldilocks Zone. I’ve talked a lot about cooling temperatures, but operating temperatures are just as important, you don’t want to run the compressor too hot or too cold.

Knowing how the compressed air system is running makes a huge difference in understanding and managing the system. Just like the production, monitoring key items will give indications on how well or poorly the compressor, dryer, and overall compressed air system is running. Certainly, you want to get alarms and warnings, but system pressure, airend discharge temperature, and motor temperature, as well as the state of all the equipment, compressors, dryers, and especially drains are important.

Figure 8: Operating status screen of a compressed air station.

Figure 9: Message and alarm history of a compressed air station.

Always make sure that there is enough service space for the compressed air equipment, that it is getting enough ventilation, and assume worst case conditions when designing the system. Trim coolers may be a good option depending on where in the world the equipment is located, but keep in mind the overall costs and benefits. Depending on the time of year and location in the world, compressed air will have some or a lot of moisture. Drains are a critical part of every compressed air system. A simple drain failure could ruin production, so make sure these are accounted for in the design.

► Size your compressor for your conditions

► Keep your air compressor comfortable

► Measure and monitor

► Look for trends in reports

► Be proactive

 

About the Author

Neil Mehltretter is the Technical Director for Kaeser Compressors, Inc., email: neil.mehltretter@kaeser.com, tel: 540-898-5500.

About Kaeser Compressors, Inc.

Kaeser Compressors is a leader in reliable, energy efficient compressed air equipment and system design. We offer a complete line of superior quality industrial air compressors as well as dryers, filters, SmartPipe™, master controls, and other system accessories. Kaeser also offers blowers, vacuum pumps, and portable gasoline and diesel screw compressors. Our national service network provides installation, rentals, maintenance, repair, and system audits. Kaeser is an ENERGY STAR Partner. For more information, visit https://us.kaeser.com/.

All photos courtesy of Kaeser. Compressors, Inc.

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