Industrial Utility Efficiency    

What’s Possible When Operating Rotary Screw Air Compressors in Hot Ambient Conditions?

There are times when rotary screw air compressors must operate in high ambient temperatures, leaving questions about the impact on these vital machines. What follows is an overview of what’s possible in these conditions, along with advice for ensuring the optimal performance of these air compressors in hot ambient conditions.

Sullair 7500

Shown is an air compressor room with proper ducting, which takes hot discharge air out of the room to avoid hot air re-circulation.

A Close Look at Rotary Screw Air Compressor Airends

Before going into detail, we need to first set a baseline. Although all air compressors compress air, there is a difference in how centrifugal, oil flooded rotary screw, oil free rotary screw, piston and scroll machines compress air. Because this article focuses on rotary screw air compressors, it’s important to know the difference between oil free and oil flooded air compressors. This can be understood in part by looking at the actual air compressor component, which is often referred to the “airend” (See Figure 1).

Hitachi DSP Air End

Figure 1: An oil free rotary screw air compressor airend.

The rotary screws that compress the air are in the center of the image shown in Figure 1. In oil flooded air compressors, that pocket with the rotary screws is filled with oil, and the oil acts as a seal to help with both the compression and to pull heat out of the compressed air. In oil free rotary screw air compressors, the pocket has no oil and is roughly ~100+ °F hotter than oil flooded air compressors. In both air compressors increasing the ambient temperature will increase working temperature of the air compressor. Like most machinery, air compressors can be prone to failure when continuously operating above the designed ambient temperature.

There is also a jacket around the screws as shown in Figure 1, which only exists in oil free air compressors/airends. This jacket has a coolant acting as a medium to pull as much heat from the airend as possible. The general rule for the cooling medium in modern airends is that oil is used in air-cooled air compressors, or water if the compressor is water cooled. Some air-cooled airends have a special OEM coolant for this jacket, or they simply use fins like an old-style radiator to dissipate the heat. In the end, whether you have an oil free or oil flooded air compressor, make sure you are using OEM engineered oil (and other coolants if required by the OEM) to ensure you are taking as much heat out of the compressed air as possible.

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Operating above the Ambient Design Point

The potential impact on rotary screw air compressors of operating above their ambient design point is that it could shorten the life of the motors or cause them to fail. Operating any electric motor above its thermal design will likely cause the windings and/or bearings to fail. The grease used on the main motors are designed for a specific maximum ambient temperature. Operating above that design point will require re-greasing in more frequent intervals or an optimum grease to handle the heat buildup to avoid bearing failure. Sullair and most major OEM air compressor manufacturers typically have a “self-preservation” mode on the air compressor that shuts it down based on the hot air or oil discharge temperature. Using a non-OEM oil may not cool the air compressor properly. This will cause the air compressor to operate at an elevated internal temperature and over time lead to failure. This could also lead to internal varnishing of the airend.

A good analogy of varnishing in an air compressor is like plaque building up on teeth. Varnishing, or dirt/particles, build up inside the air compressor. Non-OEM oil will cause buildup on the orifices, rotor/stator clearances, rotors, etc. This can shorten the life of the airend and cause a premature re-build or replacement. This means potentially facing a long and unplanned shutdown with unexpected maintenance costs.

 

The Impact of Ambient Temperature, Relative Humidity (RH) and Pressure

Now that we understand the basics, we need to look at how air compressors react to ambient temperature, relative humidity (RH) and pressure. We can also look at ways to ensure your air compressors can operate efficiently in hot ambient conditions.

Figure 2 looks to be an intimidating equation, but when you boil it, down an air compressor can make a certain amount of airflow – cubic foot per minute (cfm) – at a certain pressure.

Conversion of scfm to acfm

Figure 2.

It’s important to take the marketing promises on CAGI Sheets or OEM marketing material and re-use this formula to calculate the air compressor’s capabilities based on the ambient conditions  of your actual air compressor room. This doesn’t mean the local weather forecast of your plant, but the actual conditions of the air compressor room’s installation. I recommend re-calculating the air compressor capabilities for the hottest temperature (including RH) and coldest air temperature in the air compressor room to ensure your air compressor meets your demand of airflow. CAGI sheets are reporting airflow and power consumption at one set of standard conditions. This needs to be recalculated if you are not at the standard pressure-temperature-humidity. Rotary screw air compressors are more efficient in cooler ambient temperatures.

Another important factor that will help maintain the efficiency of your air compressor in a hot room is to properly ventilate the air compressor room. When you have any industrial machinery generating heat, you need to make sure that the room has proper airflow so as not to build a hot box. Having hotter air in the air compressor room will cause extra stress on the air compressor components and potentially lead to a component failure. This will also negatively affect the performance of the air compressor. Figure 3 illustrates one way of taking hot air created by air compressors out of the room and avoiding the hot air being re-circulated into the air compressor.

Many OEMs also offer an option to have a remote air inlet. This is a great option to feed the compressor’s inlet cool air to operate efficiently, but the air compressor room still needs to be properly ventilated to remove the heat generated by the air compressor.

Proper ventilation example

Figure 3. An air compressor room with proper ventilation provides cool air for the inlet of the air compressor. It also includes ducting to take the discharge heat out of the room.

Factoring in Minimum and Maximum Temperatures

As a user of compressed air, it’s important to understand the capabilities of your air compressor. Both oil free and oil flooded air compressors, like any air compressor, have a design point for maximum and minimum ambient temperatures.

On the minimum side, most manufacturers require the air compressor to be above freezing unless you add a cold weather package. This package typically consists of heaters, often located on the sump, moisture separates, and in the control box for the electronics. The package also insulates the control and condensate lines in the package to allow water and other fluids to avoid freezing. Lack of a cold weather package option in freezing conditions can cause cold-start failure. The fluids need to be insulated and heated – for example, water in moisture drains – and will expand and cause the moisture drains to leak or crack.

On the maximum ambient temperature side, there are many interesting solutions. Most air compressors are designed for a maximum ambient temperature of 40 °C (104 °F). This should be discussed with your OEM provider as some air compressors will void the warranty if you operate above 40 °C. Some OEMs offer high-temperature air compressor packages for 50 °C (121 °F) and some extreme packages for 55 °C (131 °F). Sullair and other OEMs will consider an engineered solution for almost any design point to meet the customer’s needs. The main components evaluated for 50/55 °C are simple. Here’s a review of the three main component modules for high-temperature air compressors:       

  • Cooling system – Fans can change in pitch, size or speed to allow more cooling airflow across the cooler pack. The auxiliary motor for the fan could increase in size and require more power consumption than a standard 40 °C air compressor. The coolers will also be double checked to ensure proper cooling. If not properly done, the air compressor will shut down to high discharge temperature, or it could potentially have a catastrophic failure.
  • All motors – I highly recommend you review the main motor technical data sheet. The standard off-the-shelf motor is a 40 °C motor. Almost all motor manufacturers make 50/55 °C motors at an additional cost. Make sure the main motor can handle the environment, as some OEMs use a 40 °C  motor in a 50 °C package. Also ensure when replacing the main or auxiliary motor, you replace it with the right ambient temperature rating. The main motor is one of the costliest components in an air compressor. There is a value in monitoring and trending the heat of the motor internally. This can be done with Resistance Temperature Detectors (RTDs) on the windings/bearings. Paying for this inexpensive option is a great long-term strategy to guard against problems.
  • All electronics should be reviewed for their ability to handle the heat. This includes Variable Speed Drives (VSDs), PLCs, solenoids, etc. In some extreme cases, control cabinets are equipped with special ventilation or air conditioning modules, but most air compressors do not require this.

When discussing electronics, it is important to note variable displacement and variable speed options can be used in hot environments in a safe manner. If you have a hot air compressor room, it is recommended to use a remote VSD and wire it near the air compressor room in an air-conditioned room.

Some might dismiss this and say it isn’t needed. However, the remote VSD in a cool room will help the costly VSD last as long as possible, so you don’t have to replace it. I highly recommend you work closely with the technicians installing the remote VSD to avoid electronic noise, or harmonic frequencies. If you have a hot air compressor room, OEMs like Sullair often have other advanced solutions available, such as spiral valve. These other solutions are more practical and better options for customers in hot, humid or dirty environments over a VSD.

A spiral valve allows you to have the turndown of a VSD without the pains of the electronic VSD. Spiral valves often last longer in heat than a VSD. Almost all VSDs are built to operate at 40 °C (~104 °F) The spiral valve is also verified by CAGI as a form of variable displacement. In very simple terms, there is a patented design to increase and decrease the length of the rotor to allow turndown. This is done in a mechanical way instead of a VSD controlling motor speeds based on certain parameters. In turn, users can save on maintenance costs and time using a spiral valve instead of a VSD.

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What to Know When Purchasing an Air Compressor

Here are suggestions to consider when purchasing a new air compressor, while taking important factors related to ambient conditions into account:

  • Know the air compressor package rating. Is it 40 °C? 50 °C? 55 °C? If operating below 32 °F, you should specify your air compressor needs a cold weather package to avoid condensate drains from rupturing, while also allowing proper start up for the motor and electronics during a cold start.
  • Confirm the main/fan motor meets the ambient temperature requirements.
  • Understand special maintenance needed for operating above 40 °C/104 °F. Do the oil or filters have any special intervals in hot environments? Update all costs in your ROI.
  • Vet the types of drains on the package. Make sure you upgrade to a zero loss drain. These ensure no air is lost while draining the unwanted liquid. The advanced zero loss drains also drain based on volume of liquid not based on time. Avoid timer and manual drains to automate the process of draining liquids properly to avoid water being sent down stream of the air compressor.
  • Understand the impact of ambient conditions on dryers. If it’s an oil free unit with a Heat of Compressor (HOC) dryer, it’s important to know the impacts on dryer temperature inlet. HOC dryers are designed for certain discharge temperatures. Changing the air compressor discharge temperature could cause the dryer to not hit the pressure dewpoint properly and leave downstream moisture in the compressed air system.
  • For all air compressors, know how the air compressor approach temperature affects the dryers. Dryers are designed for 100 °F, 100 PSI, and 100% RH. If your air compressor has a hotter discharge temperature, you need to buy a larger sized dryer to adequately cool the air for your needs.
  • If you decide to use a water-cooled air compressor, review water quality requirements. This includes gallons of water per minute , pressure, temperature, and water quality parts per million (ppm) requirements, etc.

Some additional items to consider for maintenance:

  • Understand the actual air compressor room temperature. Monitor the changes over the seasons. Most are surprised by the actual temperature inside the room and how it changes throughout the seasons.
  • Double check the oil system. Make sure you use the right oil and check the oil level and sump filter pressure differential.
  • Conduct oil sampling. This is a great indicator on whether an air compressor may fail in the future or if the oil is properly working in your conditions.
  • Pay attention to the airend. Is there an abnormal noise or vibration? Ensure the shaft seal is not leaking.
  • Understand the maintenance required for the main motor. If it uses a VSD, inspect the grounding brush and clean it regularly. Monitor bearing and/or winding RTDs, if applicable.
  • Ensure a clean air compressor cooling system, aided by a ventilation system.

These recommendations and considerations associated with a rotary screw air compressor should help your air compressor deliver the best possible performance in what can be hot and difficult conditions.

 

About the Author

Manhar Grewal is a R&D  Commercial Systems Product Manager for Sullair. He joined Sullair in 2015 and previously served as an application engineer and an oil free air compressor product manager, email: manhar.grewal@sullair.com.

About Sullair

Sullair, founded in Michigan City, Indiana, in 1965, is a leading global provider of compressed air solutions. The company was one of the first to execute rotary screw technology in air compressors, and its machines are known throughout the world for their reliability, durability and performance. Sullair became a Hitachi Group Company in July 2017. For more information, visit https://sullair.com/en.

All photos courtesy of Sullair.

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