How to minimise air motor freezing

Using the same basic principle as household refrigerators, air motor icing or freezing occurs when air is compressed and then rapidly depressurised. Air motor freezing can appear in subtle ways. However, with the ability to effectively recognize the signs, a contractor can avoid costly downtime while promoting increased production. A user of pneumatic equipment is likely to have a freezing issue if explicit conditions exist, such as : 

• The pump is experiencing erratic pressure fluctuations
• The motor or muffler is heavily frosted
• The compressed air being used is moisture-laden because little or no drying equipment is installed
• The air motor stops, then restarts suddenly (once the ice melts)
• Operation of the equipment takes an extended period of time
• A noticeable air supply or fluid control adjustments need to be made to maintain a constant cycle rate

Internal air valve motor freezing is a possible cause for slow motor operation and loss of pressure. Nonetheless, external icing is common and harmless to air motors and does not indicate a problem.

When air is pressurised in a sprayer or when the air gets exhausted in the air motor at 100 psi (6.9 bar) and is released to the atmosphere at 14.7 psi (1.0 bar), the equipment is going to quickly freeze due to the rapid expansion of those molecules.

Various environmental factors influence air motor freezing including the compressed air temperature, compressed air dew point, compressed air pressure, ambient air flow, ambient air temperature and ambient dew point. However, these factors can be both unpredictable and temporary, which is especially relevant to seasonal fluctuations in relative humidity.

Some areas that are prone to this type of air motor behaviour are in the Pacific Northwest and along the Gulf Coast. When there is a lot of humidity and moisture in the air, freezing of equipment can be a fairly regular occurrence.

Furthermore, several non-environmental factors contribute to air motor freezing such as the motor air valve material, motor air valve geometry, motor pressure demand, length in seconds of the cycle run, cycle rate of motor and exhaust port restrictions.

The advantage of a pneumatic motor is that it has a simple, robust design. If a contractor experiences air motor freezing, there are several solutions. These solutions include changing the operating parameters, changing the exhaust configuration, installing an air line heater and substituting equipment.

• Changing the operating parameters means that the user decreases the pump air supply pressure. This method will change the output rate of the fluid and lower the pump line restrictions.
• When changing the exhaust configuration, the contractor can put a muffler on the air motor to restrict the exhaust. However, this may lower the performance of the pump. The contractor can also attach a tube to the air exhaust port and exhaust the air to a remote location.
• Installing an air line heater is a prominent solution for eliminating air motor icing. The air line heater heats the air supplied to the pump to 130°F. This raises the exhaust air temperature to above 32°F, eliminating icing. Only install these heaters in areas not prone to explosions or use an approved explosion-proof heater.
• If air motor freezing has occurred as a result of non-environmental factors, such as a high pressure demand, high cycle rate and long duty cycle, the contractor might consider installing a larger sized compressor.
• If none of the other solutions solve the air motor freezing issue, investing in a new motor that prevents icing may be the best option.

 

Although the previously mentioned practices have proven effective in eradicate icing and recovering equipment to its optimal operating condition, there are methods that can be implemented to greatly minimise air motor freezing. 

Prior to using pneumatic equipment, it is important to ensure that the air supply and surrounding ambient air is completely dry to avoid freezing. Therefore, utilising refrigeration system air dryers, membrane air dryers and desiccant air dryers are most effective for this strategy. 

An after-cooler can also be used to ensure a dry air supply. The after-cooler may be satisfactory for intermittent use, while a pump running continuously may require refrigeration, a membrane or desiccant air dryers.

In the past, heat tape has also been wrapped around motors to help prevent air motor freezing. However, because technology has advanced in the meantime and is consistently changing, this method is no longer as effective as it once was in preventing ice from forming.

The compressor must supply clean, dry, compressed air at the proper pressure and volume to the air drive pumps to achieve an adequate level of function. To meet these performance requirements, preventative maintenance may be necessary. Seen overall, proper treatment of the air in the compressor system is critical to reduce wear arising from dirty air and air motor freezing.

To reduce air motor freezing, Graco recommends that the air supplied to air motors be at a dew point at 55°F (11°C) and an air temperature of 70°F (21°C) to minimise air motor icing conditions. If one is using an older generation Graco NXT™ air motor, there is a "de-icing" feature with a bleed air valve installed that will send air through the motor and reduce icing.

However, Graco has developed the new King motor that reduces icing potential by up to 70% compared to previous Graco motors. The sprayer's thermally isolated poppets are insulated from motor castings that can often drop below freezing point. Furthermore, the newly designed poppets and external pilot lines virtually eliminate pilot valve freezing that can result in an instant stop. The components are easily accessed for service or replacement if necessary.

To learn more about the Graco King motor, visit graco.com/king. If you think you may have an air motor freezing problem and are seeking the best solution for your specific application, contact your local Graco Distributor. They will be able to assess the problem and provide you with the most cost-efficient, practical solution.