Maintenance Tech Tip: Calculating Compressor Room Heat

In the past we shared tips for determining where to house your compressor. Once you’ve found the ideal home for your compressor, you need be mindful of a few key factors to ensure it continues working properly and efficiently.

Housing your compressor in a separate room saves space on the work floor and can reduce noise, but a few measures need to be taken to guarantee this room remains a safe space for your system. One of the most important issues is excess heat in compressor rooms.

Kinetic energy and heat in compressor rooms

We all know compressors generate heat. But how does that happen? Kinetic energy – the energy of movement – manifests as temperature. When the temperature of an object is lower than the ambient temperature, the difference is perceived as heat. This can be seen in many daily examples from the agitated water molecules that make your coffee hot to the warmth you feel from rubbing your hands together on a winter day.

In compressor rooms, heat can come from various sources such as sunshine, people, lights and motors, as well as any other particular sources of heat. Some of these sources, like sunshine, lights and motors, are described as sensible heat sources. Sensible heat is the heat associated with a change in temperature, while latent heat is usually associated with phase changes in water vapor. The heat load from people is part sensible and part latent.

Calculating sensible heat

Sensible heat sources make the biggest impact on compressor room heat. When deciding how much sensible heat to allow, each source must be considered. A temperature rise of 10 degrees is very common for compressor room environments. The sensible heat rise can be determined by the equation:

H_s = Q_s x ρ x C_p x ΔT

where

H_s = sensible heat gain (BTU/hr)

Q_s = volumetric flow for sensible heat (CFM)

ρ = density of air (lb/ft³)

C_p = specific heat of the air (BTU/lb-deg. F)

ΔT = change in temperature (degrees F)

To calculate the required general ventilation (also described as the required volumetric flow), the acceptable temperature or humidity rise must be determined. To do this, the volumetric flow required for dissipating the sensible heat and latent heat must be calculated. The necessary volumetric flow is usually the larger of the two numbers, and in compressor room applications that is always the sensible heat.

The type of compressor used can affect the amount of heat in the compressor room. Ventilation air from air-cooled compressors transmits almost 100 percent of motor energy in the form of heat, while water-cooled compressors only retain about 10 percent.

Effects of improper compressor room ventilation

But why go through all this trouble to cool off a room you probably won’t be in that often? While heat is a normal byproduct of kinetic energy, too much heat in a compressor room can cause problems.

Air-cooled compressors with high discharge temperatures can cause dryer inlet temperatures to exceed the 100 degrees F maximum and ambient temperatures of air-cooled dryers to exceed 100 degrees F. Both of these issues will reduce the rating of the dryer to a point that will impact the rated pressure dew point and possibly cause more moisture to condense. Higher inlet temperatures reduce the air density, which reduces the mass flow of the compressor.

High temperatures can also cause the lubricant in oil-injected compressors to varnish, and oil-injected rotary screw compressors will shut down when temperatures exceed about 230 degrees F. To avoid these risks, consult an expert to make sure your compressor environment is properly ventilated.