March 2, 2018

Thermo King Return Air Filters for Bus HVAC Applications


The purpose of the return air filter is to capture air-borne particles thus keeping the particles from collecting on the wet evaporator fin surfaces or passing through the coil and then becoming circulated within the passenger compartment.

To be an effective air filter for a bus application, the media must be able to capture particles ranging in size from 20 µm down to 3 µm. The media must maintain its filtering ability throughout its life, which is determined by the time it takes to fill the media to the point where airflow is adversely affected. The airflow is deemed adversely affected when the resistance to flow through the filter exceeds 1.0” WG.  At this point, the unit airflow is reduced such that there is insufficient warm air flowing through the evaporator coil and incomplete evaporation of the refrigerant.  The thermal expansion bulb senses the lack of super heat in the refrigerant and begins to close, reducing the evaporation temperature of the refrigerant.  As this condition continues, freezing of the condensate on the bottom of the coil could begin.  Also, the closing of the expansion valve reduces the refrigerant flow back to the compressor causing poor oil return and reduced cooling of the compressor, and could result in eventual compressor failure.

Thermo King return air filters are available in several filtering efficiencies with different dust holding capacities. The transit authority must determine the tradeoff between improved filtering capabilities and filter life so that proper filtering can be continuously provided to maintain proper air-conditioning performance. On the one hand, operating an air-conditioning system with a filter that has initial high filtering capability and low dust holding capacity leads to increased compressor operation to achieve the desired interior air temperature and increased frequency of replacing the filter.  On the other hand, operating the system with a filter that does not provide good filtering capability leads to dirty evaporator coils resulting in reduced cooling ability, reduced airflow, and increased frequency of coil cleaning. 

Because the filter media is used with the passenger compartment of a bus, the media must meet requirements of Federal Motor Vehicle Safety Standard 302 (49 CFR 571.302). This standard requires that all materials within 13 mm of the passenger compartment be tested to and comply with the standard. This standard was put in place to protect passengers in the event of a thermal event.  The standard specifies that in such an event, the materials used in the filter must be burn resistant. This requires the materials to either self-extinguish or burn at a very slow rate in order to provide protection for passengers. Thermo King return air filters meet the standard set forth by the Federal Motor Vehicle Safety Standard 302 (49 CFR 571.302).