Thermo King Compressors for Bus HVAC Applications

Compressors are essential in keeping buses cool and refrigerant flowing

The purpose of the Thermo King AC compressor is to force and move the refrigerant charge through the HVAC system to provide the desired level of cool air within the passenger compartment of a bus. The compressor's function is to elevate discharge pressure above ambient temperature so heat can be rejected and the refrigerant can be condensed into a liquid. At the same time, the compressor has to lower the suction pressure to a point that the refrigerant changes from a liquid to a vapor.  

To be an effective AC compressor, the Thermo King AC compressor was designed to operate to the capacities required that are optimal for the operating conditions of the HVAC system. The compressor is the workhorse of the AC system and is designed to withstand the high speeds and high temperatures required for long life. The core of the design is the key internal components within the compressor. The piston and piston sleeves are made of specific alloys that work together to eliminate the need for rings, which can break, drag and fail. Reducing drag reduces friction significantly which requires less horsepower and ultimately saves on fuel consumption. The shaft seal is made of a very specific bronze alloy with stainless steel bellows and a carbide hard ring that eliminates the potential of shedding particles that can contaminate the system and unseat the seal and cause seal failure. 

An improperly functioning compressor will not cool the system and provide the desired temperatures in the passenger area of the bus. But just as important, the less efficiently the compressor operates, the more time and power are required to reach the set-point. The increased time to reach the set-point requires more horsepower and consumes more fuel. At the same time, within the system, the thermal expansion bulb senses the lack of superheat in the refrigerant and begins to close, reducing the evaporation temperature of the refrigerant (high discharge pressure).  As this condition continues, freezing of the condensate on the bottom of the coil could begin (low suction pressure). Also, the closing of the expansion valve reduces the refrigerant flow back to the compressor causes poor oil return and reduced cooling of the compressor, and could result in eventual compressor failure.

Thermo King New Compressors

Thermo King provides new compressors, they are either sold as new, or sold under a reman number to accommodate customers that prefer to exchange cores. The Thermo King compressors sold as a reman with the core exchange are new, they are not rebuilt and not remanufactured. The compressors meet the original design and output specifications.

Competitive Remanufactured Compressors

There are competitors that remanufacture Thermo King Compressors to offer to customers. While most competitors do a fair job of rebuilding the compressors, they do not always use genuine Thermo King OE for internal parts replacements. In many cases, they will use inferior replacement parts, poorly reconditioned, or used parts. As a result, competitors that remanufacture the compressors may provide a functional compressor but one that may not be functioning at full capacity and have an unpredictable service life. 

Remanufactured Compressor Analysis

An analysis of 2 competitive remanufactured compressors was conducted and compared to the Thermo King Compressor specifications. Capacity testing showed an average loss of 16% capacity. In both cases, there was evidence of excessive wear of the bearings, crankshaft journals which resulted in discoloration of the oil, and evidence of metal shavings in the system. (Figures 1-4) Other findings were improperly honed cylinder sleeves, worn oil pump drive shafts, and oil pump assemblies.  The most extreme example was the failure of the compressor within 15 minutes of operation caused by the over-torquing of the oil pressure regulating valve (Figure 5). Leak testing showed improperly machined sealing surfaces. 

Figure 1. Rod bearings showed evidence of excessive wear. Discolored oil, and bearing material particles in the compressor oil.

Figure 2. Crank shaft Journal showed evidence of excessive wear. Crank shaft particles in the compressor oil.

Figure 3. The drive end of crankshaft showed excessive wear, and very poorly reconditioned surfaces.

Figure 4. Discolored compressor oil, and material particles from the crankshaft and connecting rod bearings.

Figure 5. Elongation and twisting are signs of material failure from over torqueing.

The transit authority must determine the trade-off between a somewhat higher upfront cost of a compressor that is efficient and durable enough to maintain proper air-conditioning performance and the potential of a less efficient compressor that can lead to loss of fuel efficiency, additional repairs and unscheduled loss of service.