NATE Air Conditioning and Heat Pumps Practice Exam

Heat pump compressors sometimes run hotter than compressors used in cooling-only units because heat pump compressors ____.

1. exhibit low refrigerant density at low suction pressures during the heating cycle

2. pump liquid refrigerant during the cooling cycle

3. must prevent ice formation during the defrost cycle

4. must heat indoor spaces in the winter

The correct answer is: exhibit low refrigerant density at low suction pressures during the heating cycle

The correct choice highlights that heat pump compressors experience lower refrigerant density at low suction pressures during the heating cycle. This phenomenon occurs because heat pumps often operate by extracting heat from outdoor air, even when temperatures are low. During this process, particularly in colder conditions, the refrigerant entering the compressor has a lower density, which can lead to less efficient heat transfer and increased operating temperatures within the compressor. Understanding this helps explain the challenges heat pump compressors face compared to those in cooling-only systems. In cooling applications, the compressors primarily deal with more consistent refrigerant densities since they are designed to work in warmer ambient conditions, focusing on heat removal rather than heat transfer in lower temperatures. In the context of the other options, while they pertain to specific operational aspects of heat pumps, they do not directly address the reason behind the higher operating temperatures of heat pump compressors. For example, the pumping of liquid refrigerant is more an issue related to the design of the system and can lead to damage but does not inherently cause the compressor to run hotter during normal operation. Similarly, preventing ice formation and heating indoor spaces are operational necessities of heat pumps, but they don’t specifically pertain to the compressor's thermal management as intricately as the aspects of refrigerant density