Thoughts on compressors for heat pump applications - Heat pumps for domestic applications

With the current discussion on climate changes, heat pumps have moved into focus, so many people in the process of building homes decide to use heat pumps for heating purposes and/or for hot water supply.

Ongoing discussions concerning the usage and the prices of fossil fuels have pushed the usage of heat pumps for existing buildings as well.

Heat sources for heat pump applications are primarily the air and the ground (often via brine systems). Depending on the heat source and the heating system in the building (e.g. floor heating or radiators), the consequences will be different operating conditions for the heat pumps.

If air is used as the energy source, the geographical location of the heat pump will have a major influence on the main operating conditions. As an example, the average outdoor temperatures according to Deutschem Wetterdienst, Offenbach (

• Hof: 1.1°C
• Frankfurt Airport: 4.9°C
• List / Sylt: 6.9°C

When comparing cities across Europe the results will be even larger differences in average outdoor temperatures. Consequently, an air/water heat pump used in the southern Rhone valley will show markedly different operating hours and conditions than a heat pump installed in an environment such as Gjovik (150 km north of Oslo, Norway).

Compressors used in heat pumps are optimized for certain operating conditions, but can also be used in conditions far away from the optimization point. However, the compressor COP usually decreases rapidly the farther away the operating conditions are from the compressor optimization point. Furthermore the compressor technology (e.g. rotary, piston, scroll, screw,..) influences the compressor performance and COP at the optimization point.

This lead to the conclusion that for some heat pumps, a piston compressor might be the best choice, while other heat pumps should be equipped with scroll compressors - depending on geographical location, usage, heat source, and building heating system, etc.

Focusing on one particular compressor technology only would waste the possible advantages of those compressor technologies not considered.

To reach the best results for a certain heat pump (or heat pump series) during the whole heating season, all compressor technologies should be taken into consideration and the best fit should be used depending on the planned application.

More and more customers ask for air conditioning systems with heat pump function for larger buildings. Such buildings often have high thermal loads due to solar heat, computer waste heat, lighting systems, machines, etc.

This usually means that the units will be operated in cooling mode most of the year.

During winter time, cloudy weather, nights, or weekends, it may be necessary to heat these buildings. It often makes sense to build the A/C units so that it can be operated in heating mode (heat pump mode) as well. One possible solution is to reverse the system.

To achieve a high COP during the whole year with these units, compressors optimized for cooling mode are often applied - since the main mode is cooling mode. For the shorter heating mode periods, lower COP due too the less than optimal conditions has to be accepted.

With the right choice of compressors, it is possible to realize savings due to higher performance and COP in the heating mode as well.

Danfoss offers Performer® Scroll compressors optimized for different condensing temperatures (38°C and 54°C) in order to reach the best performance depending on the operating mode (heating or cooling). Please refer to Fig. 3 to see the application envelopes for the different optimization points.


Fig. 3. Danfoss Performer® Scroll application envelopes for different optimization points.

The application envelopes for different optimization points are not the same, but both compressor types can be used for cooling and heating mode in larger A/C units.

Which optimization point should be used depends on the planned application, location etc. The right choice of optimization point assures the best possible COP in cooling as well as heating mode.

During heating mode, low condensing temperatures are sufficient in many units, since the air inlet temperatures are often only slightly above the room temperatures.

In this case, a compressor with an optimization point of 38°C (e.g. Danfoss Scroll Compressor type HRH054U4LP6 for R410A) would be a good choice. This compressor will run close to the optimization point most of the year and the A/C unit with heat pump mode will reach best performance and high COP all year around, thus ensuring low operating costs.

Refrigerant compressors are used in many different applications. Some of these applications are special heat pump applications. One example is a dry cleaning machine, i.e. a professional application for drying of clothes.

The general purpose and function of a household washing machine should be widely known, so here is an ultra short technical summary: Water and detergents are mixed to create suds. The suds dissolve the dirt from the clothes. Suds and dirt are pumped into the sink. Fresh water is used to rinse the clothes to remove the remaining suds.

Dry cleaning machines use a chemical substance (instead of suds) to remove the dirt (Hence the name "dry cleaning": no water is used).


Fig. 4. Dry cleaning machine

The chemical substance Per (Tetrachlorehten) and increasingly a mixture of different carbon hydrates is used as solvent. The substances reach best cleaning results below normal room temperature. Therefore, refrigeration systems are used to cool the solvent. Water cooled condensers are often used in dry cleaning machines.

For obvious environmental reasons is not feasible to pump the solvent into the sink like the suds from the household washing machine. The dirt is removed from these substances by evaporating a part of the solvent in a separate tank. The solvent evaporates, the dirt stays behind. The refrigeration system is used to condensate and to recycle the solvent. Until this point, the refrigeration system is used only for cooling: one evaporator is cooling the solvent, another evaporator is used to recycle the solvent. The refrigerant usually condenses in a water cooled condenser.

Considerable amounts of solvent would remain in the clothes if they were taken out of the dry cleaning machine without additional processing. A so-called 'desodoration phase' is therefore initiated to remove the remaining solvent: warm air is blown through the clothes. The warm air evaporates the solvent and removes it from the clothes. The solvent is recycled using an air cooled heat exchanger with low surface temperatures, where the solvent is condensed. The warm air with solvent is guided to the air cooler. The air cooler is operated with low evaporating temperatures to recycle as much solvent as possible. The cold air is guided back into the dry cleaning machine via a second heat exchanger to heat it up again.

As an alternative to water cooled refrigerant condenser, an air cooled condenser is often used in the process. The heat from the condenser is used to heat the air for the desodoration phase. In this optimized setup the unit works in a heat pump cycle with double effect. Both sides of the refrigeration system are used: the cold side to recycle the solvent, the warm side to heat the air for the desodoration phase.

The different phases of a dry cleaning process result in different loads and demands for the compressor :

• Medium to high evaporating temperatures during the cleaning phase (solvent is cooled, condensing temperature depends on the set point of the water valve of the water cooled condenser),
• Low evaporating temperatures with medium to high condensing temperatures during the desodoration phase (heat pump mode with air cooled condenser).

It is necessary to have a reliable compressor operation under all these conditions.

Many compressors are equipped with internal motor protection systems.

Danfoss H-Series Performer® Scrolls are equipped with an innovative protection system which enhances the compressor protection at low evaporating temperatures.

This makes the Danfoss H-Series Performer® Scroll compressor a very good solution for this kind of application (e.g. a dry cleaning machine)

An internal motor protection should be placed directly onto the motor winding to react directly on the motor temperature. This gives a fast reaction and the optimum protection.


Fig. 5. Cut away Performer® Scroll Compressor HR-series

Figure 5 shows a cut away of the upper part of a Danfoss Performer® Scroll HR-Series.

In addition to the normal internal motor protection, the green plastic tube guides the oil coming from the cranks shaft and the bearings to the motor protector.

The rotating counter weight (coloured in magenta) creates a certain gas flow in the compressor. At high to medium evaporating temperatures, the gas density is high and the gas flow blows away the oil from the protector. This pre-empts an early and unnecessary cut out due to warm oil. In addition, the gas flow can transport heat from moving parts (e.g. bearings, moving scroll) in the compressor to the internal protector, which enables an indirect protection of these parts.

At low evaporating temperatures, the refrigerant gas density is low. The heat transport capacity of the gas flow is too low to ensure a sufficient indirect protection of moving parts.

However, the low density gas flow is no longer capable of blowing away the oil from the motor protector. Now the oil is used to "transport" the heat from the moving part to the motor protector to ensure the protection.

The end result is a high class compressor protection - even if the operating conditions change quickly like those of dry cleaning machines.

The examples mentioned above show that sound concepts, innovative ideas, and the right compressor choice for a given application lead to energy efficient and environmentally friendly solutions.