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Components for heat pumps (introduction)

  • Monday, 16 January, 2012

Heat pumps are currently in fashion, and are even subsidised by plenty of state, which is quite sensible, because not only do they run on electricity that is emission-free at the point of use, they are also comparatively economical and draw their power from the national grid and centralized generating stations. In general, it will always be more efficient to use centrally generated power to operate equipment such as heat pumps rather than to generate heat locally by burning fossil fuels such as oil or gas, although decentralised solutions such as solar panels, can also be use to operate heat pumps. Heat pumps can be broadly subdivided into three basic systems, air–air, air–water and water (brine)–water. In this instalment you can read about the components that manufacturers use in these heat pump systems.

Heat pumps are currently in fashion, and are even subsidized by plenty of state, which is quite sensible, because not only do they run on electricity that is emission-free at the point of use, they are also comparatively economical and draw their power from the national grid and centralized generating stations. In general, it will always be more efficient to use centrally generated power to operate equipment such as heat pumps rather than to generate heat locally by burning fossil fuels such as oil or gas, although decentralized solutions such as solar panels, can also be use to operate heat pumps. Heat pumps can be broadly subdivided into three basic systems, air–air, air–water and water (brine)–water. In this installment you can read about the components that manufacturers use in these heat pump systems.

Function

A heat pump transports heat. This is the defining characteristic of all heat pumps. It functions in the same way as a compression refrigeration system, except that the principal focus is not on the evaporator side, i.e. the cool side, but on the condenser side where the heating effect is. The essential principle remains the same: evaporation of refrigerant in the evaporator, increase of pressure and hence temperature in the compressor, dissipation of the gained heat by liquefying the refrigerant in the condenser, and subsequent expansion of the refrigerant through the throttle valve. The refrigerants used are also hardly any different from those used in an ordinary refrigeration plant. The preferred refrigerants for heat pump applications are R407C, R410A and R134a.

Types of heat pump

Heat pumps are designated according to the media at the evaporator and condenser, so in an air to air heat pump, the heat is extracted from the surrounding air via the evaporator and passed, in the form of a temperature increase, via the heat pump cycle to the air of the room that is being heated. This is the principle that will generally be found in simple and inexpensive split air-conditioning units with a cycle-reverse function. In the case of an air to water heat pump, the heat is taken from the surrounding air, as for the air to air system, but on the secondary side it is released into the hot water or heating circuit. In the case of a water (brine) to water heat pump, the heat is generally extracted from the ground to be released into the hot water network. Water or brine can be used depending on the expected temperatures. Where pure water is used, special consideration must be given to possible operating conditions at subzero temperatures and during wintertime. If in doubt, the addition of antifreeze (e.g. glycol or some other alcohol) may be a solution. Where the primary circuit runs at markedly subzero temperatures, brine is to be preferred. Water (brine) water heat pumps can be further subdivided into groundwater or well-water heat pumps and heat pumps with horizontal and vertical collectors. For a groundwater or well-water heat pump the heat is taken from the groundwater or from a well. Water (brine) to water heat pumps with horizontal collectors are very popular for houses with plenty of garden. In this case the collectors (plastic or copper pipes) are distributed over the area at a reasonable depth. The third variant for water (brine) to water heat pumps has a vertical collector. This does not require a large area of land but does call for deep drilling, which is a considerable cost factor and needs to be reconciled with the geological situation. There is no general rule of thumb here. For example, in some cases, rocky ground may be more suitable for drilling than clay soil. The following rule applies to all types of heat pumps: Each Kelvin of higher temperature in the heat source produces a higher temperature in the evaporator and therefore leads to higher efficiency in the heat pump.

Compressors

In subsequent instalments of this series we shall be taking a closer look at the components used by heat pump manufacturers. The compressor has a central role to play. Two compressors technologies are particularly commonly used for heat pumps in a domestic or commercial setting: reciprocating compressors and scroll compressors. Reciprocating compressors are the classic indestructible solution, while scroll compressors are a more modern variant that is becoming more and more popular. When choosing a compressor for a heat pump it is important to consider the refrigerant and the field of application. The compressor must be approved for the desired refrigerant and must be usable for a wide range of applications. The range on the low-pressure side should include higher and lower evaporation temperatures so that the heat pump is always ready for use in spite of changes in the source temperature over the course of the year. If the heat pump is used only during interseasonal periods and some other form of heating such as gas or oil is used in winter, then the compressor will not need to be ready for extra low evaporation temperatures. When choosing a compressor for the condenser side we have to consider the necessary input temperature for the hot water. Underfloor heating is ideal for a heat pump and equally so for the compressor. Underfloor heating systems require only low input temperatures, so they only need low condenser temperatures. This increases the efficiency of the heat pump. The worst case is where old cast-iron radiators are installed in the building. These call for the highest input temperatures – modern radiators will usually be content with 10K lower. In this case the compressor needs to be geared for condenser temperatures of up to 65°C, because it still needs to give up its heat to the heating circuit. Not least, the compressor should have a good coefficient of performance (COP) under the expected operating conditions. The COP of a heat pump compressor is the quotient of the delivered heating capacity to the power consumption. A high COP value is always better than a low one. An even more meaningful indicator for the compressor of a heat pump is its seasonal coefficient of performance, which is the COP value averaged out or measured over an entire year. The seasonal coefficient of performance ensures that the compressor is not merely optimized for a specific design point (pressure ratio), rapidly losing efficiency if the conditions deviate slightly from this.

Throttle device

Which throttle device should be used? Here you can basically use any of the three classic solutions – capillary tube, thermostatic or electronic expansion valve. In view of the especial significance now accorded to energy efficiency the capillary tube type is no longer considered state-of-the-art. Thermostatic expansion valves are one possible option, but electronic injection is unbeatable and is the best solution for progressive, highly optimized systems. A thermostatic expansion valve is an automatic mechanical controller with no external energy source. This means that it does not need an electrical connection in order to function, which is a clear advantage for the heat pump manufacturer. Thermostatic expansion valves are designed to always guarantee sufficient super-heating at the evaporator outlet. This prevents damage to the compressor and ensures that the evaporator is always well supplied with refrigerant. Thermostatic injection solutions may thus be the right choice for heat pump systems, although thermostatic expansion valves always operate with the same target (super-heating) value. An electronic injection controller can do this better because a pressure sensor and a sensitive temperature sensor continually provide it with feedback about the current super-heating level in the evaporator. This means that the controller can take appropriate measures to ensure the lowest possible level of super-heat and modify the target super-heat value to reflect conditions in the plant. This adaptive controlling of the refrigerant injection results in optimum use of the evaporator and therefore the highest evaporation pressures that can be realized in the particular system, and this in turn leads to higher COP values.

Heat exchangers

Obedient to the call for ever-lower superheating and higher evaporation temperatures, heat exchangers used in heat pumps always aim to minimise their temperature difference. For evaporators in air to water and air to air systems this can be achieved by making the heat exchanger sufficiently large, and the same applies to air-cooled condensers in air to air systems. For water (brine) to refrigerant heat exchangers it is at the same time possible to save weight and refrigerant volume without having to pay for this with higher temperature differences in the heat exchanger. Using compact brazed plate heat exchangers improves the flow through the plates, making it possible to optimise the utilization of the heat exchanger surfaces in both fluids. The more advantageous relationship between the maximum and minimum flow speeds also serves to improve heat transfer. The result is higher efficiency with a low heat flux density within the very narrow temperature ranges that characterize heat pump systems. Optimised series H compact plate heat exchangers ("H" as in "heat pump") are equally well suited for use as evaporators or compressors. Micro plate heat exchangers (MPHEs), with their special point channel pattern, achieve markedly better heat transference than the formerly used herringbone pattern used in traditional plate heat exchangers.

Four-way reversing valves

Four-way valves are used to reverse the cycle of "one-to-one" heat pump systems. This makes the evaporator into a condenser and the condenser into an evaporator. Such a circuit is used, for example, in split-system air-conditioners, which provide cooling during the summer and heat during the interseasonal periods. When they are used for heating these units become air to air heat pumps. A four-way reversing valve has four pipe connections, three of them on one side and the fourth on the opposite side, whereby the three copper connectors have a larger diameter than the one on the opposite side. The middle one of the three large connectors is permanently on the suction side, and the single smaller connector permanently on the pressure side. Since the other two may be on the suction or the pressure side depending on how the unit is switched, they are dimensioned as for the permanent suction connection in order to allow only a minimal pressure drop. A four-way valve also possesses a pilot solenoid valve with a coil that can be powered to change the direction of flow of the refrigerant. There are small-diameter pilot connections between the small-diameter valve connection and the pilot solenoid valve and from there to the central large diameter connector.

Filter driers and bi-flow driers

Filter driers are generally installed in the liquid lines of heat pump systems, where they carry out a dual role. Their purpose is, firstly, to catch coarse particles of dirt and copper filings, and secondly, to bind moisture in the system. For this purpose, for optimum drying and filtering performance, modern filter driers for heat pumps are equipped with a 100% molecular sieve dry block. The filter drier should be replaced each time any work is done on the heat pump system. There is a wide range of different types of filter driers available. Standard driers of type DML are encountered most frequently in standard heat pump systems. These have a solid material core and form an inseparable unit with their compact housing. These standard driers are designed for only one flow direction, which is usually entirely sufficient. If you really want bi-flow operation – as for instance in a heat pump with a four-way reversing valve – then you can choose a bi-flow drier of type DMB. Bi-flow driers can be used with the flow in either direction without risking previously filtered material being sent straight back into the system.

Pressure controls

Pressure controls are used in heat pumps and can be subdivided into high-pressure and low-pressure controls. High-pressure controls (switches) are used as cut-outs for the compressor, which is the principal pressure source in a heat pump. Low-pressure switches protect the compressor from low pressures due to e.g. insufficient refrigerant or – in a water to water heat pump – failure of the primary circuit pump (and a faulty flow switch). There are two basic types of pressure controls, standard adjustable wall-mount and cartridge pressure controls. The wall-mount pressure control of type KP is particularly popular with fitters. It enables trip thresholds to be adjusted and its weight is not entirely borne by the pipe. It is also possible to locate the pressure control in a compartment at the front of the equipment, making it much easier to access and maintain. Cartridge pressure controls of type ACB are, however, the series manufacturer's preferred solution, because their fixed settings cannot be modified by unauthorized persons in the field. Cartridge pressure controls are also usually extremely reasonably priced.

Controllers

Heat pump manufacturers like to use freely programmable controllers as these can easily be used to implement not only all the usual functions but also functions of the manufacturers' own devising. Danfoss MCX controllers are one such solution. They are compatible with open standards and ideal for controlling classical heat pump installations. They offer manufacturers a unique level of versatility on the basis of a standard hardware platform, which serves to guarantee the availability of replacement parts. Their support for open programming standards facilitates a wide range of control options and extensive scope in the design of heat pump applications. In spite of being a standard component they make it possible to individualize the end product, so that the heat pump manufacturer remains in control of after-sales service business. Unlike many manufacturer-specific solutions, the MCX controller makes use of the well-known standard programming language C++, which makes the heat pump manufacturer almost entirely independent of the manufacturer of the control.

Conclusion

A lot of factors need to be taken into consideration in the construction of heat pump systems. You will be able to read more about heat pump components in subsequent installments. Only correctly chosen and configured components can interact properly and give you an energy-efficient and affordable heat pump.