Refrigerator Troubleshooting Diagram

Archive for the ‘Refrigerator Heat Pumps’ Category

Air for Heat Source of Heat Pump

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While ambient air is free and widely available, there are a number of problems associated with its use as a heat source. In the cooler and more humid climates, some residual frost tends to accumulate on the outdoor heat transfer coil as the temperature falls below the 2-5°C range, leading to a reduction in the capacity of the heat pump. Coil defrosting can be achieved by reversing the heat pump cycle or by other less energy efficient means. This results in a small energy penalty because during the defrost cycles cool air is circulated in the building. Provided the defrost cycle is of short duration, this is not significant. In addition, for thermodynamic reasons the capacity and performance of the heat pump fall in any case with decreasing temperature. As the heating load is greatest at this time, a supplementary heating source is required. This device could be an existing oil, gas or electric furnace or electric resistance heating; the latter is usually part of the heat pump system. The alternative to the provision of a supplementary heating device is to ensure that the capacity of the heat pump is adequate to cope with the most extreme weather conditions. This can result in over-sizing of the unit at a high additional capital cost and is not costeffective compared with the cost of supplementary heating devices.

Exhaust (ventilation) air is a common heat source for heat pumps in residential and commercial buildings. The heat pump recovers heat from the ventilation air, and provides water and/or space heating. Continuous operation of the ventilation system is required during the heating season or throughout the year. Some units are also designed to utilize both exhaust air and ambient air. For large buildings exhaust air heat pumps are often used in combination with air-to-air heat recovery units.

Outside ambient air is the most interesting heat source as far as availability is concerned. Unfortunately when the space heating load is highest the air temperature is lowest. However, temperatures are not stable. The COP of vapor-compression heat pumps decreases with decreasing cold source temperature. In addition at evaporator temperatures below 5°C air humidity is deposited on the evaporator surface in the form of ice. This does not improve the heat transfer and leads to lower working fluid temperatures and therefore lower COP values, depending upon the temperature of the air flowing over the evaporator. If ice formation occurs periodic de-icing of the evaporator surface has to be applied. This invariably leads to decreased values of the overall system COP (5–10%).

Written by sam

January 30th, 2010 at 5:45 pm

Heat Pumps Heat Sources

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To understand the basic principle of the heat pump, one must realize that heat is a form of energy, the quantity of which is quite independent of the temperature which happens to exist at the time. In air, soil and water, in air extracted from buildings, and in waste water of any kind, there are enormous quantities of heat which are useless only because the temperature is too low. From all these sources, heat can be extracted, and with a small expenditure of additional, high-grade energy a heat pump can upgrade the waste heat to a temperature suitable for room heating.

The primary heat sources include air, water and soil. In practice, air is the most common source for heat pumps while water- and soil-source systems are less commonly applicable. In general, air, soil and ground water are considered practicable as heat sources for small heat pump systems while surface water, sea water and geothermal are more suited to larger heat pump systems. As far as low-temperature sources is concerned, ground or surface water, air and soil are most commonly used.

The technical and economic performance of a heat pump is closely related to the characteristics of the heat source. An ideal heat source for heat pumps in buildings has a high and stable temperature during the heating season, is abundantly available, is not corrosive or polluted, has favorable thermophysical properties, and requires low investment and operational costs. In most cases, however, the availability of the heat source is the key factor determining its use. Table 4.4 presents commonly used heat sources. Ambient and exhaust air, soil and ground water are practical heat sources for small heat pump systems, while sea/lake/river water, rock (geothermal) and waste water are used for large heat pump systems.

Several heat pump configurations can be visualized utilizing a seemingly inexhaustible number of energy sources. Some of these energy sources are outside air, sensible heat from stream or well water, latent heat diffusion from water (ice formation), warm discharge effluents from industry, fireplace waste heat, and heat generation in sewage. Most of these energy sources are not widely available to the general public. Four types of heat pump systems are in common use in practice:

• single-package heat pumps using an air source,
• split-system heat pumps using an air source,
• single-package heat pumps using a water source, and
• split-system heat pumps using a water source.

Single-package heat pumps have all the essential components contained within a single unit while split-system heat pumps house the essential components in two separate units (i.e. one unit outdoors and one unit indoors).

Written by sam

January 27th, 2010 at 8:17 pm

Heat Pump Applications in Industry

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The most common waste heat streams in industry are cooling water, effluent, condensate, moisture, and condenser heat from refrigeration plants. Because of the fluctuation in waste heat supply, it can be necessary to use large storage tanks for accumulation to ensure stable operation of the heat pump. Some common applications can be summarized as follows

• Space heating: Heat pumps can utilize conventional heat sources for heating of greenhouses and industrial buildings, or they can recover industrial waste heat that could not be used directly, and provide a low- to medium temperature heat that can be utilized internally or externally for space heating. Mainly electric closed-cycle compression heat pumps are used.
• Process water heating and cooling: In many industries, warm process water in the temperature range from 40-90°C is needed particularly for washing, sanitation and cleaning purposes. Heat pumps offer good potential for such applications and may be a part of an integrated system that provides both cooling and heating. Although electric closed-cycle compression heat pumps are mainly installed, some absorption heat pumps and heat transformers may find application.
• Steam production: In industrial processes, vast amounts of low-, medium- and highpressure steam in the temperature range from 100-200°C are consumed. In the market, at present, the high-temperature heat pumps can produce steam up to 300°C. In this regard, open and semi-open MVR systems, closed-cycle compression heat pumps, cascade systems and some heat transformers are employed.
• Drying and dehumidification process: Heat pumps are used extensively in industrial dehumidification and drying processes at low and moderate temperatures (maximum 100°C). The main applications are drying of pulp and paper, various food products, wood and lumber. Drying of temperature-sensitive products is also interesting. Heat pump dryers generally have high coefficient of performance (with COP of 5 to 7). and often improve the quality of the dried products as compared with traditional drying methods. Because the drying is executed in a closed system, odors from the drying of food products etc. are reduced. Both closed-cycle compression heat pumps and MVR systems are used.
• Evaporation, distillation and concentration processes: Evaporation, distillation and concentration are energy-intensive processes, and most heat pumps are installed in these processes in the chemical and food industries. In evaporation processes the residue is the main product, while the vapor (distillate) is the main product in distillation processes. Most systems are open or semi-open MVRs, but closed-cycle compression heat pumps are also applied. Small temperature lifts result in high performance with COPs ranging from 6 to 30.

In addition to the above mentioned processes, there are some significant applications of heat pumps, covering a very wide range, from connected loads of a few watts for the thermoelectric heating/cooling units in the food industry, to loads of several megawatts for large vapor compression plants in industry, including:

• Small, mass-produced, hot water heaters, sometimes combined with refrigerators and with connected loads between 200 and 800 W.
• Heating heat pumps for individual rooms, single-family houses, smaller office buildings, restaurants and similar projects. Package heat pumps (in closed casings) are also available as split units with an indoor and an outdoor section for installation in the open. Mass-produced, sometimes on a large scale, these have a heat output often with supplementary heating (electric, liquefied gas, warm water) up to about 120 kW, and connected loads from 2 to 30 kW.
• Heat pumps for heating and heat recovery for large air-conditioning plants in office blocks, department stores and similar projects. Appropriately adapted mass-produced chilled water units as well as systems individually assembled from the usual components for large refrigeration plants are used. Heat output is up to more than 1200 kW and the connected load is between 20 and 400 kW. If the heat pump is also used for cooling in summer, it is often better to use it to recover heat from the extract air in winter than to use an additional recuperative heat exchanger.
• Heating-cooling heat pumps for cooling and heating of rooms, objects of mass flows. The main task of these plants, also determining the control, is usually for either cooling or heating, not both, since the other effect is an additional gain which is not available during non-operational periods of the system and can only be supplied by a store, e.g. a hot water boiler.
• Waste heat utilization heat pumps for utilizing or reusing discharged heat which cannot be reused immediately because of its low temperature. This is so, for example, in drying processes in which the waste heat contained in the extracted water vapor is used for heating the drying air, or in laundries where practically all the applied heat energy is discharged with the waste water and can be recovered by a heat pump. The plants are controlled by heat demand, often combined with the storing of waste heat.

Written by sam

January 26th, 2010 at 9:40 pm

Large Heat Pumps for District Heating and Cooling

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Many large electrically driven heat pumps are running all over the world today and even more are ordered and planned for the future. Large heat pumps are defined as equipment with an output of about 500 kW or more. These are particularly used for district heating and cooling applications.

The point has often been made that the heat pump is competitive and is well established in markets where cooling is required, too. These markets are:

• simultaneous production of cold and heat (double utilization) such as in more recent HVAC applications or in the classical commercial cases of skating rink plus swimming pool, or refrigeration plus hot tap water production; and

• consecutive production of cold and heat in HVAC plants with the same equipment, known as the heating/cooling heat pump (air cooling and dehumidifying in the summer season and heating and possibly humidifying during the winter season).

The large heat pump for district heating and cooling use proves to be well-suited:

• for base load coverage in systems without combined heat and power generation (CHP),
• for low load, low temperature summer operation for domestic hot water production,
• where supply and/or return temperatures are low,
• where water is available as a heat source, for instance cleared sewage water, industrial waste water, lake or sea water. (There are also plants with a heat capacity up to 2.5 MW using ambient air as a heat source.)

Written by sam

December 8th, 2009 at 10:22 am

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Sectoral Heat Pump Utilization

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As mentioned earlier, a heat pump is a device that gets heat at a certain temperature and releases this heat at a higher temperature. When operated to provide heat (e.g. for space heating or water heating), the heat pump is said to operate in the heating mode; when operated to remove heat (e.g. for air-conditioning), it is said to operate in the cooling mode. In both cases, additional energy has to be provided to drive the pump. Overall, this operation becomes energetically attractive if the total energy output is greater than the energy used to drive the heat pump and economically attractive if the total life-cycle cost (including installation, maintenance and operating costs) is lower than for a competing device.

The common heat source for a heat pump is air, although water is also used in many applications. During the past decade ground or geothermal resources have received increasing attention to be used as a heat source, particularly in residential applications. From the sectoral utilization point of view, air is considered the most common distribution medium where the heat pump provides both heating and cooling. For heating only, air is also a common medium, except in those regions where many water distribution systems are installed in the residential sector. The energy needed to drive a heat pump is normally provided by electricity or fossil fuels, such as oil or gas.

The general characteristics of some typical commercially available heat pump systems are listed in Table 4.1 for the residential, commercial and industrial sectors. For the commercial sector, all the basic characteristics are similar to those in the residential sector except for the fuel drive. In the former sector, a greater variety of fuels can be used because of the larger-scale operation which suits fossil engine systems. In industry, large-scale uses also result in greater fuel flexibility and the heat source is usually waste hot water, steam or humid air. The type of heat sink will depend on the particular industrial process.

The heating and cooling of single and multi-family houses has become the most successful application of heat pumps thus far. A large variety of systems exists depending upon whether they are intended for both heating and cooling or only heating, the nature of the low temperature source and the medium distributing the heat (cold) to the building (air, water, etc.).

The heating-only heat pump is applicable to the residential sector in many countries where there is no air-conditioning load. Units can be installed separately or as add-on devices. While performance tends to be higher than for existing systems, the major difficulty is that the higher first-cost of the unit can be recovered only over the heating season, in contrast to heating and cooling units which operate throughout the year. As indicated earlier, the electric add-on heat pump is a system that can be used in conjunction with fossil fuel-fired furnaces or with central electric warm air furnaces.

For the residential sector, output requirements from a heat pump vary according to the use to which the output is applied, as indicated in Table 4.2. The requirements of a single family residence will range from 4 to 30 kW depending on the size, type and degree of insulation of the building. Multi-family building needs range from 20 kW for a two-family residence, to 400 kW for an apartment block, although non-central installations involve smaller size units. Depending on the size of the grid, district heating schemes can range from 400 kW for a localized application to 10 MW for a large-scale system. The output needs of the commercial sector range from 20 kW for shops and small offices to 1 MW for large commercial centers. A greater range, from 100 kW to 30 MW, is found in the industrial sector. The delivery temperature also varies with the requirements of a particular application. Table 4.3 summarizes the temperature requirements for a number of uses.

table-4-2

table-4-3

Heat pumps for residential heating and cooling can be classified into four main categories depending on their operational function:

• Heating-only heat pumps for space heating and/or water heating applications.
• Heating and cooling heat pumps for both space heating and cooling applications.
• Integrated heat pump systems for space heating, cooling, water heating and sometimes exhaust air heat recovery.
• Heat pump water heaters for water heating.

In residential applications room heat pumps can be reversible air-to-air heat pumps (ductless packaged or split type units). The heat pump can also be integrated in a forced-air duct system or a hydronic heat distribution system with floor heating or radiators (central system).

They often use air from the immediate surroundings as heat source, but can also be exhaust-air heat pumps, or desuperheaters on air-to-air and water-to-air heat pumps. Heat pumps can be both monovalent and bivalent, where monovalent heat pumps meet the annual heating and cooling demand alone, while bivalent heat pumps are sized for 20-60% of the maximum heat load and meet around 50–95% of the annual heating demand. The peak load is met by an auxiliary heating system, often a gas or oil boiler. In larger buildings the heat pump may be used in tandem with a cogeneration system.

In commercial/institutional buildings the heat pump system can be a central installation connected to an air duct or hydronic system, or a multi-zone system where multiple heat pump units are placed in different zones of the building to provide individual space conditioning. Efficient in large buildings is the water-loop heat pump system, which involves a closed water loop with multiple heat pumps linked to the loop to provide heating and cooling, with a cooling tower and auxiliary heat source as backup.

In residential, commercial and institutional buildings, recently, there is an increasing interest in room type controlled heat pumps (Figure 4.1). In addition to some benefits such as greater comfort, reduced noise and reduced energy use, some features of this type of system are:

• preventing operation when connection is made to the wrong supply voltage or if the wiring is incorrect,
• preventing overheating of the compressor, fan motor and power transistor,
• detecting refrigerant undercharge and evaporator freeze-up, and
• maintaining the pressure balance by controlling the on/off switching cycle of the compressor.

room-heat-pump

Written by sam

November 29th, 2009 at 9:13 pm

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Refrigerator Heat Pump Efficiencies

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There are four different criteria used to describe heat pump efficiency. In all of these criteria, the higher the number the higher the efficiency of the system. Heat pump efficiency is determined by comparing the amount of energy delivered by the heat pump to the amount of energy it consumes. It is important to highlight that efficiency measurements are based on laboratory tests and do not necessarily measure how the heat pump performs in actual use.

Coefficient of Performance (COP)

The COP is the most common measurement used to rate heat pump efficiency. COP is the ratio of the heat pump’s heat output to the electrical energy input, as follows:

COP = Heat Output/Electrical Energy Input

For example, air-source heat pumps generally have COPs of 2 to 4; they deliver 2 to 4 times more energy than they consume. Water and ground source heat pumps normally have COPs of 3 to 5. The COP of air-source heat pumps decreases as the outside temperature drops. Therefore, two COP ratings are usually given for a system: one at 8.3°C (47°F) and the other at -9.4°C (17°F). When comparing COPs, make sure ratings are based on the same outside air temperature. COPs for ground- and water-source heat pumps do not vary as much because ground and water temperatures are more constant than air temperatures.

While comparing COPs is helpful, it does not provide the whole picture. When the outside temperature drops below 4.4°C (40°F), the outdoor coils of a heat pump must be defrosted periodically. It is actually possible for the outdoor coil temperature to be below freezing when a heat pump is in the heating cycle. Under these conditions, any moisture in the air will freeze on the surface of the cold coil. Eventually the frost could build up enough to keep air from passing over the coil and the coil would then lose efficiency. When the coil efficiency is reduced enough to appreciably affect system capacity, the frost must be eliminated. To defrost the coils, the heat pump reverses its cycle and moves heat from the house to the outdoor coil to melt the ice. This reduces the average COP significantly.

In fact, some heat pump units have an energy saving feature that will allow the unit to defrost only when necessary. Others will go into a defrost cycle at set intervals whenever the unit is in the heating mode. Another factor which lowers the overall efficiency of air-to-air heat pumps is their inability to provide enough heat on the coldest days of the winter. This means a back-up heating system is required. This back-up is often electric resistance heat, which has a COP of only one. Whenever the temperature drops into the -3.8°C to -1.1°C range, or whatever its balance point is, and this electric resistance heat kicks in, overall system efficiency drops.

Primary Energy Ratio (PER)

Heat pumps may be activated either electrically or by engines (like internal combustion engines or gas motors). Unless electricity comes from an alternative source (e.g. hydro, wind, solar, etc.), heat pumps also utilize primary energy sources upstream like a thermo-electric plant or on-spot like a natural gas motor. When comparing heat pump systems driven by different energy sources it is more appropriate to use the PER, as defined by Holland et al. (1982), as the ratio of useful heat delivered to primary energy input. So this can be related to the COP by the following equation:

PER = n • COP

where n is the efficiency with which the primary energy input is converted into work up to the shaft of the compressor.

However, due to high COP, the PER, as given below, becomes high relative to conventional fossil fuel fired systems.

In the case of an electrically driven compressor where the electricity is generated from a coal burning power plant, the efficiency r may be as low as 0.25 or 25%. The above equation indicates that gas engine driven heat pumps are very attractive from a primary energy ratio point of view since values for r of 0.75 or better can be obtained. However, heat recovery systems tend to be judged on their potential money savings rather than their potential energy savings.

(EER)potential energy savings.

The EER is used for evaluating a heat pump’s efficiency in the cooling cycle. The same rating system is used for air conditioners, making it easy to compare different units. In practice, EER ratings higher than 10 are the most desirable. EER is the ratio of cooling capacity provided to electricity consumed as follows:

EER = Cooling Capacity/Electrical Energy Input

Heating Season Performance Factor (HSPF)

A heat pump’s performance varies depending on the weather and how much supplementary heat is required. Therefore, a more realistic measurement, especially for air-to-air heat pumps, is calculated on a seasonal basis. These measurements are referred to as the Heating Season Performance Factor (HSPF) for the heating cycle. The industry  standard test for overall heating efficiency provides a rating known as HSPF. Such laboratory test attempts to take into account the reductions in efficiency caused by defrosting, temperature fluctuations, supplemental heat, fans and on/off cycling. HSPF is the estimated seasonal heating output divided by the seasonal power consumption, as follows:

HSPF = Total Seasonal Heating Output/Total Electrical Energy Input

It can be thought of as the ‘average COP’ for the entire heating system. An HSPF of 6.8 corresponds roughly with an average COP of 2. HSPFs of 5-7 are considered good. The higher the HSPF. the more efficient the heat pump. To estimate the average COP, divide the HSPF by 3.4.

Most utility-sponsored heat pump programs require that heat pumps have an HSPF of at least 6.8. Many heat pumps meet this requirement. Some heat pumps have HSPF ratings above 9. In general, more efficient heat pumps are more expensive. Compare the energy savings to the added cost.

Seasonal Energy Efficiency Ratio (SEER)

As explained above, a heat pump’s performance varies depending on the weather and the amount of supplementary heat required. Thus, a more realistic measurement, particularly for air-to-air heat pumps, is calculated on a seasonal basis. These measurements are referred to as the Seasonal Energy Efficiency Ratio (SEER) for the cooling cycle. Therefore SEER is rating the seasonal cooling performance of the heat pump. The SEER is the ratio of the total cooling of the heat pump to the total electrical energy input during the same period.

SEER = Total Seasonal Cooling Output/Total Electrical Energy Input

Naturally, the SEER for a unit will vary depending on where in the country it is located. SEERs of 8-10 are considered good. The higher the SEER the more efficiently the heat pump cools. The SEER is the ratio of heat energy removed from the house compared to the energy used to operate the heat pump, including fans. The SEER is usually noticeably higher than the HSPF since defrosting is not needed and there is no need for expensive supplemental heat during air conditioning weather.

Written by sam

November 29th, 2009 at 7:25 am

Refrigerator Heat Pump

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Heat pumps have enormous potential for saving energy, particularly in industrial processes. They are the only heat recovery systems which enable the temperature of waste heat to be raised to more useful levels. Although the principle of the heat pump has been known since the middle of the nineteenth century, there was little incentive to develop them in a time of cheap and abundant energy.

Recent research and development has indicated that heat pump performance is likely to improve over the coming years. Improvements in component design and in use of waste heat sources will raise heat pump performance. With regard to technical aspects, the many years of experience that have brought about important findings for the planning and design of heat pump systems can be used. Moreover, new ideas and equipment appearing in the last decade have simplified the construction of the heat pump heating and cooling systems.

Heat pumps look and operate very much like air conditioners (only for forced-air systems) with the notable exception that they provide both heating and cooling. While heat pumps and air conditioners require the use of some different components, they both operate on the same basic principles.

Heat flows naturally from a higher to a lower temperature. Heat pumps, however, are able to force the heat flow in the other direction, using a relatively small amount of high quality drive energy (electricity, fuel, or high-temperature waste heat). Thus, heat pumps can transfer heat from natural heat sources in the surroundings, such as the air, ground or water, or from man-made heat sources such as industrial or domestic waste, to a building or an industrial application. Heat pumps can also be used for cooling. Heat is then transferred in the opposite direction, from the application that is cooled, to surroundings at a higher temperature. Sometimes the excess heat from cooling is used to meet a simultaneous heat demand.

Almost all heat pumps currently in operation are based either on a vapor compression, or on an absorption cycle. Theoretically, heat pumping can be achieved by many more thermodynamic cycles and processes, including Stirling and Vuilleumier cycles, singlephase cycles (e.g. with air, CO2 or noble gases), solid-vapor sorption systems, hybrid systems (notably combining the vapor compression and absorption cycle), thermoelectric cycle and electromagnetic and acoustic processes. Some of these are entering the market or have reached technical maturity, and are expected to become significant in the future.

A heat pump is essentially a heat engine operating in reverse and can be defined as a device that moves heat from a region of low temperature to a region of higher temperature. The residential air-to-air heat pump, the type most commonly in use, extracts heat from low temperature outside air and delivers this heat indoors. To accomplish this and in accordance with the second law of thermodynamics work is done on the working fluid (i.e. a refrigerant) of the heat pump.

In order to transport heat from a heat source to a heat sink, external energy is needed to drive the heat pump. Theoretically, the total heat delivered by the heat pump is equal to the heat extracted from the heat source, plus the amount of drive energy supplied. Electricallydriven heat pumps for heating buildings typically supply 100 kWh of heat with just 20-40 kWh of electricity. Many industrial heat pumps can achieve even higher performance, and supply the same amount of heat with only 3-10 kWh of electricity.

For large-scale applications, heat pumps using a combustion furnace for supplemental heat and/or temperature peaking have become popular due to:

• their applicability to the retrofit market as add-on units to existing oil or gas furnaces and boilers, and
• the improved performance of the combined system compared with electric-resistance heat-supplemented heat pumps.

In this regard, heat pumps operating with supplementary heat are often said to be operating in a bivalent mode. A heat pump operating with electric resistance heating or without other back-up is said to be operating in a monovalent mode. With the exception of certain control components designed to regulate compressor and furnace operation, essentially standard heat pump components are used. The system is operated in the heat pump mode down to a predetermined temperature called the balance point and the furnace is switched on when supplementary heat is required or, in the case of air distribution systems, during heat pump defrosting. Some systems switch the compressor off completely below the balance point while others allow parallel heat pump and furnace operation down to –10°C for an air source heat pump. The heat pump technology is of special interest in colder climates where the traditional means of heating existing buildings is gas or oil and a requirement for some air-conditioning as an add-on arises. The systems can also be used for heating alone in conjunction with conventional furnaces. Even in the coldest climates there are a sufficient number of heating days above the balance point of an existing heat pump to make this combination worthy of consideration.

Written by sam

November 29th, 2009 at 6:56 am

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