Refrigerator Troubleshooting Diagram

Archive for the ‘Heat Pump’ tag

Heat Pump Usage

without comments

In the last fifty years, several large office buildings and small college campuses have been constructed using water-to-water heat pumps. Their capacities were up to several hundred tons. These systems usually use well water. That means two wells are used. One is used for supply and one for disposal. A possible arrangement is shown in Fig. 17-23.

The supply and disposal wells are manually selected. Well water and return water are mixed, for both evaporator and condenser. This is done on a temperature basis. Under some conditions, this system can become an internal source heat pump. That is, when the exterior-zone heating and interior-zone cooling loads are in balance, or nearly so, little or no well water is needed.

Internal source heat pumps without wells are used where there is sufficient internal cooling load to supply the net heating requirements under all conditions. Excess heat can be disposed of through cooling towers.

A problem with these systems is related to a high electrical load for the pumping system. A variety of variable-flow piping schemes have been devised to overcome this problem.

Written by sam

February 7th, 2011 at 1:54 pm

Posted in Air Conditioning

Tagged with

Heat Pump Outdoor Thermostat

without comments

In a straight heat pump or supplementary electric heater application, at least one outdoor thermostat is required to cycle the heaters as the outdoor temperature drops. In the Fuelmaster system, the indoor thermostat controls the supplemental heat source (furnace). The outdoor thermostat is not required. Since the furnace is serving as the secondary heat source, the Fuelmaster system does not require the home rewiring usually associated with supplemental electric strip heating.

Written by sam

February 7th, 2011 at 1:49 pm

Heat Pump Defrost

without comments

During a defrost cycle, the heat pump switches from heating to cooling. To prevent cool air from being circulated when heating is needed, the control automatically turns on the furnace to compensate for the heat pump defrost cycle. (Most modern heat pump systems do the same thing with strip heating.) When supply air temperature climbs above 110 to 120°F (43.3 to 48.9°C), the defrost limit control turns off the furnace and keeps indoor air from getting too warm.

After a defrost cycle, the air temperature downstream of the coil may go above the 115°F (46.1°C) closing point of the heat pump delay. Then, the compressor will stop until the heat exchanger has cooled to 90 to 100°F (32.2 to 37.8°C), as it does during normal cycling operation between furnace and heat pump.

Written by sam

February 7th, 2011 at 1:48 pm

Posted in Air Conditioning

Tagged with ,

Heat Pump Operation

without comments

On mild temperature heating days, the heat pump handles all heating needs. When the outdoor temperature reaches the “balance point” of the home (heat loss equals heat pump heating capacity), the two-stage indoor thermostat activates the furnace (secondary heat source). When the furnace fires, a heat relay deenergizes the heat pump.

When the second stage (furnace) need is satisfied and plenum temperature has cooled to 90 to 100°F (32.2 to 37.8°C), the heat pump delay turns the heat pump back on. It controls the conditioned space until the second stage (full heat) operation is required again.

When outdoor temperature drops below the setting of the low-temperature compressor monitor (field installed option) the control shuts out the heat pump. The furnace handles all of the heating need. The low temperature compressor monitor is standard on models dated 1974 and after.

During the cooling season the heat pump operates in its normal cooling mode. It uses the furnace blower as the primary air mover. See Fig. 17-21.

Written by sam

February 7th, 2011 at 1:47 pm

Posted in Air Conditioning

Tagged with

Solar for Heat Source of Heat Pump

without comments

Solar energy, as either direct or diffuse radiation, is similar to air in its characteristics. A solar-source heat pump or a combined solar/heat pump heating system has all the disadvantages of the air-source heat pump, low performance and extreme variability with the additional disadvantage of high capital cost, particularly as in all cases a heat-store or back-up system is required. In areas with high daily irradiation, this may not be the case.

Each of the above mentioned heat sources for heat pumps presents some drawbacks. Presently considerable research is devoted to the technical problems involved and alternative heat sources. Also solar energy may provide a suitable heat source. Unfortunately, solar systems presently are very costly. Furthermore, the intermittent character of solar energy requires the use of large and costly storage volumes.

Written by sam

January 30th, 2010 at 6:06 pm

Soil and geothermal for Heat Source of Heat Pump

without comments

Soil or sub-soil (ground source) systems are used for residential and commercial applications and have similar advantages to water-source systems, due to the relatively high and constant annual temperatures resulting in high performance. Generally the heat can be extracted from pipes laid horizontally or sunk vertically in the soil. The latter system appears to be suitable for larger heat pump systems. In the former case, adequate spacing between the coils is necessary, and the availability of suitably large areas (about double the area to be heated) may restrict the number of applications. For the vertical systems, variable or unknown geological structures and soil thermal properties can cause considerable difficulties. Due to the removal of heat from the soil, the soil temperature may fall during the heating season. Depending on the depth of the coils, recharging may be necessary during the warm months to raise the ground temperature to its normal levels. This can be achieved by passive (e.g. solar irradiation) or active means. In the later case, this can increase the overall cost of the system. Leakage from the coils may also pose problems. Both the horizontal and vertical systems tend to be expensive to design and install and, moreover, involve different types of experts (one for heating and cooling and the other for laying the pipe work).

Rock (geothermal heat) can be used in regions with no or negligible occurrence of ground water. Typical bore hole depth ranges from 100 to 200 m. When large thermal capacity is needed the drilled holes are inclined to reach a large rock volume. This type of heat pump is always connected to a brine system with welded plastic pipes extracting heat from the rock. Some rock-coupled systems in commercial buildings use the rock for heat and cold storage. Because of the relatively high cost of the drilling operation, rock is seldom economically attractive for domestic use.

The ground constitutes a suitable heat source for a heat pump in many countries. At small depth temperatures remain above freezing. Furthermore the seasonal temperature fluctuations are much smaller than those of the air. Heat is extracted from the soil by means of a glycol solution flowing through tubing embedded in the ground. If a horizontal grid of tubing is utilized, several hundred square meters of surface area are needed to heat a single family building. In urban areas such space is rarely available. In addition considerable costs are involved. For these reasons vertical ground heat exchangers are more preferred presently.

Geothermal heat sources for heat pumps are currently utilized in various countries, particularly in the USA, Canada and France. These resources are generally localized and do not usually coincide with areas of high-density population. In addition, the water often has a high salt component which leads to difficulties with the heat exchangers. Due to the high and constant temperatures of these resources, the performance is generally high.

Written by sam

January 30th, 2010 at 6:05 pm

Water for Heat Source of Heat Pump

without comments

Water-source units are common in applied or built-up installations where internal heat sources or heat or cold reclamation is possible. In addition, solar or off-peak thermal storage systems can be used. These sources have a more stable temperature, compared to air. The combination of a high first-cost solar device with a heat pump is not generally an attractive economic proposition on either a first-cost or a life-cycle cost basis.

Ground water is available with stable temperatures between 4°C and 10°C in many regions. Open or closed systems are used to tap into this heat source. In open systems the ground water is pumped up, cooled and then reinjected in a separate well or returned to surface water. Open systems should be carefully designed to avoid problems such as
freezing, corrosion and fouling. Closed systems can be either direct expansion systems, with the working fluid evaporating in underground heat exchanger pipes, or brine loop systems. Due to the extra internal temperature difference, heat pump brine systems generally have a lower performance, but are easier to maintain. A major disadvantage of ground water heat pumps is the cost of installing the heat source. Additionally, local regulations may impose severe constraints regarding interference with the water table and the possibility of soil pollution.

Most ground water at depths more than 10 m is available throughout the year at temperatures high enough (e.g. 10°C) to be used as low temperature source for heat pumps. Its temperature remains practically constant over the year and makes it possible to achieve high seasonal heating COPs (3 and more). The pump energy necessary to pump up this water has a considerable effect upon COP (10% reduction per 20 m pumping height). It is necessary to pump the evaporator water back into the ground to avoid depletion of ground water layers.

The ground water has to be of a purity almost up to the level of drinking water to be usable directly in the evaporator. The rather large consumption of water of high purity limits the number of heat pump systems which can make use of this source. Also surface waters constitute a heat source which can be used only for a limited number of applications.

Ground water at considerable depth (aquifers) may offer interesting possibilities for direct heating or for heating with heat pump systems. The drilling and operating costs involved require large-scale applications of this heat source. The quality of these waters often presents serious limitations to their use (corrosive salt content).

Ground water (i.e. water at depths of up to 80 m) is available in most areas with temperatures generally in the 5-18°C range. One of the main difficulties with these sources is that often the water has a high dissolved solids content producing fouling or corrosion problems with heat exchangers. In addition, the flow rate required for a single-family house is high, and ground water systems are difficult to apply widely in densely populated areas. The inclusion of the cost of providing the heat source has a significant impact on the economic attractiveness of these systems. A rule of thumb seems to be that such systems are economic if both the supply and the reinjection sources are available, marginally economic if one is available and not cost-competitive if neither source is available. In addition, if a well has to be sunk, the necessity for drilling teams to act in coordination with heating and ventilation contractors can pose problems. Also many local legislatures impose severe constraints when it comes to interfering with the water table and this can pose difficulties for reinjection wells.

Surface water as rivers and lakes is in principle a very good heat source, but suffers from the major disadvantage that either the source freezes in winter or the temperature can be very close to 0°C (typically 2-4°C). As a result, great care is needed to avoid freezing on the evaporator. Where the water is thermally polluted by industry or by power stations, the situation is somewhat improved.

Sea water appears to be an excellent heat source under certain conditions, and is mainly used for medium-sized and large heat pump installations. At a depth of 25-50 m. the sea temperature is constant (5-8°C), and ice formation is generally no problem (freezing point -1°C to -2°C). Both direct expansion systems and brine systems can be used. It is
important to use corrosion-resistant heat exchangers and pumps and to minimize organic fouling in sea water pipelines, heat exchangers and evaporators, etc. Where salinity is low, however, the freezing point may be near 0°C, and the situation can be similar to that for rivers and lakes in regard to freezing.

Waste water and effluent are characterized by a relatively high and constant temperature throughout the year. Examples of possible heat sources in this category are effluent from public sewers (treated and untreated sewage water) in a temperature range of 10-20°C throughout the year, industrial effluent, cooling water from industrial processes or electricity generation, and condenser heat from refrigeration plants. Condenser cooling water for electricity generation or industrial effluent could also be used as heat sources. The major constraints for use in residential and commercial buildings are, in general, the distance to the user, and the variable availability of the waste heat flow. However, waste water and effluent serve as an ideal heat source for industrial heat pumps to achieve energy savings in industry.

Apart from surface water systems which may be prone to freezing, water-source systems generally do not suffer from the low-temperature problems of air-source heat pumps because of the higher year-round average temperature. This ensures that the temperature difference between the source and sink is smaller and results in an improvement of the performance of the heat pump. The evaporator must, however, be cleaned regularly. The heat transfer at the evaporator can drop by as much as 75% within approximately five months if it is not kept properly clean. The costs of cleaning become relatively low for larger projects so that the use of this source may become economic.

Written by sam

January 30th, 2010 at 5:54 pm

Air for Heat Source of Heat Pump

without comments

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 Pump Applications in Industry

without comments

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

Sectoral Heat Pump Utilization

without comments

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

Posted in Refrigerator Heat Pumps

Tagged with

You might also likeclose