Refrigerator Troubleshooting Diagram

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Evaporators

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The evaporator is that part of the system where the useful cooling is accomplished by removing the heat from the area to be cooled. This is done by bringing the temperature of the liquid refrigerant below the temperature of the surrounding medium. The heat passes into the liquid refrigerant and is absorbed as latent heat, changing the state of the refrigerant from a liquid into a vapor. It is then withdrawn by action of the compressor.

There are two general types of evaporators: dry and flooded. In the dry type, the refrigerant enters in the liquid state, and the design provides for complete evaporation with the vapors slightly superheated. In the flooded type, not all of the refrigerant is evaporated, and the liquid/vapor mixture leaving the evaporator flows into a surge drain from which the vapors are drawn into the compressor suction line; the liquid is recirculated through the evaporator.

Written by sam

February 13th, 2011 at 11:31 pm

Posted in Compressors

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Evaporators for Add On Residental Use

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One of the more efficient ways of adding whole-house air conditioning is by adding an evaporator coil in the furnace. The evaporator coil becomes an important part of the whole system. It can be added to the existing furnace to make a total air conditioning and heating package. There are two types of evaporators—down-flow and up-flow.

The down-flow evaporator is installed beneath a down-flow furnace. See Fig. 15-11. Lennox makes down-flow models in 3-, 4-, and 5-ton sizes with an inverted “A” coil. See Fig. 15-12. Condensate runs down the slanted side to the drain pan. This unit can be installed in a closet or utility room wherever the furnace is located. This type of unit is shipped factory assembled and tested.

Up-flow evaporators are installed on top of the furnace. They are used in basement installations and in closet installations. See Fig. 15-13. The adapter base and the coil in Fig. 15-14, are shown, as they would fit onto the top of an up-flow existing furnace. The plenum must be removed and replaced once the coil has been placed on top of the furnace.

In most cases, use of an add-on top evaporator means the fan motor must be changed to a higher horsepower rating. The evaporator in the plenum makes it more difficult to force air through the heating system. In some cases, the pulley size on the blower and the motor must be changed to increase the cfm (cubic feet per minute) moving past the evaporator.

Some motors have sealed bearings. Some blower assemblies, such as that shown in Fig. 15-15, have sealed bearings. However, some have sleeve bearings. In such cases, the owner should know that the motor and blower must be oiled periodically to operate efficiently.

Figure 15-16 shows how the evaporator coil sits on top of the furnace, making the up-flow type of air conditioning operate properly. The blower motor is located below the heater and plenum.

The evaporator is not useful unless it is connected to a compressor and condenser. These are usually located outside the house. Figure 15-17 shows the usual outdoor compressor and condenser unit. This unit is capable of furnishing 2.4 to 5 tons of air conditioning, ranging in capacities from 27,000 to 58,000 Btu. Note that this particular unit has a U-shaped condenser coil that forms three vertical sides. The extra surface area is designed to make the unit more efficient in heat transfer. The fan, which is thermostatically operated, has two-speeds. It changes to low speed when the outside temperature is below 75°F (23.9°C).

Like most compressors designed for residential use, this compressor is hermetically sealed. The following safety devices are built in: a suction-cooled overload protector, a pressure-relief valve, and a crankcase heater. Controls include high- and low-pressure switches. They automatically shut off the unit if discharge pressure becomes excessive or suction pressure falls too low.

In apartments where space is at a premium, a different type of unit is used. It differs only in size. See Fig. 15-18. These compact units have a blower, filter, and evaporator coil contained in a small package. They have electric-heat coils on top. In some cases, hot water is used to heat in the winter.

Figure 15-19 shows the various ways in which these units may be mounted. The capacity is usually 18,000 to 28,000 Btu. Note location of the control box. This is important since most of the maintenance problems are caused by electrical, rather than refrigerant, malfunctions. This type of unit allows each apartment tenant to have his or her own controls.

Written by sam

February 7th, 2011 at 5:12 am

Posted in Air Conditioning

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Refrigerating Air and Gas Coolers

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These coolers are generally called direct expansion coils and consist of a series of tubes through which refrigerant flows (Figure 3.21). The tubes, which are finned to increase the heat transfer rate from the medium to be cooled (e.g. air) to the boiling point, are normally arranged into a number of parallel circuits fed from a single throttling valve. The hot refrigerant vapor is accumulated in the outlet (suction) gas header. These direct expansion coils are used only in the positive displacement compressor systems, owing to quite lowpressure ratios. Like liquid coolers, these coolers are also classified as flooded and dry types. In a flooded coil, a float valve is used to maintain the preset level in the coil, keeping the evaporator coil close to full of the liquid refrigerant. This full contact of the liquid with the tube walls provides a high heat transfer rate. In practical applications, flooded type evaporators are not preferable, because they require large amounts of refrigerant. A dry coil requires only a small amount of refrigerant and this reduces the cost of the refrigerant charge. Sometimes a metering device (thermal expansion valve) regulates the amount of the liquid entering the coil to maintain a predetermined amount of superheat in the refrigerant at the coil outlet. The dry expansion coil contains mostly liquid at the inlet and only superheated vapor at the outlet, after absorbing heat from the medium to be cooled. In the air coolers, when the surface temperatures fall below 0°C, frosting occurs. Thick layers of frost act as insulation and reduce the air flow rate (in the forced convection coils) and the available inner space.

Several methods are used for defrosting, e.g. hot-gas defrost and water defrost. But recently frost-free refrigeration systems have become popular because of the problems mentioned above.

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Written by sam

November 19th, 2009 at 10:19 pm

Refrigerating Liquid Coolers

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Shell and tube type heat exchangers (Figure 3.20) are the more common form of evaporation units for water cooling and chilling applications. These are utilized to cool liquids, which can be used as the secondary refrigerant or to cool the final products directly. In practice, these types of heat exchangers are known as liquid coolers or chillers.

Some example applications in food and refrigeration industry are:

• chilling of drinkable water,
• chilling of water for air conditioning coils,
• chilling of milk after pasteurization, and
• process cooling operations.

Chilled water systems can use either a flooded evaporator or a direct-expansion evaporator which are typically shell and tube type heat exchangers. In a flooded evaporator, refrigerant floods the shell side of the heat exchanger and is controlled by a level valve. Water being chilled passes through the tubes. Conversely, in a direct-expansion evaporator, water is carried in the shell and refrigerant is boiled inside the tubes. The rate of refrigerant flow is throttled to insure that only refrigerant gas exits the evaporator. Copper tubes mounted within a carbon steel shell is the most common construction used for chilled water evaporators.

It is important to note that if the refrigerant vaporizes on the outside surface of the tubes the evaporator is a flooded cooler; if it vaporizes inside the tubes the evaporator is a dry cooler (note that in this more common type, the mixture of liquid and vapor is evaporated completely, usually with some degree of superheating (Hewitt et al., 1994). In a flooded cooler the water or brine is circulated through the tubes, which are usually finned to provide an increment in the heat transfer rate and a decrease in the evaporator size. In a dry cooler the liquid refrigerant is contained within the tubes, and water or brine is circulated through the shell of the cooler, which serves as an evaporator. Flooded coolers are often specified for applications where shell-side vaporization of refrigerant of other liquids is desirable. Due to rapid boiling in the shell, in order to obtain high purity vapors, a vapor disengagement vessel is often welded to the main shell. Flooded coolers are particularly employed in multiple compressor systems.

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Written by sam

November 19th, 2009 at 10:16 pm

Refrigerating Evaporators

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Evaporator can be considered the point of heat capture in a refrigeration system and provides the cooling effect required for any particular application. There are almost as many different types of evaporators as there are applications of heat exchangers. However, evaporators are divided into two categories such as (i) direct cooler evaporators that cool air that, in turn, cools the product and (ii) indirect cooler evaporators that cool a liquid such as brine solution that, in turn, cools the product. Normally, the proper evaporator comes with the system. However, there may be an occasion when designing a system, so one will need to determine the requirements and select the proper evaporator from a manufacturer’s catalog or manual.

In practice, the following evaporators are commonly used for cooling, refrigerating, freezing, and air conditioning applications:

• liquid coolers,
• air coolers, and/or gas coolers.

Written by sam

November 19th, 2009 at 10:11 pm