Refrigerator Troubleshooting Diagram

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Two Stage Cascade Refrigerating System

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A two-stage cascade system employs two vapor-compression units working separately with different refrigerants, and interconnected in such a way that the evaporator of one system is used to serve as condenser to a lower temperature system (i.e. the evaporator from the first unit cools the condenser of the second unit). In practice, an alternative arrangement utilizes a common condenser with a booster circuit to provide two separate evaporator temperatures.

In fact, the cascade arrangement allows one of the units to be operated at a lower temperature and pressure than would otherwise be possible with the same type and size of single-stage system. It also allows two different refrigerants to be used, and it can produce temperatures below -150°C. Figure 3.38 shows a two-stage cascade refrigeration system, where condenser B of system 1 is being cooled by evaporator C of system 2. This arrangement enables to reach ultralow temperatures in evaporator A of the system.

A practical two-stage cascade refrigeration system.

A practical two-stage cascade refrigeration system.

For a schematic system shown in Figure 3.39, the condenser of system I, called the first or high-pressure stage, is usually fan cooled by the ambient air. In some cases a water supply may be used but air-cooling is much more common. The evaporator of system I is used to cool the condenser of system II called the second or low-pressure stage. The unit that makes up the evaporator of system I and the condenser of system II is often referred to as the inter-stage or cascade condenser. As stated earlier, cascade systems generally use two different refrigerants (i.e. one in  each stage). One type is used for the low stage and a different one for the high stage. The reason why two refrigeration systems are used is that a single system cannot economically achieve the high compression ratios necessary to obtain the proper evaporating and condensing temperatures. It is clear from the T-s diagram of the two-stage cascade refrigeration system as shown in Figure 3.39 that the compressor work decreases and the amount of refrigeration load (capacity) in the evaporator increases as a result of cascading (Cengel and Boles, 1998). Therefore, cascading improves the COP.

schematic-twostage-cascade-refrigerating-system

Written by sam

November 22nd, 2009 at 5:51 pm

Refrigerating Air Purging Methods

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Basically, there are two ways to purge a system of air: manual or automatic. To purge manually, a properly positioned valve is opened by hand, allowing the air to escape. It is a common misconception that when a cloud of refrigerant gas is seen being discharged to atmosphere, the system has been purged of air. Air can still be trapped in the system.

Therefore, many refrigeration system users prefer automatic purging. Refrigeration systems include the compressor, condenser, receiver, evaporator and purger (see Figure 3.35). Of these components, the purger is perhaps the least understood and appreciated. The purger’s job is to remove air from the system, thus improving compressor and condenser operating efficiency.

multipoint-purger

Two types of automatic purgers are used as follows : (i) nonelectrical mechanical and (ii) automatic electronic purgers. Determining the type of automatic purger to use is a matter of whether electricity is available at the purger location and if it safe to allow electrical components to be used. The nonelectrical mechanical units are used primarily in applications where electricity is not available at the point of use or in hazardous applications where electric components are not allowed. They remove air by sensing the density difference between the liquid refrigerant and gases. An operator opens and closes valves to start and stop the purging operation and ensure its efficiency.

Electronic automatic refrigeration purgers are classed as single-point and multipoint purgers. The single-point electronic refrigerated purger has a mechanical-purge operation with a temperature/gas level monitor that controls the discharge to atmosphere. The purging sequence is performed manually. A multipoint refrigerated purger will purge a number of points using the same unit. However, each purge point is purged individually, and the multipoint purger offers total automation, including startup, shutdown and alarm features. With this purger, it is important to choose a purger designed for the total tonnage of your system. Undersized purgers may cost less initially but may adversely impact the system’s efficiencies and payback period. Some multipoint purgers include a microprocessor-based programmable controller rather than a clock timer. The fuzzy logic controller can ‘learn’ as it cycles through the system. As the purger accumulates air and purges, the controller records and prioritizes each purge point in its memory, thus removing air more efficiently.

Written by sam

November 22nd, 2009 at 3:45 pm

Refrigerating Defrosting

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One of the most common applications of refrigeration systems is to produce and maintain space temperatures by circulating air through a refrigerated coil. If the temperature of the refrigerant in the coil is below 0°C, water in the air freezes and accumulates on the coil. The ice blocks airflow and acts as an insulator, penalizing coil performance. For efficient performance, the coil must be defrosted periodically. The defrost cycle is a necessary and important part of the design of the refrigeration system.

Over the years, various defrost methods have been used. One of the first methods was to arrange the coil in such a manner that it could be isolated from the cold room. Warm air was circulated over it until the ice melted. Another method is to run water over the coil. Careful design of the water lines into and out of the cold room prevents freezing of the defrost water. Electric heater rods inserted into formed holes through aluminum fins work effectively, and this type is common for halocarbon systems. All of these have been used for ammonia coils, but the most common method is hot gas from the compressor discharge. Hot gas defrost is simple and effective, it removes ice rapidly, and is relatively inexpensive to install. However, the control valves selection and the sequence of operation must be correct for reliable and efficient defrosts.

Defrost systems vary with the size and type of evaporator, with some choices possible for the larger size coils. Electrical heating defrost via elements in the drip trays under the evaporator or as elements through the coil fins are the most common and economical for small evaporators. Hot gas systems that pump hot refrigerant gas through the coils or defrosting by running ambient water over the coils are more common on larger systems.

Auto cycle defrost is not as complicated as it sounds. In fact, cycle defrost systems are the least complex in operation and most effective defrost systems available. Cycle defrost systems are feasible only on all-refrigerator units because these units do not contain a freezer compartment. Cycle defrost units contain a special thermostat which senses the evaporator plate temperature. At the completion of each compressor run cycle, the thermostat disconnects the electrical power and turns off the compressor. The thermostat will not connect the electrical power again to initiate the next compressor run cycle until the evaporator plate reaches a preset temperature well above freezing.

During this evaporator warm-up period, the frost which has accumulated during the previous compressor run cycle melts and becomes water droplets. These water droplets run down the vertical surface of the evaporator and drop off into the drip tray located just underneath the evaporator, which then empties into a drain tube. The drain tube discharges the water droplets into the condensation pan located in the mechanical assembly under and outside the refrigerator compartment. There, the compressor heat and the air flow from the condenser fan evaporate the moisture.

Written by sam

November 22nd, 2009 at 3:38 pm

Refrigerating Superheating and Subcooling

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Superheating (referring to superheating of the refrigerant vapor leaving evaporator) and subcooling (referring to subcooling of refrigerant liquid leaving the condenser) are apparently two significant processes in practical vapor-compression refrigeration systems and are applied to provide better efficiency (COP) and to avoid some technical problems, as will be explained below.

vapor-compression-system

Superheating

During the evaporation process the refrigerant is completely vaporized partway through the evaporator. As the cool refrigerant vapor continues through the evaporator, additional heat is absorbed to superheat the vapor. Under some conditions such pressure losses caused by friction increase the amount of superheat. If the superheating takes place in the evaporator, the enthalpy of the refrigerant is raised, extracting additional heat and increasing the refrigeration effect of the evaporator. If it is provided in the compressor suction piping, no useful cooling occurs. In some refrigeration systems, liquid-vapor heat exchangers can be employed to superheat the saturated refrigerant vapor from the evaporator with the refrigerant liquid coming from the condenser (Figure 3.32). As can be seen from Figure 3.32, the heat exchanger can provide high system COP. Refrigerant superheating can also be obtained in the compressor. In this case, the saturated refrigerant vapor enters the compressor and is superheated by increasing the pressure, leading to the temperature increase. Superheating obtained from the compression process does not improve the cycle efficiency, but results in larger condensing equipment and large compressor discharge piping. The increase in the refrigeration effect obtained by superheating in the evaporator is usually offset by a decrease in the refrigeration effect in the compressor. Because the volumetric flow rate of a compressor is constant, the mass flow rate and the refrigeration effect are reduced by decreases in the refrigerant density caused by the superheating. In practice, it is well known that there is a loss in the refrigerating capacity of 1% for every 2.5°C of superheating in the suction line. Insulation of the suction lines is a solution to minimize undesirable heat gain. The desuperheating is a process to remove excess heat from superheated refrigerant vapor, and if accomplished by using an external effect it will be more useful to the COP. Desuperheating is often considered impractical, owing to the low temperatures (less than 10°C) and small amount of available energy.

Subcooling

This is a process of cooling the refrigerant liquid below its condensing temperature at a given pressure (Figure 3.32). Subcooling provides 100% refrigerant liquid to enter the expansion device, preventing vapor bubbles from impeding the flow of refrigerant through the expansion valve. If the subcooling is caused by a heat transfer method external to the refrigeration cycle, the refrigerant effect of the system is increased, because the subcooled liquid has less enthalpy than the saturated liquid. Subcooling is accomplished by refrigerating the liquid line of the system, using a higher temperature system. Simply we can state, subcooling cools the refrigerant more and provides the following accordingly:

• increase in energy loading,
• decrease in electrical usage,
• reducing pulldown time,
• more uniform refrigerating temperatures, and
• reduction in the initial cost.

Note that the performance of a simple vapor-compression refrigeration system can be significantly improved by further cooling the liquid refrigerant leaving the condenser coil. This subcooling of the liquid refrigerant can be accomplished by adding a mechanicalsubcooling loop in a conventional vapor compression cycle. The subcooling system can be either a dedicated mechanical-subcooling system or an integrated mechanical-subcooling system (Khan and Zubair, 2000). In a dedicated mechanical-subcooling system, there are two condensers, one for each of the main cycle and the subcooler cycle, whereas for an integrated mechanical-subcooling system, there is only one condenser serving both the main cycle and the subcooler cycle.

For example, subcooling of R-22 by 13°C increases the refrigeration effect by about 11%. If subcooling is obtained from outside the cycle, each degree increment in subcooling will improve the system capacity (approximately 1%). Subcooling from within the cycle may not be as effective because of offsetting effects in other parts of the cycle. Mechanical subcooling can be added to existing systems or designed into new ones. It is ideal for any refrigeration process in which more capacity may be necessary or operating costs must be lowered. It has proved cost efficient in a variety of applications and is recommended for large supermarkets, warehouses, plants, etc. Figure 3.33 shows a typical subcooler for commercial refrigeration applications.

subcooler

Written by sam

November 22nd, 2009 at 3:35 pm

Refrigerating Defrost Controllers

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A defrost controller with timer (Figure 3.27) operates various control valves and fan relays to quickly and efficiently remove frost and ice accumulation from evaporator surfaces. There are four easy-to-set defrost steps:

• pump out,
• hot gas,
• equalize, and
• fan delay.

This controller uses reliable, solid-state electronics with a precision quartz time clock and time interval adjusting slide knobs to sequentially operate through the four steps for smooth defrosting. Each step is clearly indicated by a bright LED during operation. Terminals for optional sensor defrost initiation and termination are provided. A 24-hour quartz time clock facilitates simple setting in 15-minute increments of defrost start times. A 7-day quartz time clock for weekly scheduling is also available. All time clocks have 72-hour battery backup in case of short-term power failure. Because of its time-adjustable 4-step defrost operation, this controller is suitable for almost every defrost application including top and bottom feed unit coolers, blast freezer evaporators, ice makers, etc.

refrigerator-defrost-controller

Written by sam

November 21st, 2009 at 9:13 am

Refrigerating System Receivers

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Some of the refrigeration units have enough space within the condenser to accommodate the entire refrigerant charge of the system. If the condenser does not have sufficient space, a receiver tank should be provided. The amount of refrigerant required for proper operation of the system determines whether or not a receiver is required. In practice, when proper unit operation requires approximately 3.6 kg or more of refrigerant, the use of a receiver is essential (Langley, 1982).

Receivers (Figure 3.25) are required on refrigeration systems which use an expansion valve for refrigerant control. The receiver provides a place to store the excess refrigerant in the system when the expansion valve restricts the flow to the evaporator. Receivers are not required, however, when using a capillary metering system. In addition to accommodating fluctuations in the refrigerant charge, the receiver aims to maintain the condenser drained of liquid, thereby preventing the liquid level from building up in the condenser and reducing the amount of effective condenser surface area.

refrigerator-receivers

Written by sam

November 21st, 2009 at 9:10 am

Refrigerating Accumulators

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It is well known that compressors are designed to compress vapors, not liquids. Many refrigeration systems are subject to the return of excessive quantities of liquid refrigerant to the compressor. Liquid refrigerant returning to the compressor dilutes the oil, washes out the bearings, and in some cases causes complete loss of oil in the compressor crankcase. This condition is known as oil pumping or slugging and results in broken valve reeds, pistons, rods, crankshafts, and the like. The purpose of the accumulator is to act as a reservoir to temporarily hold the excess oil-refrigerant mixture and to return it at a rate that the compressor can safely handle. Some accumulators include a heat-exchanger coil to aid in boiling off the liquid refrigerant while subcooling the refrigerant in the liquid line (see Figure 3.24), thus helping the system to operate more efficiently. Note that proper installation of a suction accumulator in the suction line just after the reversing valve and before the compressor helps eliminate the possible damage.

In large holdover plate refrigerator and freezer systems, refrigerant can accumulate in the plates and suction line when  the compressor is not running. On start-up, this liquid refrigerant can be suddenly dumped into the compressor, creating a situation due to the liquid slugging of refrigerant and oil. This can cause damage to the compressor. When installed in the suction line of the compressor, a suction accumulator protects the compressor from this liquid slugging by gradually feeding liquid refrigerant into the compressor.

Note that accumulators should be selected according to the tonnage, evaporator temperature and holding capacity.

refrigerator-accumulator

Written by sam

November 21st, 2009 at 9:01 am

Refrigerating Thermostatic Expansion Valves

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The thermostatic expansion valves are essentially reducing valves between the highpressure side and the low-pressure side of the system. These valves, which are the most widely used devices, automatically control the liquid-refrigerant flow to the evaporator at a rate that matches the system capacity to the actual load. They operate by sensing the temperature of the superheated refrigerant vapor leaving the evaporator. For a given valve type and refrigerant, the associated orifice assembly is suitable for all versions of the valve body and in all evaporating temperature ranges.

When the thermostatic expansion valve is operating properly, the temperature at the outlet side of the valve is much lower than that at the inlet side. If this temperature difference does not exist when the system is in operation, the valve seat is probably dirty and clogged with foreign matter. Once a valve is properly adjusted, further adjustment should not be necessary. The major problem can usually be traced to moisture or dirt collecting at the valve seat and orifice. Figure 3.22 shows a common type of electrically driven expansion valve.

electronics-expansion-valve

Written by sam

November 19th, 2009 at 10:24 pm

Refrigerating Throttling Devices

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In practice, throttling devices, called either expansion valves or throttling valves, are used to reduce the refrigerant condensing pressure (high pressure) to the evaporating pressure (low pressure) by a throttling operation and regulate the liquid-refrigerant flow to the evaporator to match the equipment and load characteristics. These devices are designed to proportion the rate at which the refrigerant enters the cooling coil to the rate of evaporation of the liquid refrigerant in the coil; the amount depends, of course, on the amount of heat being removed from the refrigerated space. The most common throttling devices are as follows :

• thermostatic expansion valves,
• constant pressure expansion valves,
• float valves, and
• capillary tubes.

Note that a practical refrigeration system may consist of a large range of mechanical and electronic expansion valves and other flow control devices for small- and large-scale refrigeration systems, comprising thermostatic expansion valves, solenoid valves, thermostats and pressostats, modulating pressure regulators, filter driers, liquid indicators, non-return valves and water valves, and furthermore, decentralized electronic systems for full regulation and control.

Written by sam

November 19th, 2009 at 10:21 pm

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.

air-coolers-evaporator

Written by sam

November 19th, 2009 at 10:19 pm