Analysis of Reversed Carnot Cycle,

Consider 1 kg of air, taken in a cylinder which is fitted with a piston. Its state is represented by point c.

c-b: Adiabatic Expansion

Air expands adiabatically. Temperature reduces from T_c to T_a.

b-a: Isothermal Expansion

Further expansion of air takes place at constant temperature (T_c). It abstracts heat from the bodies to be cooled.

a-d: Adiabatic Conipression

Air is compressed adiabatically using compressor i.e., work is supplied to air. Temperature rises to T_c.

d-c: Isothermal Compression

Further compression of air takes place at constant temperature (T_c). Air losses heat to any cooling medium.

Ideal COP of the System,

The ratio of refrigeration effect produced by a refrigerating machine in a given tulle to the work supplied during the same time is known as Coefficient of Performance (COP) of the machine.

Let,

R = Refrigerating effect

W = Work input to the machine

\[\text{C}\text{.O}\text{.P}=\frac{R}{W}\]

The coefficient of performance of a refrigeration system is always greater than one. Refrigerating effect per cycle,

$\oint{N}$ = Heat abstracted by air during process ba (From T- s diagram).

= Area under (ba)

\[\oint{N}={{T}_{a}}\times \Delta s\]

Work supplied per cycle,

$\oint{W}$ = Heat rejected by air – Heat abstracted by air

= Area under cb – Area under ba

\[{{T}_{c}}\Delta s-{{T}_{a}}\Delta s=\text{Area cbad}\]

\[\oint{W=}\text{ }({{T}_{c}}-{{T}_{a}})\Delta s\]

Coefficient of performance,

\[\text{C}\text{.O}\text{.P}=\frac{\oint{N}}{\oint{W}}\]

\[=\frac{\text{Heat supplied}}{\text{Workdone}}=\frac{{{T}_{a}}\times \Delta s}{({{T}_{c}}-{{T}_{a}})\times \Delta s}\]

\[\text{C}\text{.O}\text{.P}=\frac{{{T}_{a}}}{{{T}_{c}}-{{T}_{a}}}\]

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Performance Factor (or) Coefficient of Performance of Heat Pump

Figure : Heat Pump

Coefficient of Performance (C.O.P) is used to express the performance of the heat pump. It is defined as the ratio of heat supplied by the heat pump from the surrounding to the system at higher temperature to the net work supplied to run the heat pump. Mathematically it is written as.

\[\text{C}\text{.O}\text{.P}=\frac{\text{Heat supplied by heat pump to the system}}{\text{Net work supplied to heat pump}}\]

Applying first law of thermodynamics,

Workdone,

\[W={{Q}_{B}}-{{Q}_{A}}\]

\[{{\left( \text{C}\text{.O}\text{.P} \right)}_{Heat\text{ }pump}}\text{=}\frac{Desired\text{ }effect}{Work\text{ }done}=\frac{{{Q}_{B}}}{{{Q}_{B}}-{{Q}_{A}}}\]

\[{{\left( \text{C}\text{.O}\text{.P} \right)}_{Heat\text{ }pump}}=\frac{{{Q}_{B}}}{{{Q}_{B}}-{{Q}_{A}}}\]

Performance Factor (or) Coefficient of Performance of Refrigerator

Figure : Refrigerator

It is defined as the ratio of heat extracted from the surroundings to be cooled to the net work supplied.

Applying first law of thermodynamics,

Workdone,

\[W={{Q}_{B}}-{{Q}_{A}}\]

Then,

\[{{\left( \text{C}\text{.O}\text{.P} \right)}_{refrigerator}}\text{=}\frac{Desired\text{ }effect}{Work\text{ }done}\]

\[=\frac{{{Q}_{A}}}{W}=\frac{{{Q}_{B}}}{{{Q}_{B}}-{{Q}_{A}}}\]

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Figure 1: Bell Coleman Cycle.

COP for Bell Coleman air-refrigerator

Figure 1: Bell Coleman Cycle on P-V and T-s Diagrams.

Isentropic Compression Process

The cold air from the refrigerator is drawn into the compressor cylinder where it is compressed isentropically. During the compression stroke, pressure and temperature increases from P₁ to P₂ and T₁ to T₂ whereas volume decreases from V₁ to V₂.

Constant Pressure Cooling Process

The warm air from the compressor is then passed into the heat exchanger (or) cooler where it is cooled with the help of cooling medium i.e., water, at constant pressure. The temperature and volume decrease from T₂ to T₃ and V₂ to V₃ whereas pressure remains constant.

Isentropic Expansion Process

The air from the heat exchanger is now drawn into the expander cylinder where it is expanded isentropically. Pressure and temperature decrease from P₃ to P₄ and T₃ to T₄ respectively, whereas volume increases from V₃ to V₄.

Constant Pressure Expansion Process

The cold air from the expander is now passed to the refrigerator where it is expanded at constant pressure. The temperature and volume increase from T₄ to T₁ and V₄ to V₁ whereas pressure remains constant.

COP for Bell Coleman Air Refrigeration Cycle

Consider,

C – Specific heat at constant pressure.

COP is given by,

\[COP=\frac{R}{W}\]

Where,

R – Refrigeration effect

W – Workdone or work supplied.

Refrigeration effect,

\[R={{C}_{p}}({{T}_{1}}-{{T}_{4}})\text{ KJ/kg}\]

Work supplied,

W= Heat rejected – Heat absorbed

\[={{C}_{p}}({{T}_{2}}-{{T}_{3}})-{{C}_{p}}({{T}_{1}}-{{T}_{4}})\]

\[COP=\frac{{{C}_{p}}({{T}_{1}}-{{T}_{4}})}{{{C}_{p}}({{T}_{2}}-{{T}_{3}})-{{C}_{p}}({{T}_{1}}-{{T}_{4}})}\]

\[COP=\frac{({{T}_{1}}-{{T}_{4}})}{({{T}_{2}}-{{T}_{3}})-({{T}_{1}}-{{T}_{4}})}…(1)\]

For adiabatic compression,

\[\frac{{{T}_{1}}}{{{T}_{2}}}={{\left( \frac{{{P}_{2}}}{{{P}_{1}}} \right)}^{\frac{\gamma -1}{\gamma }}}…(2)\]

For adiabatic expansion,

\[\frac{{{T}_{3}}}{{{T}_{4}}}={{\left( \frac{{{P}_{3}}}{{{P}_{4}}} \right)}^{\frac{\gamma -1}{\gamma }}}={{\left( \frac{{{P}_{2}}}{{{P}_{1}}} \right)}^{\frac{\gamma -1}{\gamma }}}…(3)\]

Since, for constant pressure process, P₃ = P₂ and P₄ = P_1.

From equation (2) and (3)

\[\frac{{{T}_{2}}}{{{T}_{1}}}=\frac{{{T}_{3}}}{{{T}_{4}}}\]

Or

\[{{T}_{2}}={{T}_{1}}\times \frac{{{T}_{3}}}{{{T}_{4}}}\]

\[COP=\frac{({{T}_{1}}-{{T}_{4}})}{({{T}_{1}}.\frac{{{T}_{3}}}{{{T}_{4}}}-{{T}_{3}})-({{T}_{1}}-{{T}_{4}})}\]

\[=\frac{({{T}_{1}}-{{T}_{4}})}{\frac{{{T}_{3}}}{{{T}_{4}}}({{T}_{1}}-{{T}_{4}})-({{T}_{1}}-{{T}_{4}})}\]

\[=\frac{{{T}_{4}}({{T}_{1}}-{{T}_{4}})}{({{T}_{1}}-{{T}_{4}})-({{T}_{3}}-{{T}_{4}})}\]

\[COP=\frac{{{T}_{4}}}{{{T}_{3}}-{{T}_{4}}}\]

Advantages of Bell Coleman Air Refrigeration Cycle

1. Refrigerant used is air, which is free of cost.
2. Safe operation in NH₃ machine is possible. Since, air is non-flammable.
3. In aircrafts, the selection of system depends on the ratio of weight of air refrigeration system to toil of refrigeration. This ratio is too smaller than that of other refrigeration systems.
4. Small amount of air leakage is permissible.
5. There is no need of extra space of extra compressor because the compression of air source is handled by the maul compressor.

Disadvantages of Bell Coleman Air Refrigeration Cycle

1. It requires more quantity of air to be circulated, when compared to other refrigerants.
2. It has low COP value.
3. Operating costs are higher.

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Figure 1 : Carnot Engine T-s Diagram & P-V Diagram.

The Carnot heat engine cycle with T-s and P-V diagrams are shown in the figure 1. The Carnot engine cycle consists of four types of thermodynamic processes in which two are frictionless adiabatic (Isentropic) and the other two are constant temperature operations (Isothermal) processes. These processes operate between the two temperature limits T₁ and T₂.

In the operation ‘ab’, boiling water is heated at temperature ‘T₁’ to obtained a dryness fraction ‘x₁’ from the steam. During this operation heat is being absorbed at constant temperature (T₁). In the operation ‘bc’, the temperature of the steam decreases isentropically and changes to temperature (T₂). The point ‘c’ is the point of steam expansion. Now, the operation shifts to ‘cd’. In this case, the heat is removed at and temperature (T₂).

When the steam is completely used it gets cooled down. In the operation ‘dc’. The steam again retains its temperature and pressure through compression and the cycle completes. Now, heat supplied during tile operation ‘ab’ at constant temperature ‘T₁’ is given by,

\[\text{Area of  abfe}={{T}_{1}}({{S}_{2}}-{{S}_{3}})\]

Heat removed during the operation ‘cd’ at constant temperature is given by,

\[\text{Area of  cdef }={{T}_{2}}({{S}_{2}}-{{S}_{3}})\]

During isentropic operation ‘bc’ and ‘da’. There is no heat produced.

Therefore,

Net Workdone = Heat supplied – Heat removed

\[={{T}_{1}}({{S}_{1}}-{{S}_{4}})-{{T}_{2}}({{S}_{2}}-{{S}_{3}})\]

\[=({{T}_{1}}-{{T}_{2}})({{S}_{2}}-{{S}_{3}})\]

The Carnot efficiency is a function of the temperature ‘T₁’ and T₂ and is given by..
Cannot cycle,

\[\text{ }\!\!\eta\!\!\text{ }=\frac{\text{Workdone}}{\text{Heat supplied}}\]

\[=\frac{({{T}_{1}}-{{T}_{2}})({{S}_{2}}-{{S}_{3}})}{{{T}_{1}}({{S}_{2}}-{{S}_{3}})}\]

\[\text{ }\!\!\eta\!\!\text{ }=\frac{{{T}_{1}}-{{T}_{2}}}{{{T}_{1}}}\]

or

\[\text{ }\!\!\eta\!\!\text{ }=\text{1}-\frac{{{T}_{2}}}{{{T}_{1}}}\]

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(a) Outer view of Air Cooler.

(b) Cross section of air cooler.

Fig. 1:  Air cooler.

Water is filled in the tank of the cooler, the level of water in the tank may be controlled by floats. A pump is installed, which is run by a motor, to lift water from the tank. The waterfalls on pads are made of wood fibers. Such pads are provided on three sides of the cooler. The pads also act as an air filters.

A blower draws in hot air from the atmosphere through the wetted pads. This cooled air is thrown into the room or the space to be cooled through the fourth side of the cooler, which is kept, and opened.

(a) Cooler Shutters.

(b) Cooler kit.

Fig. 2: Cooler Shutters and Kit.

The coolers may have a single blower, double blower, and a single motor for blower and pump, or separate motors for both, etc. Fig. 2 shows two designs of cooler shutters and the cooler kit.

Difference between Air conditioner and Cooler

The post What is an Air Cooler? Working Principle, Parts & Diagram appeared first on ElectricalWorkbook.

Fig. 1: Fridge Voltage Stabilizer (Refrigerator Stabilizer).

To stabilize the voltage, a voltage regulator or voltage stabilizer is used with domestic refrigerators. The voltage regulator may be manual or automatic. In manual voltage regulator, one has to check the voltmeter reading and adjust it. This is simply a tap-changing transformer. The automatic voltage regulator employs relays and other electronic circuitry which automatically keeps the output voltage within very narrow range. The generally available automatic voltage stabilizers are of the following ranges:

The automatic regulator is costly as compared to the manual but is more advantageous because it automatically takes care of the refrigerator during nighttime and other periods of neglect. The voltmeter of such regulators, need not to be switched on all the time. The 1 shows the outlook of an automatic voltage stabilizer and Fig. 2 shows the block diagram of its circuitry.

Fridge Voltage Stabilizer (Refrigerator Stabilizer) Block Diagram

Fig. 2: Fridge Voltage Stabilizer (Refrigerator Stabilizer) block diagram.

The 230 V/50 V supply is given to an autotransformer and also to the auxiliary transformer, which operates relays. The autotransformer can provide step-up or step-down voltage according to the need. The auxiliary transformer is a step-down transformer of rating 230 V/12 V. It has an output current of 500 mA to operate the relays. The rectifier rectifies output ac from the auxiliary transformer to be fed to the relay system. The rectifier’s output is given to a circuit, which operates relays. This has two NPN transistors that operate respective relays.

The relay system has two relays each having a voltage setting of 12 V. The relays have one normally open and one normally closed contact. The output of each relay is connected to the autotransformers tapping. The output is taken from the autotransformer and connected to the output socket of the stabilizer.

The post What is Fridge Voltage Stabilizer (Refrigerator Stabilizer)? Working & Block Diagram appeared first on ElectricalWorkbook.

Axial flow fans produce an air flow in a direction parallel to the axis of rotation, i.e., the air flow is parallel to the axis of the Impeller, hence the name. These fans can handle large volume of air but their operation creates noise, hence used in industries, where noise has little considerations.

These fans have best performance where air flow required against low resistance. These fans give good efficiency and tolerable noise level in air-conditioning work. When there is no duct work, axial flow or Impeller fans can be used. These fans are very much used as exhaust fans in kitchens, lavatories, bathrooms to remove odor and at many other places where no duct work is required.

In axial flow fan, airflow is parallel to the axis of impeller. In other words, when suction and delivery are in the same direction, i.e. parallel to axis of fan, then it is called an axial flow fan.

 Types of Axial Flow Fan

They are of three types:

1. Propeller type.
2. Tube axial type.
3. Vane axial type.

Propeller type:

Fig. 1: Propeller type axial flow fan.

It consists of a propeller or disc type wheel, which operates within a cylindrical housing. Blades are twisted and projected radially outwards from the hub (see Figure 1). The number of blades may vary from two to six. The quantity of air delivered depends upon the number of blades. A propeller fan is low-pressure high capacity fan, which requires less power than centrifugal fan.

Tube axial flow fan:

Fig. 2: Tube axial flow fan.

It consists of a heavy-duty propeller wheel, which operates within a simple cylindrical housing. It is more efficient than propeller fan. The air delivered by tube axial fan follows a spiral path, when it leaves the cylindrical housing (see Figure 2). Tube axial flow fans are duct-mounted, whereas the propeller fans are wall or diaphragm mounted. Since tube axial flow fans produce high noise level, therefore, their use is limited to applications, where noise level is not a major problem.

Vane axial flow fan:

Fig. 3: Vane axial flow fan.

It is also called as ‘tube axial fan with vanes’. A vane axial flow fan is mounted in a cylindrical housing with a set of air guide vanes (see Figure 3). Since the vanes are located at the delivery side of wheel, it helps to straighten out the path of discharged air. Due to such an arrangement, the fan eliminates spiral flow of the discharge air.

Advantages of Axial Flow Fan

1. Reducing turbulence of flow.
2. Silent operation due to straight flow.
3. Working efficiency and pressure characteristics of vane axial flow fan are better than tube axial fan.

Disadvantages of Axial Flow Fan

1. The only disadvantage of vane axial flow fan is non-ability to develop high pressures. Therefore, they are not preferred for duct air conditioning systems.

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Types of Water Cooler

The three types of water coolers are

1. Storage type
2. Instantaneous type
3. Bottle type.

Storage Type Water Cooler

Figure 1: Storage type water cooler.

In storage type, tap water (water to be cooled) is stored in large size storage tank, surrounded by cooling coil (see Figure 1). It takes more time in the beginning to lower the temperature of water. It is generally used in schools, offices, hospitals etc.

Cycle used: Vapour compression cycle.

Refrigerant: R-134a.

The storage type water cooler has an evaporator coil soldered on the outside surface of the wall. The tank is made of stainless steel or GI sheet. The water level in the water tank is maintained and controlled by a float valve.

Construction:

Water cooler has a metal sheet cabinet. It consists of a hermetically sealed compressor, condenser, capillary tube, accumulator, refrigerant coil, water cooling coil, thermostat, relay, overload protection etc. Thermostat is provided to control the temperature of water. Separate inlet and outlet connections of water are provided to storage tank. Water level is maintained with the help of float valve to minimize the wastage of refrigerated water. A push type water tap is generally provided for drawing cold water.

Working:

When the vapour compression system starts to operate, the heat of water is taken up by the refrigerant flow through the evaporator coil and gets evaporated. This vapour refrigerant is sucked by the compressor, where it is compressed to high pressure, high temperature and is sent to condenser. In an air-cooled condenser, the heat content of refrigerant is rejected to atmosphere. A condenser fan placed in front of the condenser coils to produce artificial draught of air. This increases rate of cooling and refrigerant and condensation of vapour refrigerant into liquid refrigerant occurs in short time. From condenser outlet, the liquid refrigerant passes through a capillary tube to the evaporator coil and the cycle is repeated number of times, till the desired cooling of water occurs. As soon as, the desired temperature is attained by water in the storage tank, the compressor is cut-off by thermostat.

Applications: Used in offices, schools, hospitals, factories etc.

Instantaneous Type Water Cooler

Figure 2: Instantaneous Type Water Cooler.

Instantaneous type of water cooler gives cold water as and when required. The cooling coil is directly wounded on the tap water line. Therefore, as soon as water to be cooled enters the tap water line, it gets cooled instantaneously. The major drawback of instantaneous water cooler is slow delivery rate. This happens, when the cold water tap is opened more frequently and for long time. Here, water delivery rate is slow.

In case of instantaneous type of water cooler, evaporator consists of “two separate cylindrically wounded coils made-up of copper or stainless Steel”.

One coil is cooled as cooling coil and other one is water coil. In the Fig. 2, the cooling coil and water coil are shown separately for the illustration purpose. The liquid refrigerant received from capillary tube flows through the evaporator coil, whereas, water to be cooled is made to pass in the water coil through a filter and a flow regulator. Both coils are located closed to each other to permit heat transfer by conduction. Thus, the liquid refrigerant absorbs heat from the water by conduction reducing its temperature. Thermostat controls the ON/OFF operation of compressor to maintain the water temperature within the required limits. For this purpose, the feeler bulb of thermostat is located on the water pipe at its outlet end. In case of Instantaneous water cooler, it Is very important to control the flow rate of water as per the needed capacity of evaporating coil. If the rate of water is higher, the system will not be able to bring down the temperature of water to desired or required or set value. Therefore, high flow rate of cold water is required or desired, then high capacity evaporator should be selected.

Bottle Type Water Cooler

In bottle type, cold water is supplied from an inserted bottle, which is kept in the water cooler. Capacity of bottle is in few litres.

Capacity of Water Cooler

1. Capacity of the water cooler is specified on the basis of volume capacity of the storage tank. It is specified in litres. For example: 50, 100, 200, 500 litres etc.
2. Capacity of the water cooler is also specified on the basis of its capacity to cool a continuous flow rate of water at 10°C to 20°C from a specified temperature of the incoming water (say 32°C) under an ambient condition. In such cases, capacity of water cooler ranges from 45 to 190 litres of flow rate per hour.

The post What is Water Cooler? Working, Diagram & Types appeared first on ElectricalWorkbook.

In primary circuit, ammonia is used as refrigerant, whereas in case of secondary circuit, brine solution is used as refrigerant. As ammonia (primary refrigerant) is toxic in nature, so it should not come in direct contact with the water. Therefore, a secondary refrigerant (brine solution) is used. This brine solution is circulated around the galvanized iron cans containing the water circulating with the help of brine pump. Brine solution takes the heat from the water to convert it into ice. This heat absorbed by brine solution is ejected to primary refrigerant (ammonia) in the heat exchanger.

Figure 1: Ice Plant.

Construction of Ice PIant

Commercial ice is produced by freezing water in standard cans placed in rectangular tank insulated from all sites. The tank is filled with chilled brine, which is continuously circulated in secondary circuit. Brine tank is fabricated from 7 mm thick mild steel plates. The depth of brine tank and insertion of ice cans are so adjusted that, brine level is at 25 mm more than the water level in the cans. Insulated wooden lids are provided to cover the top to facilitate the removal of ice cans. Ice cans are tapered by about 10 to 12 mm in their height for easy removal of ice produced from the Ice cans. To get clear transparent Ice, the water in the can is agitated by the use of low pressure air through the tubes suspended from the top.

Working of Ice PIant

Primary circuit consists of four basic components, namely compressor, condenser, expansion device and evaporator. The vapour ammonia coming out from the evaporator is compressed in the compressor and gets converted into high pressure, high temperature vapour ammonia refrigerant. This high pressure, high temperature vapour ammonia (refrigerant) is condensed in condenser and gets converted into high pressure, low temperature liquid ammonia refrigerant. This high pressure, low temperature liquid ammonia refrigerant is made to pass through expansion device, where it and gets converted into low pressure, low temperature liquid ammonia refrigerant. This low pressure, low temperature liquid ammonia refrigerant comes in the evaporator, where it absorbs heat from brine solution and gets converted into low pressure, low temperature vapour ammonia refrigerant and then enters to compressor of primary circuit. Also, we can see that, the brine solution flowing through the secondary circuit is cooled. This cooled brine solution is circulated around the water cans to absorb heat from water. Now, the same cycle is repeated again and again till the ice of desired temperature is formed.

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Figure 1: Cold Storage.

Construction of Cold Storage

Fig. 1 shows the layout of a cold storage system consisting of four essential components: (1) Compressor, (2) Condenser, (3) Expansion device, and (4) Evaporator. Reciprocating compressors with flood mounted Air Handling Units (AHU) with finned or bare tube cooling coils are used in the refrigeration plant of cold storages.

Refrigerants used: Ammonia (NH₃), R-22, R-134.

Categories of Cold Storage

Cold storage is divided into two categories:

1. Cold storages for product, which can be protected at temperatures of 0°C and above.
2. Low temperature applications requiring subzero (below 0°C) temperature for preservation of certain commodities.

The food products can be stored for periods ranging from fifteen days to several months. Even though, does not improve the quality of foods products; but its ability to slow down the deterioration rate, makes possible to preserve them for long duration.

Advantages of Cold Storage

The following are the main advantages of cold storage:

1. Substances such as potatoes, butter etc. can be stored, when their supply is more than demand in market. The stored substances can be sold out during less supply of substances due to adverse environment effects or any other reason and demand in the market can be fulfilled, that too at reasonable price.
2. Due to reduction of spoilage, great saving is made.
3. Transportation of perishable commodities from distant places (located at large distances) is made possible.

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