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Chemical – II Unit V -

CHEMCAL ENGINEERING THERMODYNAMICS – II

UNIT V – REFRIEGERATION

Principles of refrigeration Methods of producing Refrigeration Liquefaction Process Coefficient of performance Evaluation of the performance of vapor compression and refrigeration cycles

REFRIGERATION

Refrigeration is process of producing and maintaining a below that of the surrounding atmosphere. This requires continuous absorption of at a low temperature level, usually accomplished by evaporation of a liquid in a flow process. The vapor formed may be returned to its original liquid state for re-evaporation, either by compressing and condensing or by absorbing it with a liquid of low volatility from which it is subsequently separated at high . Thus refrigeration is essentially an operation involving the pumping of heat from one temperature to a higher temperature. The complete series of processes that the - the – undergoes constitute a refrigeration cycle. A typical refrigeration cycle includes, evaporation of the liquid refrigerant, compression of the refrigerant vapor, of the vapor into liquid and finally expansion of the liquid.

CARNOT

A refrigeration cycle is a reversed heat cycle. Heat is transferred from a low temperature level to a higher temperature level. According second law of thermodynamics this requires an external source of . The ideal refrigerator like the ideal operates on a , consisting of two isothermal steps in which heat lQcl is is absorbed at a lower temperature level Tc and heat lQHl is rejected at higher temperature TH and two adiabatic cycles steps. The cycle requires the addition of net w to the . Since ∆U of the working fluid is zero from the first law of thermodynamics.,

W = lQHl –lQcl (1)

COEFFICIENT OF PERFORMANCE

It is defined as the measure of the performance of a refrigerator and is the ratio of heat absorbed at the lower temperature to the net work. Heat abosorbed at the lower temperature ω = ------Net work lQcl = ------(2) W

Dividing equation (1) by (Qc)., w QH ---- = ------1 Qc Qc

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Chemical Engineering thermodynamics – II Unit V - Refrigeration

lQHl TH But for Carnot refrigerator ------= ------lQcl Tc

w TH TH - Tc 1 Therefore ----- = ------1 = ------= ---- lQcl Tc Tc ω

Tc ω = ------

TH – Tc

Applicable to a refrigerator, operating on a Carnot cycle.

According to this refrigeration effect per unit of work decreases as the temperature of the refrigerator Tc decreases, and as the temperature of heat rejection TH increases. For refrigeration at a temperature level of 5⁰C and a surroundings of 30⁰C the value of ω for a Carnot refrigerator is 5 + 273.15 ω = ------= 11.13 (30 +273.15) – (5 + 273.15)

THE VAPOR COMPRESSION CYCLE

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Chemical Engineering thermodynamics – II Unit V - Refrigeration

A liquid evaporating at constant pressure provides a means of heat absorption at constant temperature. Similarly condensation of the vapor after compression to a higher pressure provides for the rejection of heat at constant temperature. The liquid from the is returned to its original state by an expansion process using a turbine or an expander. When the compression and expansion are isentropic, the sequence of processes constitutes the cycle as shown above. This is just similar to Carnot cycle except that the superheated vapor from the must be cooled to its saturation temperature before condensation begins.

On the basis of unit mass of fluid, the heat absorbed in the is

lQcl = ∆H = H2 – H1

Heat rejected lQHl = H3 – H4

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Chemical Engineering thermodynamics – II Unit V - Refrigeration

But W = lQHl – lQcl = (H3 – H4) – (H2 – H1)

Qc H2 – H1 COP = ω = ------= ------

W (H3 – H4) – (H2 – H1)

In small units expansion is accomplished by throttling the liquid from the condenser though a partly opened valve.

The pressure drop in this results from fluid friction in the valve. But still because of its simplicity and lower cost, it outweighs the energy savings possible with a turbine. The throttling process occurs at constant . The vapor compression cycle incorporating an expansion valve is shown above. Line 4- 1 represents the constant enthalpy throttling process. Line 2 - 3 represents an sloping in the direction of increasing .

The dashed line 2 – 3’ is the path of isentropic compression. For this cycle

H2 – H1 COP = ω = ------

(H3 – H4) – (H2 – H1)

But H4 = H1 throttling being a H2 – H1 COP = ω = ------

(H3 – H2)

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Chemical Engineering thermodynamics – II Unit V - Refrigeration

Design of the evaporator, compressor condenser requires the knowledge of the rate of circulation of refrigerant m lQcl Mass flow rate of refrigerant = ------

H2 – H1

The vapor compression cycle shown above can also be represented on P H diagram as shown above. P H diagrams will give directly. Although the evaporation and condensation processes are represented by constant pressure paths, small pressure drops do occur because of fluid friction.

Usually refrigeration equipments are commonly rated in tons of refrigeration.

A is defined as heat absorption at the rate of 12, 000 BTU per hour. This rate corresponds to the rate of heat removal that is required to freeze 1 ton of water at 273 K in a day.

One ton of refrigeration is equivalent to a refrigeration rate of 12, 600 KJ/h in SI units.

A Carnot refrigerator will have highest value of COP for a given set of values Tc and TH. A vapor compression cycle with expansion in a throttle valve has somewhat lower value and this is further reduced when compression in not isentropic.

THE CHOICE OF THE REFRIGERANT

Efficiency of Carnot engine and COP of Carnot refrigerator is independent of the refrigerant, however irreversibilities inherent in the vapor compression cycle cause the COP of the practical depend on refrigerant.

Toxicity, Flammability, Cost, properties and in relation to temperature are of greater importance in the choice of refrigerant.

The most widely used are a group of halogenated hydrocarbons marketed under the various proprietary names – 12, genetron, arctron, isotron, frigen, mafron etc. These are either based or ethane based, where the hydrogen atoms are replaced by chlorine or fluorine atoms.

For methane based refrigerants, the name is represented by two digits. First digit minus 1 represents the number of hydrogen atoms, and 2 digit represents the number of fluorine atoms.

Eg: R – 12  CCl2F2 Dichlro difluoro methane. R – 10  CCl4 Carbon tetrachloride.

For ethane based refrigerants, a three digit number is assigned where the first digit is always 1, the second minus 1 represents hydrogen atoms and the third digit represents number of fluorine atoms.

Eg: R-113  C2F3Cl3 Trifluoro trichloro ethane. R – 142  C2H3F2Cl Difluoro chloro ethane.

But these halogenated hydrocarbons being largely insoluble in water will move up and react with ozone layer and deplete it. ia also a popular refrigerant but it is toxic and inflammable.

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Chemical Engineering thermodynamics – II Unit V - Refrigeration

MULTI-STAGE VAPOR COMPRESSION

Limit placed on the operating of the evaporator and condenser of a refrigeration system also limit the temperature difference TH – Tc over which a vapor compression cycle can operate. TH is fixed by the temperature of the surroundings ie., the temperature of the water available. This limit in

TH – Tc can be handled by the operation of two or more refrigeration cycles employing different refrigerators, in a cascade. A two stage cascade is shown in figure. Here the two cycles operate so that heat absorbed by the refrigerant of the higher temperature cycle (cycle 2) in the interchanger serves to condense the refrigerant of the lower temperature cycle (cycle 1). The two refrigerators are so chosen, so that at the required temperature levels each cycle operates at reasonable pressures.

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Chemical Engineering thermodynamics – II Unit V - Refrigeration

ABSORPTION REFRIGERATION

The work that is needed to operate a refrigerator cycle can also be obtained by operating a heat engine at a higher temperature level.

The work required by a Carnot refrigerator absorbing heat at temperature Tc and rejection heat at the temperature of the surroundings TS lQcl Tc ω = ------= ------W Ts – Tc

Ts –Tc W = ------lQcl (1) Tc

When a source heat is available at a temperature above that of surroundings TH, then work can be obtained from a Carnot engine operating between this temperature and the surroundings temperature Ts. The heat required lQHl for the production of work lWl is found as follows

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Chemical Engineering thermodynamics – II Unit V - Refrigeration

lWl Ts TH - Ts η = ----- = 1 ------= ------

lQHl TH TH

TH lQHl = lWl ------(2) TH – Ts Substituting for lWl from (2).,

Ts - Tc TH lQHl = lQcl------Tc TH – Ts

The value of lQHl/lQcl given by this equation is of course a minimum, because Carnot cycles can not be achieved in practice.

When Tc = - 263.15 K, Ts = 303.5 K and TH = 373.15 K

Then lQHl / lQcl = 0.81.

For actual , the value would be on the order of three times.

The absorption refrigeration system is a heat operated unit which uses a refrigerant that is alternately absorbed and liberated from the absorbent that is . The section to the right accomplishes compression, is equivalent to a heat engine. Refrigerant as vapor from the evaporator is absorbed in a relatively non volatile liquid solvent at the pressure of the evaporator and at relatively low temperature. The heat given off in the process is discarded to the surroundings at Ts. This is the lower temperature level of the heat engine. The liquid solution from the absorber which contains relatively high concentration of refrigerant passes to a which raises the pressure of the liquid to that of the condenser. Heat from the higher temperature source at TH is transferred to the compressed liquid solution, raising its temperature and evaporating the refrigerant from the solvent. Vapor passes from the regenerator to the condenser, and solvent returns to the absorber.

Eg: Solvent: Water Refrigerant : Ammonia -10⁰C Solution Water <0⁰C

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Chemical Engineering thermodynamics – II Unit V - Refrigeration

LIQUEFACTION PROCESSES

Liquefied are in common use for a variety of purposes. For example liquid in cylinders serves as a domestic , liquid is carried in rackets, natural gas in liquefied for ocean transport and liquid is used for low temperature refrigeration. Gas mixtures are liquefied for separation in to their component species by fractionation.

Liquefaction results when a gas is cooled to a temperature in a two region. This may be accomplished in several ways.

1. By heat exchange at constant pressure.

2. By an expansion process from which work is obtained.

3. By a throttling process.

The first method requires a at a temperature lower than that to which the gas is cooled and is most commonly used to precool a gas prior to its liquefaction by the other two methods. An external refrigerator is required for a gas temperature below that of the surroundings. The constant pressure process (1)., approches the two phase region most closely for a given drop in temperature.

The throttling process (3)., does not result in liquefaction unless the initial state is at a high pressure and low enough temperature for constant enthalpy process to cut into the two phase region. This does not occur when the initial state is A. If initial state is A’ where the temperature is the same but the pressure is higher than at A, then isenthalpic expansion by process 3’ does result in the formation of liquid. The change of state from A to A’ is most easily accomplished by compression of the gas to the final is most easily accomplished by compression of the gas to the final pressures B, followed by constant pressure cooling to A’. Liquefaction by isentropic expansion along process (2) may be accomplished from lower pressures than by throttling.

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Chemical Engineering thermodynamics – II Unit V - Refrigeration

The throttling process (3) is the one commonly employed in small scale commercial liquefaction plants. Here the temperature must be low enough and pressure should be high enough prior to throttling , that the constant enthalpy path cuts into the two phase region.

Ex: Air with a pressure of 100 atm and a temperature of 170 K can be liquefied by throttling.

The most economical way to cool a gas for liquefaction is by countercurrent heat exchange with the portion of the gas that does not liquefy in the throttling process.

LINDE LIQUEFACTIN PROCESS

The linde liquefaction process which depends solely on throttling expansion is shown in the above figure. After compression, the gas is precooled to ambient temperature . It may be even further cooled by refrigeration. The lower the temperature of the gas entering the throttle valve, the greater the fraciton of gas that is liquefied.

CLAUDE LIQUEFACTION PROCESS

Replacing the throttle valve by an expander increases the efficiency of the liquefaction process. The Claude process is based on this idea. In claude process, gas at an intermediate temperature is extracted from the system and passed though a expander from which it exhausts as a saturated or slightly superheated vapor. The remaining gas is further cooled and throttled through a valve to produce liquefaction as in the linde process. The unliquified portion, which is saturated mixes with the expander exhaust and returns for recycle through the heat exchanger system.

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Chemical Engineering thermodynamics – II Unit V - Refrigeration

POWER CYCLES

RANKINE CYCLE

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Chemical Engineering thermodynamics – II Unit V - Refrigeration

Steam plants operate on a closed cycle, wherin working fluid undergoes a series of operations and returns to the initial state. The thermodynamic analysis of power plants is done by comparing the performance of the actual cycles with certain idealised cycles.

The components of a also known as standard vapor power cycle are shown in the figures. In this water at low temperature and pressure is compressed isentropically to the pressure by a pump and is superheated in the boiler. The superheated vapor is expanded in a turbine, isentropically to the condenser pressure- (3-4). In the condenser the low pressure exhasut steam from the turbine gives out its heat and condeses to saturated liquid which enters the pump again.

Let Q1 be the heat absorbed and Q2 be the heat discarded.

Obviously efficiency of the Rankine cycle is less than that of a Carnot cycle operating between the same thermal reservoirs. In the case of Carnot cycle, which operates with the input of saturated steam and discharges a mixture of vapor and liquid leads to severe erosion problems. Also it is difficult to operate a pump which takes in a two phase mixture and is charges a saturated liquid. The Rankine cycle is free from these problems and is therefore accepted as a model for the actual vapor power cycle.

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Chemical Engineering thermodynamics – II Unit V - Refrigeration

INTERNAL COMBUSION

Internal combustion engines are operated on open cycles, unlike steam power plants which are operated on closed cycles. In IC engines the working fluid a mixture of fuel and air is burnt and after getting work, combustion products are discarded. The high temperature and the absence of surfaces are the main advantages of IC engines over steam generation systems. Here the working fluid does not undergo a cycle of changes but the engine operates on a closed mechanical cycle.

The thermodynamic analysis of internal combustion engine is made possible by devising ideal closed cycles with air as the working fluid and by comparing the performance of actual cycles with these ideal air-standard cycles. The assumptions involved are: 1. The working fluid is assumed to behave as an without any chemical change. 2. The combustion process in the actual cycle is replaced by a heat transfer process in the ideal cycle. Heat is assumed to be transferred from a external source. 3. The exhaust stroke in the actual engine is replaced by a heat rejection step in the ideal cycle. Heat is assumed to be transferred to the surroundings. 4. The air is assumed to have constant specific heat and all the processes are internally reversible.

OTTO CYCLE AND

Otto and Diesel cycle are the two important air-standard cycles used for the analysis of internal combustion engines.

Process 1-2: Insentropically air is compressed during the inward stroke of the , during which

Temperature of the gas increases fromT1 to T2

Process 2-3: Heat Q1 is supplied , due to which temperature increses from T2 to T3. Pressure is also incresed drastically from P2 to P3 and entropy also increases from S2 to S3.

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Chemical Engineering thermodynamics – II Unit V - Refrigeration

Process 3-4: The air is expanded isentrophically during the outward stroke of the piston during which both T and P decreses. In actual engines the products of combustion at very high pressure and temperature expands, approximately adiabatically during this step.

Process 4-1: Heat Q2 is transferred from the system reversibly at constant to a low temperature reservoir. The T,P and S decreses during this stage. In actual engines, exhaust valve opens and the pressure falls rapidly at nearly constant volume.

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Chemical Engineering thermodynamics – II Unit V - Refrigeration

Diesel cycle

Air standard diesel cycle

In diesel engines the temperature at the end of compression exceeds the fuel ignition temperature and the combustion of fuel occurs spontaneously.

Process 1-2: Isentropic compression to high pressure and temperature.

Process 2-3: Heat Q1 is supplied at constant pressure. (Fuel in injected and burnt spontaneously)

Process 3-4: Isentropic expansion which results in the drop of temperature to T4

Process 4-1: Heat rejection Q2, at constant volume, which decreases the T and P.

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Chemical Engineering thermodynamics – II Unit V - Refrigeration

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Chemical Engineering thermodynamics – II Unit V - Refrigeration

DUAL CYCLE

In a air standard dual cycle, combustion is neither a constant volume process nor a at constant pressure process. It is a compromise between ideal and an ideal diesel cycle. The P-V and T-S diagrams for a dual cycle is as shown in the figure.

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