Refrigeration

Refrigeration is the process of producing and maintaining temperature below that of the surrounding atmosphere.

Refrigerant

It is a substance used for producing lower temperature. e.g. NH3, Water, R11, R134. There are two types of refrigerants:

• Primary Refrigerant: Those refrigerant which absorbs heat directly from storage by undergoing in a cycle. It absorbs latent heat. e.g. NH3, R11,R22, R134.
• Secondary Refrigerant: These refrigerants are first cooled by primary refrigerants and then used for cooling at the required space. It will not undergo in a cycle and absorbs sensible heat. e.g. Water, brines, glycols and sometimes even halocarbons.

Designation of Refrigerants

1. For saturated hydrocarbons \( C_mH_nF_pCl_q\) designation is given by:

R (m-1) (n+1)p     and n + p + q = 2m + 2

     for example: $CHF_2Cl$   $\Rightarrow$   R22
                             $CF_2Cl_2$     $\Rightarrow$   R12
                             $C_2H_6$     $\Rightarrow$   R170

    2. For saturated hydrocarbons \( C_mH_nF_pCl_q\) designation is given by:

R 1 (m-1) (n+1)p     and n + p + q = 2m

     for example: $C_2H_4$   $\Rightarrow$   R1150

    3. For inorganic compounds designation is given by:

R  (700 + Molecular weight of refrigerant)

     for example:    $NH_3$   $\Rightarrow$   R717
                             $H_2O$     $\Rightarrow$   R718
                             $CO_2$     $\Rightarrow$   R744

Desirable Properties of Refrigerant

Thermodynamic Properties

• High critical temperature for high COP.
• Freezing point should be low as it must operate in the cycle above its freezing point.
• Specific heat low in liquid phase and high in vapour phase.
• High thermal conductivity

Chemical Properties

• non- toxic and non flammable
• It should not act with material of construction

Physical Properties

• low viscosity
• No leakage tendency
• high dielectric strength

Refrigeration Effect and Capacity

Heat Pump / Refrigerator

Both Heat pump and refrigerator transfers heat from low temperature to high temperature but requirement of refrigerator is to cool the space, however heat pump requirement is to heating the place.

Refrigeration effect(RE) is defined as the heat which is extracted from storage in order to maintain low temperature. 

\( COP_R = \frac{Q_2}{Q_1 - Q_2}= \frac{RE}{Q_1 - Q_2} = \frac{RE}{W}\)

\( COP_{HP} = \frac{Q_1}{W} = 1 + COP_R\)

• COP signifies the running cost.
• Higher the COP lower the running cost.
• COP of heat pump is always greater than one. 

Refrigeration Capacity = Refrigeration effect x mass flow rate

generally refrigeration capacity unit is defined as ton of refrigeration (TR). A ton of refrigeration is defined as "the amount of heat to be removed from one ton (1000 kg) of water at 0°C in order to convert it into ice at 0°C in 24 hrs."

1 TR = 3.5 kJ/s  =  210 kJ/min

Ideal Refrigeration/ Reverse Carnot Cycle

It is the most efficient efficient refrigeration cycle and has highest theoretical COP. COP doesn't depends on working substance in this case.

\[ COP_R = \frac{Q_2}{Q_1 - Q_2}= \frac{T_L}{T_H - T_L}\]

Vapour Compression Refrigeration System (VCRS)

This works on reverse Rankine cycle.

• Process 1 - 2 : Isentropic Compression
• Process 2 - 3 : Constant pressure heat rejection
• Process 3 - 4: iso - enthalpic expansion (h3 = h4)
• Process 4 - 1 : Constant pressure heat addition

\[ COP = \frac{Q_2}{W} = \frac{h_1 - h_4}{h_2 - h_1} = \frac{h_1 - h_3}{h_2 - h_1}\]

Cascade Refrigeration

In a cascade system a series of refrigerants with progressively lower boiling point are used in a series of single stage units. This is uses for obtaining very low temperature and saves considerable compression work.

  

\[ COP_{overall} =  \frac{COP_1* COP_2}{1 + COP_1 + COP_2}\]

Vapour Absorption Refrigeration System (VARS)

These systems run on low grade energy thus, they are preferred where waste heat is available.  

It employs two fluids refrigerant and absorbent and low pressure refrigerant vapor is absorbed into absorbent and releases large amount of heat. This solution is pumped at high pressure generator where heat is added and added heat causes desorbs from absorbent and its vapour flows to condenser where heat is rejected. The liquid refrigerant throttled through an expansion valve to lower pressure evaporator where it absorbs heat and provides cooling. The remaining absorbent in generator pass through vale to Absorber. 

Maximum COP is given by
\[ COP_{max}  = η_{carnot} . COP_R    = \bigg( \frac{T_G - T_O}{T_G}\bigg) \bigg( \frac{T_R}{T_O - T_R} \bigg)\]

 where To is ambient temperature.

Gas Refrigeration Cycle / Reverse Brayton Cycle

This cycle also known as Joule or Bell - Colemn cycle. 

• Process 1 - 2 : Isentropic Compression
• Process 2 - 3 : Constant pressure heat rejection
• Process 3 - 4:  Isentropic expansion 
• Process 4 - 1 : Constant pressure heat addition

\[ COP = \frac{1}{\frac{T_2}{T_1} - 1}= \frac{1}{{r_p}^{\frac{γ-1}{γ}} - 1} \]

$r_p$ is pressure ratio = $\frac{P_2}{P_1}$

• Its COP is low but used in aircraft because of low weight and less costly.

Next: Psychrometry

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