BASICS OF REFRIGERATION AND AIR CONDITIONING

BASICS OF REFRIGERATION AND AIR CONDITIONING

Er .Deepak Sathe

DEE

SDE(E)

BSNL,Mumbai
2006

BASICS OF REFRIGERATION :-

The word ‘refrigerate’ means to chill or freeze a substance i.e. to lower its temperature by removing heat.

Refrigeration is the process of removing heat from a substance and rejecting the heat so removed to the atmosphere which is at a higher temperature level.

Heat always flows from a body at a high temperature to another at a lower temperature. Hence when we want heat flow in reverse direction i.e. from lower to higher temperature level or in other words when we want to lower the temperature below the surrounding atmospheric temperature it requires external energy.

Refrigeration is accomplished by various methods:- 1) Vapour compression system   2) Vapour absorption system  etc…..

Mechanical vapour compression system is practical application of Charles’s law Pressure varies proportionate to temperature. The boiling point of a liquid ( or condensing  temperature of a gas)  can be varied at will by varying pressure of vapour in a enclosed space above the liquid level (i.e. in saturated condition)

                           

Refrigerants are heat carrying mediums which during their cycle in the refrigeration system absorb heat at ( Low pressure) a low temperature level and discard the heat so absorbed at a higher level i.e. ambient temperature (at High pressure).

                                                                                

 These refrigerants have boiling points much below ordinary room temperature. So they exist as gases and are held in liquid state by keeping them under pressure such as in refrigerant cylinder.

From pressure-temperature chart (at saturation) we get boiling point and their corresponding pressure.

For R 22
Boiling point ( 0 F)	Pressure   (PSIG)
-41	0  (At atmospheric pressure)
49.1	82.5
75	132.2
120	259.9
125	277.9
127.4	286.9
130	296.9

1) Evaporator: - The process of heat removal from the substance / air to be cooled is done in evaporator. The refrigerant is in liquid state as it absorbs maximum heat during vaporization. At the inlet of evaporator, the refrigerant is predominately in the liquid form with small amount vapour formed as a result of flashing at the expansion valve.

                         SST is 9⁰ F to 10⁰ F (49.1 ^OF) corresponding pressure is 82.5 PSIG neglecting pressure losses in evaporator.

                            As refrigerant passes through the evaporator more and more liquid is vaporized by the load, when it reaches the end of evaporator it is purely in the super heated vapour state.  

D-X system which we use in package air conditioner, is a system where the air to be cooled is directly passed over the evaporator coil inside which refrigerant is boiling.

2) Compressor:- For closed cycle of  vapour compression system ,the refrigerant coming out of the evaporator ( superheated low pressure vapour) must be compressed corresponding to the saturation temperature higher than ambient air temperature ( in case of air cooled system).

The compressor acts as pump to circulate refrigerant through closed  system.

                           

Types:- a)  Open or Hermetically closed.

b)  Reciprocating, Screw, Rotary, Centrifugal or Scroll

Compressor manufacturers publish rating charts/tables showing capacity for various suction temperatures and discharge temperatures. Capacity increases with increase of suction temperature whereas it decreases with increase of discharge temperature.

                                    Discharge pressure in absolute units

Compression Ratio=

                                   Suction pressure in absolute units

Pressure in Absolute units = PSIG + 14.7

                 Refrigeration effect produced                  BTU / hour

EER     =                                                    = 

                               Power consumed                                     Watt

                             EER is maximum in case of Scroll type compressor.

3) Condenser:- The function of condenser are

                           1)  To desuperheat high pressure gas.

                           2) To condense it from gas to liquid.

                           3) To Sub cool liquid.

                           SDT is 53⁰ C (127.4 ⁰F) corresponding pressure is 286.9 PSIG neglecting pressure losses in condenser.

                          Depending upon type of cooling medium condensers are 1) Air cooled 2) Water cooled 3) Evaporative (combination of both air & water).

                          In air cooled condenser refrigerant is cooled by ambient air, the ambient air temperature is more than the temperature of water. Hence the condensing temperature is higher in case of air cooled condenser than water cooled condenser. Heat transfer efficiency increases when the medium is liquid. This makes water cooled system more efficient than air cooled but the scarcity of water compels to use air cooled system.

4) Throttling device:- (Thermostatic expansion valve) The pressure of the liquid refrigerant  coming out of condenser / receiver  has to be reduced so that it can  vaporize at desired temperature in the evaporator (SST).

                           Sufficient refrigerant has to be fed into the evaporator to meet load. Externally equalized thermostatic expansion valve is normally used, feeler bulb is placed on suction line.

Superheating:-If the temperature of the refrigerant gas is more than its saturation temperature ,the gas is said to be  in superheated condition.

TEV is adjusted for superheat of 10⁰F means the suction line where feeler bulb is mounted there is no liquid but vapour in superheated state by 10⁰F above its saturation temperature.

Superheat  provided is the difference between the temperature of the suction line ( place where feeler bulb is mounted) and the saturated temperature corresponding to the evaporator pressure( i.e. suction pressure neglecting pressure drop in evaporator or add 2⁰F for suction pressure drop).

As the superheat increases compressor capacity drops due to reduction in density of gas, HP per ton increases. But this superheat is required to safe guard compressor against liquid flood back.

                  

Sub cooling:- If the temperature of the refrigerant gas is less than its saturation temperature ,the liquid is said to be  in sub-cooled condition.

The high pressure ,high temperature liquid coming out of condenser must be cooled down and reduce its pressure before entering evaporator, this is achieved by throttling device. The cooling occurs automatically when liquid passes through TEV.A portion of liquid refrigerant boils taking latent heat of vaporization from liquid itself.

For thermodynamics tables of R22

Temp. (0F)	Enthalpy of liquid (BTU/lb)	Enthalpy of vapour (BTU/lb)	Latent heat of vaporization(BTU/lb)= Enthalpy diff between vapour & liquid
119	42.45	112.5	70.05
40	21.42	108.14	86.72

The amount of heat to be removed to cool liquid from 110⁰ F TO 40⁰ F = 42.45 – 21.42 = 21.03

 latent heat of vaporization at 40⁰F = 86.72

i.e. 24.2% of latent heat of vaporization is utilized to cool liquid refrigerant to evaporator temperature level.

The refrigeration is accomplished mainly by absorption of its latent heat of vaporization by boiling liquid refrigerant in the evaporator.

The 24.2% of  latent heat of vaporization is used in cooling down liquid while passing through TEV ,balance 75.8% gives effective refrigeration effect ,known as Net Refrigeration Effect (NRE).

Thus NRE decreases as the difference between condensing and evaporating temperature widens.

Also with sub cooling ,as the temperature from which liquid is to be cooled is reduced, the NRE increases and mass flow rate per ton reduces.

Manufacturers rating chart  gives capacity / power for wide range of SST & SDT at liquid sub cooled at 15⁰F.

For every degree increase above 15⁰F of liquid sub cooling, there is increase in compressor capacity by 0.5%.without any change in power requirement, thus increases efficiency.

Over sizing air cooled condenser ( or  by use of liquid –suction heat exchanger ) so as to take care of de-superheating, condensing and liquid sub cooling, we can increase efficiency of Refrigeration system.

Small amount of vapour  present in liquid line occupy substantial volume ,which offer high resistance to flow in liquid line. This effects capacity of refrigerant system. Vapour can be present in the liquid line 1) If condensation is not complete. 2) Shortage of refrigerant. 3) Excessive pressure drop in liquid line causing formation of flash gas.

Pressure Drop – Liquid lift:- When liquid / gas flows through pipe ,resistance is offered by pipe walls, results in pressure drop .In refrigeration ,pressure drops are expressed in temperature units. If liquid is in saturated condition (not sub cooled) even small pressure drop can cause liquid flashing resulting in formation of flash gas. Similarly where an evaporator is located at higher level than the condenser, the pressure drop due to liquid lift (lift against gravity) is very high causing heavy liquid flash.

	R22
Kg/cm2 per mtr	0.035
Psi per ft	0.5

Selection of Design criteria:-

1) Saturated suction temperature (SST):- Compressor capacity varies directly with SST. Higher SST  better will be the capacity.

Hermetic / Semi hermetic compressor -   10⁰C   or 50⁰F

Open type compressor-     15⁰C    or   59⁰F

This is governed by 1) for ensuring suction gas cooling of motor windings.

2) Saturated discharge temperature (SDT):- Compressor capacity varies inversely

 with SDT.

Depend upon i) type of condenser air cooled or water cooled.

 ii) Highest outside condition.

 ( ETD) Entering Temperature diff = (SDT – entering temp of condenser i.e.max ambient temp)

                 Normally taken as    10⁰C to 17⁰ C   ( 20⁰F  to  30⁰F)

3) Sub cooling:-  Minimum 15⁰F ( sub cooling can save energy up 10%)

4) Super heat:-  Minimum 6⁰C ( for safety of compressor)

5) Static pressure:- 35 mm WG.( for high sensible units it is better to take 25 mm WG as  cfm for is more than the units for comfort cooling and installing units in area to be air-conditioned, as power consumption of continuous operated blower is @ 25% )