ACTIVE CARBON
A. VAPOUR COMPRESSION TYPE
B. Absorption type
A. VAPOUR COMPRESSION TYPE
In vapour compression, the refrigeration cycle works on reverse Rankine (or carnot) cycle.
The refrigerant vapour is compressed by a compressor which increases the pressure so that the compressed vapour attains temp. higher than the cooling water or air and the vapour gives out heat to cooling water/air thereby the vapour condenses to liquid in the condenser as the saturation temp.
is reached due to cooling.
The condensed liquid then flows through an expansion valve where partial flashing of liquid takes place, thus cooling the remaining liquid to below the temp. of product to be cooled. The temp difference facilitates heat to be transferred from product to refrigerant, which causes the cooling of product and the refrigerant to evaporate. The hotter products give out heat for evaporation of refrigerant thereby cooling the product in refrigerated space. Heat of evaporation of refrigerant is the key factor. For refrigerators, compressor with motor is inside a sealed unit, condensation by air-cooling and refrigerant liquid flashes in freezer body tubes. The vapour of refrigerant is removed by compressor suction which causes the flow of liquid to maintain and also maintain low pressure in evaporator. The simple circuit given in Fig. 1 and layout in Fig. 2.
Refrigerants
Three types–halogenated hydro-carbons, hydro-carbons and inorganic compounds. These are generally substances having higher latent heats and low specific heats with the objective of reducing quantity of refrigerant in circulation and minimizing losses, which occur in expansion valve and other fittings. The boiling point of refrigerant establishes the refrigeration temp. at which it could be used in the particular refrigeration system. Normally refrigerants are non-inflammable or slightly inflammable and non toxic or slightly toxic.
Coefficient of performance of a refrigerant It is the ratio of
refrigeration effect
Works expended = 0
Q 5.75
Qk Qo =
−
Chapter 9
REFRIGERATION
78
where Q0 = net heat extracted per unit mass, and Qk – Qo = work of compression per unit mass
Fig. 1. Vapour compression refrigeration circuit.
This ratio indicates efficiency under certain conditions of the refrigerant selected. It is often given at standard conditions i.e., –15°C (5°F) evaporation and +30°C (86°F) condensation. The maximum value is 5.75, which is obtained on working with a, reverse Rankine cycle (or Carnot cycle) and is independent of refrigeration. For vapour compression machine, the coefficient is of the order of three, for air cycle refrigeration application one and for vapour absorption systems, it is well below one.
H.P.
TR = 4.71
µe (Imp H.P.)
and H.P.
TR = 4.78
µe (metric H.P.) Refrigerant Number R
In a numerical cooling system which defines the molecular structure of refrigerant as per designation ABCD (applicable to halo and hydrocarbons refrigerants only where A, number of double bonds: B, number of carbon atoms less one, C, the number of hydrogen atoms + one and D, the number of fluorine atoms.
Example : For R-12, A = 0, B = 1 – 1 = 0, C = 0 + 1 and D = 2.
Therefore the refrigerant number becomes R-12 (Dichloro Difluoro methane.)
Inorganic refrigerants are designated differently. They are given digits number, the first is 7 and following two numbers gives its molecular number.
b c
Lay-O ut of a Vapour C om pression-Type R efrigeration S ystem
E vaporator
B rine (C old)
R eturn
O il S eparator
C ooling w ater
C ooling w ater C ondensor
C ooling w ater
c
f
g R eceiver
(a) Float value (b) O il collector (c) S afety oil drain (d) N on-condensable gas purging device
(e) E xpansion value and charging device (f) Liquid level gauge (g) Filter
S ource : B orsig pocket book 3rd edn. 1970 a
e
R efrigeration C om pressor S uction line
D ischarge line Liquid
Fig. 2 d
Example : Ammonia-R 717
US’s ASHRAC standard 34–94 (2) classify refrigerants in 6 groups:
Group A1 = non inflammable and non toxic.
Group B1 = non inflammable but slightly toxic.
Group B2 = same as above but moderately toxic.
Group A3 = highly inflammable but non toxic.
Group B3 = highly inflammable and highly toxic.
Ozone Depletion
Both CFC and HFCs contribute ozone depletion in stratosphere and as a result, ozone hole was detected in Antarctica. Ozone depletion potential (ODP) has been fixed as 1.0 for CFC–1 and all other CFCs and HFCs are grouped accordingly.
The production of such ozone depletion compounds was decided to be phased out in 1987 as per Montreal protocol as per the following schedule:
CFCs phase out by 1.1.96 (Developed countries) HFCs phase out by 1.1.2020 (Developed countries).
However, developing countries were given further extension of 10-15 years from above dates.
In view of the above stipulations developed countries are on the look out for alternate CFCs and HFCs. Some HFC refrigerants like R 134 a, hydro-carbons, propane (R-290) and ethane (R-170) are reportedly found better replacements for CFCs and HFCs.
Table 1
Name Code Range Volume refrigerant Effect
Hydro fluoro- R-134 a –15°C evaporation 1062 (KJ/m3)
Carbons and 30°C condensation
Ethane R-170 –15°C evaporation 1811 (KJ/m3)
and 30°C condensation
Propane R-290 –15°C evaporation 2238 (KJ/m3)
and 30°C condensation
Normally mineral oil is used as a lubricant in compressor, which is miscible with refrigerant and return back to compressor for lubrication. About 2–3% lubricating oil is reqd. Details given in Table-5.
However, R-134a requires a synthetic oil, called polyster oil, which is miscible with refrigerant.
DIN 51503 gives refrigeration oil specification. For applications of various refrigerants see Table 126 in Vol. IIA.
Table 2
Refrigerant TLV. ppm ODP** GWP* 100 yr. Atmospheric Life yr.
CFC-11 1000 1 4000 60
CFC-12 1000 1 8400 130
CFC-13 1000 – 11700 400
HCFC-22 1000 – 1500 15
(Contd.)
(Table Contd.)
Refrigerant TLV. ppm ODP** GWP* 100 yr. Atmospheric Life yr.
HCFC-23 1000 0 – 310
HCF-134a 1000 0 1200 16
HFC-152a 1000 0 140 2
Propane (R-290) – 0 3 4
Ammonia (R-717) 25 0 1 1
*Global warming potential
**Ozone depletion potential Units of Refrigeration
It varies from country to country. Care should be taken as to know the type of refrigeration unit followed in the country.
A few are as follows:
1 US ton of refrigeration, TR = 200 BTU/min or 12000 BTU/hr
= 3024 Kcal/hr 1 U.K. ton of refrigeration, TR = 220 BTU/min
= 3340 Kcal/hr
1 lb/hr TR = 9 kg/hr per 1000 Kcal/hr
1 lb/min TR = 0.15 kg/min per 1000 Kcal/hr
1 BTU/min TR = 5 Kcal/hr per 10000 Kcal/hr
1 TR (Euorpe) = 211 kg/min