Modern Engineering Thermodynamics

562 CHAPTER 14: Vapor and Gas Refrigeration Cycles
Since no real system can be more efficient than a Carnot system, Eq. (14.21) represents the maximum possible
thermal efficiency of an absorption refrigeration system operating with a generator temperature T g , an evaporator
temperature T e , and an ambient temperature T a . Because absorption systems are heat rather than work driven,
their practical COP values tend to be around 1.0 or less. Common absorption refrigeration fluid systems are:
ammonia (refrigerant)-water (carrier), water (refrigerant)-lithium bromide (carrier), and water (refrigerant)-
lithium chloride (carrier). The ammonia-water system was widely used in domestic refrigerators until about
1950. The lithium salt–water systems that use water as the refrigerant cannot go below 32°F (0°C) and consequently
are mainly used in air conditioning applications.
Absorption refrigeration dominated the refrigeration market before 1875, which is remarkable considering its
inherent complexity and the fact that its design was empirical. A suitable theory for the operation of absorption
refrigeration did not appear until 1913, and today multistage regenerative absorption refrigerators can produce
temperatures as low as 65 K.
EXAMPLE 14.8
A new absorption refrigeration system with a generator temperature of 100.°C and an evaporator temperature of 5.00°C is
being designed to operate in an environment at a temperature of 20.0°C. To provide an upper limit for the operating efficiency,
determine the Carnot absorption refrigeration coefficient of performance of this system.
Solution
Use Figure 14.20 as the equipment schematic for this example. Equation (14.21) gives the COP for this system as
ðCOPÞ Carnot
= T
e
T g − T a 5:00 + 273:15
100: − 20:0
= = 3:98
T g T a − T e 100: + 273:15 20:0 − 5:00
absorption
refrigerator
Since no system can be more efficient than a Carnot system, this represents the maximum COP of any absorption system
operating under these conditions.
Exercises
22. The environmental temperature of the absorption refrigerator being designed in Example 14.8 is suddenly increased
from 20°C to30°C. Determine the new Carnot absorption refrigeration COP, assuming all the other variables remain
unchanged. Answer: (COP) Carnot absorption ref = 2.09.
23. Suppose now we want to convert the absorption refrigeration system design discussed in Example 14.8 into a cryogenic
unit with an evaporator temperature of only 65 K. What would be the maximum possible coefficient of performance of
this system assuming all the other variables remain unchanged? Answer: (COP) Carnot absorption ref = 0.06.
24. Explain why the COP given by Eq. (14.21) goes to zero as the evaporator temperature approaches absolute zero.
Answer: In this instance, as T e → 0, the evaporator cooling load _Q e also goes to zero. Since the COP is defined as the
ratio of the system cooling to the energy input, the COP must vanish as the cooling vanishes.
14.11 COMMERCIAL AND HOUSEHOLD REFRIGERATORS
Commercial and household refrigeration technology essentially developed together, because commercial refrigeration
in shops and supermarkets requires the same basic technological advances as household refrigerators. Also,
once frozen or chilled food products were purchased by the consumer, similar refrigeration needs were created in
the home. Thus. the parallel development of household and commercial refrigeration was advantageous, if the market
for chilled and frozen foods was to expand beyond the needs of a single day’s foodsupply.
Throughout the 19th century, mechanical vapor-compression refrigeration systems had been limited to largescale
industrial units powered by steam engines or internal combustion engines. Several major technical bottlenecks
prevented small commercial and household vapor-compression refrigerators from being successfully
developed. The first problem was the development of a power source suitable for use in a household. The traditional
commercial power sources (steam and internal combustion engines) were not suitable for household
use. The second problem was the enormous friction in the mechanical seals on the shaft between the power
source and the compressor. Without a complex and tight sealing system, refrigerant leaked out, causing
serious environmental and maintenance problems. The third problem was the development of an automatic