Refrigeration Thermodynamics Professor Lee Carkner Lecture 21

Refrigeration   Generally involve 4 processes   There is an input of heat and work and an output of heat W net,in + Q L = Q H   We can write the coefficient of performance as: COP R = Q L / W net,in = Q L /(Q H -Q L )

Carnot Refrigeration Cycle   Two isothermal and two isentropic processes   But serves as an upper limit on the efficiency of any refrigerator operating between two temperatures COP R,Carnot = 1 / (T H /T L -1)

Ideal Refrigeration Cycle  The ideal refrigeration cycle consists of four processes:   Isobaric heat rejection in a condenser   Isobaric heat absorption in a evaporator  Similar to a reversed Rankine cycle   Less efficient but much cheaper and simpler

Ideal Refrigerator Diagram

Household Refrigerator

COP of Refrigeration Cycle  The heat or work for each process is just  h   The heats are:   q H = (h 2 -h 3 )   The COP can be written:  COP R = q L /w in = (h 1 -h 4 )/(h 2 -h 1 )

Solving Refrigerator Problems  For the four key points in the cycle, there are only two pressures    At point 1 (before compressor) the fluid is a saturated vapor and h 1 = h g and s 1 = s g at P 1   At point 3 (before throttle) the fluid is a saturated liquid and h 3 = h f at P 3   Find all h’s to get q L, q H, w and COP

Actual Refrigerator Cycles   The fluid may leave the evaporator superheated instead of saturated   The fluid may be subcooled (a compressed rather than a saturated liquid) when it exits the condenser   Can also define the isentropic efficiency of the compressor  C = (h 2s -h 1 )/(h 2a -h 1 )

Actual Refrigerator Diagram

Refrigerants  Want a fluid with a low boiling point   Many of these fluids are toxic   First safe refrigerants were developed in the 1920’s   Unfortunately, CFCs destroy ozone which blocks out solar UV  Newer, CFC-free refrigerants (like 134a) have become very common since the 1980’s

Refrigerant Properties  Need 5-10 deg temperature difference between refrigerant and hot or cold chamber for good heat transfer   To avoid air leaks    For the condenser you need to keep a temperature above room temperature 

Next Time  Read:  Homework: Ch 11, P: 18, 31, 42, 55