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

Wnet,in + QL = QH

We can write the coefficient of performance as:

COPR = QL / Wnet,in = QL/(QH-QL)

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

COPR,Carnot = 1 / (TH/TL -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: qH = (h2-h3)

The COP can be written: COPR = qL/win = (h1-h4)/(h2-h1)

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 h1 = hg and s1 = sg at P1

At point 3 (before throttle) the fluid is a saturated liquid and h3 = hf at P3

Find all h’s to get qL, qH, 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 = (h2s-h1)/(h2a-h1)

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: 11.6-11.9 Homework: Ch 11, P: 18, 31, 42, 55