lecture 6,7,8 introduction to...

Introduction to refrigeration

Md Ferdous AlamLecturer, MEE, SUST

Lecture 6,7,8

Refrigeration is the removal of heat from a material

or space, so that it’s temperature is lower than that of

it’s surroundings.

What is saturation temperature?

“Saturation Temperature” – can be defined as the

temperature of a liquid, vapor, or a solid, where if

any heat is added or removed, a change of state takes

place.

“Latent Heat”- The heat required to change a liquid to a gas (or

the heat that must be removed from a gas to condense it to a

liquid), without any change in temperature.

• Heat is a form of energy that is transferred from

one object to another object.

• Heat Is a form of energy transferred by a

difference in temperature.

• Heat transfer can occur, when there is a

temperature difference between two or more

objects. Heat will only flow from a warm

object to a colder object.

• The heat transfer is greatest, when there is a

large temperature difference between two

objects.

Do you know the laws of thermodynamics?

First law of thermodynamics – Energy can neither be

created nor destroyed. It can only change forms. In any

process, the total energy of the universe remains the

same. For a thermodynamic cycle the net heat supplied

to the system equals the net work done by the system.

Second law of thermodynamics – The entropy of an

isolated system not in equilibrium will tend to increase

over time, approaching a maximum value at equilibrium.

A perpetual motion machine is a hypothetical

machine that can do work indefinitely without an

energy source. This kind of machine is impossible,

as it would violate the first or second law of

thermodynamics.

Never build this machines!!!!!!!!!!!

Fig: Heat Engine

Brainstorm:

Is refrigerator a Heat

pump?

Is refrigerator a heat

engine?

Do you know the

difference between

refrigerator and heat

pump?

What is enthalpy?Enthalpy is defined to be the sum of the

internal energy E plus the product of the

pressure p and volume V. Using the

symbol H for the enthalpy:

H = E + p * V

What is entropyEntropy is a measure of the uncertainty of the

energy state of the system.

The reversed Carnot cycle is the most efficient

refrigeration cycle operating between TL and TH.

evaporator

compressor

condenser

turbine

4 1

23

4 1

23

QL

QH

T

s

Replace turbine by an expansion valve in case of vapor compression cycle

Two important types of refrigeration:

Vapor compression refrigeration

Vapor absorption refrigeration

Schematic and T-s diagram for the ideal vapor-compression refrigeration cycle.

H.P. side

L.P. side

P-h diagram of refrigeration

The Vapor Compression Refrigeration Cycle:

There are four main components in a refrigeration

system:

The Compressor

The Condensing Coil

The Expansion Device

The Evaporator

Two different pressures exist in the refrigeration cycle. The

evaporator or low pressure, in the "low side" and the

condenser, or high pressure, in the "high side".

The concept of an efficiency being greater

than 100% makes people uneasy!!!!!!!!

Therefore, the conversion efficiency for a

refrigerator is called the Coefficient of

Performance (COP).

CCOPinE

cycle

Q

W

energy sought

energy that costs

may be >1 100%E E

R

1COP

/ 1

in inth

cycle out in out in

Q Q

W Q Q Q Q

For the Refrigeration cycle …

Working fluid = Refrigerant = R134a

Melting point of R134a is -103.3 degree Celsius

Refrigeration Effect and Capacity:

Refrigeration Effect:

Refrigeration

Capacity:

Compressor Work

done:

1 4inq h h

WE = m (h2 - h1)

Qin = m (h1 - h4)

COP = (h1 - h4)

(h2 - h1)

Refrigeration capacity is often expressed in tons of

refrigeration. Definition …

1 ton of refrigeration is the steady state heat

transfer rate required to melt 1 ton of ice at 0°C in

24 hours.

1 ton = 3.516 kW

Try the calculation yourself!!!!!!!!!!!!

Practice math problems from

reference books and class

lectures………