Anh Văn CN Nhiệt chapter 4

The reversed Carnot cycle refrigerator and heat pump4.1 The fundamentals of refrigeration cycle Schematic of a Carnot refrigerator and T-s diagram of the reversed Carnot cycle The refri

Trang 1

Anh văn Chuyên ngành Nhiệt

English for thermal engineering

1

Trang 2

The fundamentals of refrigeration

Trang 3

Tài liệu tham khảo

1 Fundamentals of thermal-fluid science, Y A Çengel.

2 Fundamentals of thermodynamics (sixth edition),

Sonntag, Borgnakke and van Wylen.

3 Fundamentals of engineering thermodynamics (Fifth

edition), Michael J Moran, Howard N Shapiro.

3

Trang 4

The reversed Carnot cycle (refrigerator and heat pump)

4.1 The fundamentals of refrigeration cycle

Schematic of a Carnot refrigerator and T-s diagram of

the reversed Carnot cycle

The refrigerant absorbs heat isothermally from a low-temperature

source at T L in the amount of Q L (process 1-2), is compressed isentropically to state 3 (temperature rises to T H), rejects heat isothermally

to a high-temperature sink at T H in

the amount of Q H (process 3-4), and

expands isentropically to state 1

(temperature drops to T L) The

Trang 5

Schematic of a Carnot refrigerator and T-s

diagram of the reversed Carnot cycle

The coefficients of performance of Carnot refrigerators and heat pumps are expressed in terms of temperatures as:

and

4.1 The fundamentals of refrigeration

5

Trang 6

Notes:

-Both COPs increase as the difference between the two temperatures

decreases, that is, as TL rises or TH falls;

-The reversed Carnot cycle is the most efficient refrigeration cycle operating between two specific temperature levels.

-Processes 2-3 and 4-1 cannot be approximated closely in practice since: + Process 2-3 involves the compression of a liquid–vapor mixture a compressor that will handle two phases;

Trang 7

The ideal vapor-compression refrigeration cycle

Schematic and T-s diagram for the ideal

vapor-compression refrigeration cycle

7

Trang 8

This cycle consists of 4 processes as follow:

1-2 Isentropic compression in a compressor

State 1 - Saturated vapor

State 2 - Superheated vapor State 3 - Saturated liquid

State 4 - Low-quality saturated mixture

Trang 9

The P-h diagram of an ideal

vapor-compression refrigeration cycle

All four components associated with the compression refrigeration cycle are steady-flow devices, and thus all four processes that make

vapor-up the cycle can be analyzed as steady-flow processes.

The steady-flow energy equation on a unit-mass basis reduces to:

The COPs of refrigerators and heat pumps operating on the vapor-compression refrigeration cycle can be expressed as

or

9

Trang 10

Example:

A refrigerator uses refrigerant-134a as the working fluid and operates on an

ideal vapor-compression refrigeration cycle between 0.14 and 0.8 MPa If the

mass flow rate of the refrigerant is 0.05 kg/s, determine (a) the rate of heat

removal from the refrigerated space and the power input to the compressor, (b) the rate of heat rejection to the environment, and (c) the COP of the refrigerator

Solution: From the refrigerant-134a tables, the enthalpies of the refrigerant at

all four states are determined as follows:

Trang 11

Example:

A refrigerator uses refrigerant-134a as the working fluid and operates on an

ideal vapor-compression refrigeration cycle between 0.14 and 0.8 MPa If the

mass flow rate of the refrigerant is 0.05 kg/s, determine (a) the rate of heat

removal from the refrigerated space and the power input to the compressor, (b) the rate of heat rejection to the environment, and (c) the COP of the refrigerator

Solution: From the refrigerant-134a tables, the enthalpies of the refrigerant at

all four states are determined as follows:

11

Trang 12

Solution:

a The rate of heat removal from the refrigerated space and the power input to the compressor

and

b The rate of heat rejection from the refrigerant to the environment is

c The coefficient of performance of the refrigerator is

Trang 13

The actual vapor-compression refrigeration cycle

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

13

Trang 14

Trang 15

15

Trang 16

4.2 Air conditioning system

Method of cooling air

1 Spray type washer

2 Surface type cooler:

Indirect: By heat exchange with water which has

been cooled by a refrigerant

Direct: By heat exchange in evaporator of a

refrigerator system

Trang 17

Types of system

1 Cooling only

2 Cooling or heating

3 Cooling and heating with control of humidity

(full air conditioning)

17

Trang 18

A Spray type washer

Air washer are sheet metal, or sometimes bricks or

concrete chambers, in which air is drawn through a mist caused by spray nozzles and then through eliminator to remove particles of water not evaporated into the air

The water for the spray nozzles is recirculated by a pump and can be heated or cooled A tempering heater is

installed before, and a reheating battery after the air

washer

Trang 19

B Surface type coolers

B1 Self-contained wall or window unit

Unit mounted in wall or window, evaporator inside

the room and condenser outside room

Advantages: Low cost, flexible and simple.

Disadvantages: Short life; noise; poor control; poor

filtration and air distribution; lack of fresh air supply…

Applications: Small building; individual rooms

19

Trang 20

B2 Split direct expansion unit

Air cooled condenser is separate and remote from indoor unit Compressor is in the outdoor unit

Advantages: Indoor unit can be ceiling mounted; silencers

can be incorporated for indoor unit; Multiple refrigerant

circuits give improved control; Relative simple

Disadvantages: Restriction on length of refrigerant piping

and the difference in level between indoor and outdoor

units; Limited fresh air supply

Trang 21

B2 Split direct expansion unit

21

Trang 22

B3 Packaged air conditioning unit

Package units house all the components of an air

conditioning system in one unit (the compressor and

condenser coil, evaporator coil, fans)

Types: Air cooled condenser and water cooled condenser Capacities: Fixed rate capacities of 3, 5, 7, 10 and 15 tons

Advantages: easy to control and install; initial costs is

lower than of the central systems…

Disadvantages: less flexible of air flow rates, condenser

Trang 23

Air cooled condenser, install in roof of buildings

23

Trang 24

Trang 25

B4 Variable Refrigerant Volume (VRV) or Variable

Refrigerant Flow (VRF)

The system varies the quantity of refrigerant flowing based on the demand for cooling or heating

* The units are able to provide simultaneous heating or

cooling to different indoor units

* Very efficient system.

Applications: Buildings, supermarkets… 25

Trang 26

Trang 27

27

Trang 28

B Indirect Surface type coolers

B5 Water chiller system

* A large, central compressor provides cold water to a heat

exchanger - Fan Coil Unit (FCU) -inside apartments

* Fan Coil Units (FCU) in apartments contain a fan that draws air into the unit then blows it over a cooling or heating coil The air comes out of the FCU either cooler or hotter than before.

* FCUs will generally have a chilled water coil for cooling and either a hot water coil for heating or an electric heating

Trang 29

Boiler

29

Trang 30

Trang 31

31

Trang 32

Trang 33

A Natural ventilation

4.3 Ventilation system

Relies on natural forces of wind and temperature

differences to generate flow of air

Advantages: Absence of mechanical components, no plant room needed; Reduction in building energy consumption.

Disadvantages: Close control not practicable; incoming air can not be filtered; Difficult to exclude external noise; Path for flow of air must form part of architectural building design; Cost saving of

mechanical plant may be offset by increased cost of special

building components

33

Trang 34

A Natural ventilation

Typical schemes

Trang 36

 Ps = Stack driving pressure (N/m2)

i = Density of internal air (kg/m 3 )

g = Acceleration of gravity = 9.81 m/s 2

h = Difference in height of inlet and outlet openings (m)

Trang 37

B Mechanical / forced ventilation

A building ventilation system that uses powered fans or blowers to provide fresh air to rooms when the natural

forces of air pressure and gravity are not enough to

circulate air through a building

Purposes

Mechanical ventilation is used to:

-Control indoor air quality, excess humidity, odours;

-Contaminants can often be controlled via dilution or

replacement with outside air

37

Trang 38

B Mechanical / forced ventilation

Trang 39

39

Trang 40

http://blogcongdong.com

Định dạng
Số trang	40
Dung lượng	1,07 MB