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THERMO-ELECTRIC & VORTEX TUBE REFRIGERATION CLASS NOTES

THERMO-ELECTRIC & VORTEX

TUBE REFRIGERATION CLASS NOTES

Thermo-electric refrigeration uses

semi-conductors. A semiconductor is

combination of two materials having

high electrical conductivity and low

thermal conductivity. The property

defining the quality of thermo-couple is

 ‘Figure of Merit’.

Figure of Merit = Z= α2σ/k

Where α is the Seebeck coefficient of the

two materials of the semiconductor

σ is the electrical conductivity

k is the thermal conductivity

For more cooling effect, Z should be high. For Z to be high

(i)  σ should be high

(ii) k should be low

It is sandwiching a semiconductor between two copper conductors. The semiconductor is a pellet.  Used because of the following reasons:

(i)  Optimized for more heat transfer

(ii) Choice to choose the type of charge carriers to achieve more heat transfer

SEMICONDUCTOR

A semiconductor is a combination of two materials having high electrical conductivity and low thermal conductivity. The property defining the quality of thermo-couple is ‘Figure of Merit’.

Figure of Merit = Z= α2σ/k

TYPES OF SEMICONDUCTORS

N type semi-conductor (doped with electrons)

In this, doping of a material of group IV is with a material of Group V. Therefore an extra electron on the Group V metal is free to move and acts as the charge carrier. In this, the electrons move towards the positive end of the battery. Direction of flow of electrons is the direction of heat transfer.

From the ‘side current enters, N TYPE SEMICONDUCTOR becomes hot

From the side current leaves,  N TYPE SEMICONDUCTOR becomes cold

Pb-Te (lead-Tellurium)              Z=1.5 x 10-3  K-1

Bi2 –Te (Bismuth-Tellurium) Z= 2.2 x 10-3 K-1

P type semi-conductor (doped with holes)

In this, Group III material needs an extra electron for creating “holes”. P doped semiconductors are positive charge carriers. It appears that a hole is moving. But an electron moves to fill a hole. It creates a new hole left behind where the electron was present originally. Holes and electrons move in opposite directions.

Thus holes move in a direction opposite the direction of current flow. The heat transfer is in a direction opposite the direction of current flow.

The side current enters, P TYPE SEMICONDUCTOR becomes cold

The side current leaves, P TYPE SEMICONDUCTOR becomes hot

Pb-Te (lead-Tellurium) with    Z=1.2 x 10-3  K-1

Bi2 –Te (Bismuth-Tellurium) Z = 1.8 x 10-3 K-1

Basic Principle

Most efficient configuration has p and n semi-conductors electrically in series but thermally in parallel. The device of one n type and one p type is a couple. Attach Cold side to space to cool. Connect hot side to the heat sink. It loses heat to the atmosphere by convection.

SEEBACK EFFECT IN SEMICONDUCTORS

N-Type Conductor

When it is sandwiched between two plates, the side current enters becomes hot. The side current leaves becomes cold as Fig.(a). In P-type semi-conductor, it is opposite. The side current enters a P-semiconductor becomes cold . The side the current leaves becomes hot as in Fig. (b). A combination of P and N semiconductors is shown in Fig. (c) . It becomes thermoelectric refrigeration. To have more cooling effect, connect number of P and N combinations in series.

TEMPERATURE DIFFERENCE IN THERMO-ELECTRIC REFRIGERATION

Normally 500C and Maximum can be 850C.

Higher value of  Z means more heat transfer and hence more cooling.

VORTEX TUBE REFRIGERATION

It is a non-conventional method of

refrigeration. It produces small

amount of cooling. Thus it has limited

use. Its cheap and convenient.                                   

VORTEX TUBE

Fig. VORTEX TUBE REFRIGERATION

The vortex tube, also known as the Banque – Hirsch vortex tube. It separates  compressed air into hot and cold streams. It has no moving parts. This device can produce  cooling only up to 0.5 TR.  The nozzles are fitted tangentially into a swirl chamber. Compressed air at about 7 atmospheric pressure is injected through nozzles. Two streams are formed. The outer layer is hot. The inner layer becomes cold. It is due to transfer of momentum from the inner flow to the outer flow. There it acquires a RPM of 100000. There is a conical obstruction at the right end of the tube. Portion of the compressed air escapes the right end. The remaining  gas returns back in an inner vortex of reduced diameter. The ‘cold’ air leaves from the end where nozzles are fitted. Vortex tube refrigeration achieves minimum temperature of -500C.

SAMPLE SPECIFICATIONS FOR A VORTEX TUBE

Tube Diameter                                              25 mm

Length of tube                                              500 mm

Material                                                         Stainless steel

Pressure of air                                                  7 bar

Number of nozzles                                         8

Vortex RPM generated                                   100000 RPM

Lowest temperature achieved                      -70C

COP                                                                 0.1

Cooling Capacity                                           0.5 TR.

APPLICATIONS

1. Cooling during machining

2. Spot cooling of electronic components and electronic equipment’s

3. Body cooling of the workers in mines.

4. Hot air is used in drying applications.

ADVANTAGES OF VORTEX TUBE

(i) Maintenance free (no moving parts)

(ii) Simple in design

(iii) No controls

(iv) Reliable

(v) Compact

(vi) Light weight

(vii)  Cheap

(viii) Durable because of stainless steel tube

(ix)  Adjustable temperature

(x)  Instant cooling

(xi) Uses air as a refrigerant

(xi) Initial and maintenance expenses are less. It becomes still cheaper where compressed air is already readily available

(xiii) Do not require skilled labor.

DISADVANTAGES OF VORTEX TUBE

(i) Low COP

(ii)  Limited capacity

(iii)  Only small fraction of the compressed air is available as cold air.

(iv) It has a limited use. This requires compressed air. It needs an air compressor.

 

https://www.mesubjects.net/wp-admin/post.php?post=4194&action=edit       MCQ non-con refri

 

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