THERMO-ELECTRIC & VORTEX
TUBE REFRIGERATION CLASS NOTES
Thermo-electric refrigeration uses
semi-conductors. 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
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
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 –Te3 (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 –Te3 (Bismuth-Tellurium) Z = 1.8 x 10-3 K-1
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
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.
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
Cooling Capacity 0.5 TR.
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
(vi) Light weight
(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.
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