Guided By

Prof. S.G.BORKAR

Presented By

SHRVAN HIRDE-62 B.E. VIII Semester

Department of Mechanical EngineeringG. H. Raisoni College of Engineering, Nagpur

2013-14Date: 20/2/2014ATechnical Presentation onREFERIGANT FLOW CONTROL

Under parameter TAE in the subject RAC

Hand Expansion ValvesCapillary tubeThermostatic expansion Valves- Superheat ControlAutomatic Expansion Valves - Evaporator pressure ControlElectronic Expansion ValvesThe practical refrigeration system and its temperature and pressure control*Refrigerant Flow Control

The refrigerant flow control is one of the four major components in a vapor compression refrigeration system.

The function of any refrigerant flow control is two foldto adjust the quantity of refrigerant flow into the evaporator according to the evaporator load; to create a pressure drop from the high side to the low side of the system in order to permit the refrigerant to vaporize under the desired low pressure in the evaporator while at the same time condensing at a high pressure in the condenser.

There are various types of refrigerant flow control devices, such as manual expansion valve, capillary tube, thermostatic expansion valve, float valve and electronic expansion valve and so on.

In this chapter, we will discuss five control devices about their functions, construction and applications

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13-1. Hand Expansion Valves

Hand expansion valves are also called throttle valves. The structure of a hand expansion valve is shown in Fig.13-1. *The expansion valve comprises of main body, valve seat, and hand wheel which is actuated to change the opening area around the valve seat to adjust the frictional resistance to the refrigerant flow. Fig.13-1, Hand expansion valve

*Fig.13-1, Hand expansion valveThe rate of the refrigerant flow through the valve depends on the pressure differential across the valve and opening of the valve.

Assuming that the pressure drop across the valve remains the same, the flow rate through a hand expansion valve will remain constant at all times regardless of the evaporator pressure and the evaporator load.

*The advantage of the hand expansion valve is that it is unresponsive to changes in the system load and the disadvantage is the valve must be manually readjusted each time when the load on the system changes in order to prevent either starving or overfeeding of the evaporator.

In addition, the valve must be opened and closed manfully each time when the compressor is cycled on and off.

13-2.Capillary tube

Capillary tubes are widely used as expansion devices in small vapor compression refrigeration Systems, such as household refrigerators, room air conditioners, and small package air conditioning units. *In these system, the capillary tube is wound into with coils for direct expansion.

The tube connects the outlet of condenser to the inlet of the evaporator as shown in Fig.13-2. Fig.13-2, Capillary tube

*Physically the capillary tubes are hollow tubes made with drawn copper, with internal diameters ranging between 0.51and 2 mm

Primarily there are two kinds of capillary tubes, namely adiabatic and non-adiabatic tubes.

The adiabatic capillary tube expands refrigerant from high pressure to low pressure adiabatically while in the non-adiabatic situation, the capillary tube forms a counter-flow heat exchanger with the suction line that joins the evaporator and the compressor .

The refrigerant flow inside the capillary tube is very complex, particularly in non-adiabatic situations where the capillary tubes are in thermal contact with the suction lines.

*Fig.13-3, Balance points with a reciprocating compressor and capillary tube [4]For a given condensing temperature, a suction pressure has to be decided in such a way that the mass flow rate through the capillary tube is equal to that through the compressor.

As Fig.13-3 showed, under the condensing temperature of 30, the corresponding balance point of the suction pressure was at point 1.

The point 2 and 3 were balance points under the condensing temperature 40and 50,respectively.

13-3. Thermostatic expansion Valves- Superheat Control

At present, thermostatic expansion valve is probably the most widely used refrigerant flow control device because of its high efficiency and its ready adaptability to any type of refrigeration applications.

The thermostatic expansion valve controls the mass flow rate of the refrigerant into the evaporator according to inspiration vapor degree of superheat, and at the same time throttles the liquid from condensing pressure to evaporation pressure.

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*Internally thermostatic expansion valves

Fig.13-5 is an operation diagram of the internal equalizer thermostatic expansion valve, the main parts including: a needle and seat, a pressure bellows or diaphragm, a fluid-charged remote bulb, and a spring, the tension of which is usually adjustable by an adjusting screw. Fig.13-5, The principle of internal equalizer thermostatic expansion valve

A screen or strainer is usually installed at the liquid inlet for the valve to prevent the entrance of foreign material which may cause malfunction of the valve.

The main important part of the thermostatic expansion valve is the remote bulb, which responses the superheat of the refrigeration at the outlet of the evaporator and then move to close or open the valve to throttle the flow of the liquid to the evaporator.

*Fig.13-5, The principle of internal equalizer thermostatic expansion valve

2. Externally thermostatic expansion valves

When there is significant pressure drop between the valve outlet and the evaporator exit, the thermostatic expansion valve with an internal equalizer construction will not function properly.

That is because the saturation temperature of the refrigerant is always lower at the evaporator outlet than at the evaporator inlet because of friction as it flows through the evaporator.

Then it necessitates a higher degree of suction superheat in order to bring the valve into equilibrium. *

13-5. Electronic Expansion Valves

The electronic expansion valve is used for frequency conversion air condition to automatically control refrigerant flow rate , thus to enable air conditioning system to work in the optimized conditions and realize fast freezing , precision temperature controlling and power saving. Step Motor valves

The step motor valve exerts certain logical relation of the digital signal to driven the step motor and then causes to step motor valve to actuate the valve needle through the thread forward and backward the movement, thus the goal of the flow control of the refrigerant has been achieved as shown in Fig.13-9.

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* Fig.13-9, Step motor valve

REFERENCES

Roy J. Dossat, Principles of Refrigeration, 2nd Edition, John Wiley & Sons, New York, USA, 1978Capillary tube of copper and copper alloys, GB/T 1531Appliances Components Companieshttp://www2.the-acc-group.com/java/X?cgi=cubigel.infotecnica.InformacionTecnicaArticulo2.pattern&seccion=informaciontecnica#TablasGraficosStoecker W.F., Jones J.W., Refrigeration and air conditioning, 2nd Edition, McGraw-Hill Book Company, New York, USA.,1982HE F.M.Comparison between Electronic Expansion Valve and Thermostatic Expansion ValveWorld Shipping2004, 27(5)44-45*