S3 Flow Control Note

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Refrigerant Flow Control

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Hand Expansion Valve Capillary tube Thermostatic expansion valves – superheat control Automatic Expansion Valves – Evaporator Pressure Control Low Pressure Float Control High Pressure Float Control

Refrigerant Flow Control

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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 twofold[1]： to 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.

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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.

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1. Hand Expansion Valves Hand expansion valves are also called throttle valves. The structure of a hand expansion valve is shown in Fig.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.1, Hand expansion valve

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Fig.1, Hand expansion valve

The 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.

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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.

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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.2.

Fig.2, Capillary tube

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

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 .

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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.

When the pressure of the sub-cooled liquid refrigerant flowing through the non-adiabatic capillary tubes drops below the saturation value (corresponding to its temperature), a part of the refrigerant flashes into vapor.

This results in two-phase flow while the refrigerant pressure continues to drop due to the friction and fluid expansion in the capillary tube.

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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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I. Internally thermostatic expansion valves Fig.3 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.3 The principle of internal equalizer thermostatic expansion valve

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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.3, The principle of internal equalizer thermostatic expansion valve

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In order to ensure against refrigerant liquid entering the compressor, it is common practice to have the refrigerant leave the evaporator slightly superheated.

Superheat is the difference between the temperature at the bulb and the evaporating temperature, the former is measured at the point where the remote bulb is located at the exit of the evaporator coil (seen in Fig.3).

Fig.3, The principle of internal equalizer thermostatic expansion valve

It is essential that the entire length of the remote bulb be in good thermal contact with the suction line and the outside of the remote bulb be adiabatic to ensure the temperature of the refrigerant in the suction line and the remote bulb is equal.

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Superheat is used as a signal to regulate liquid injection through the expansion valve.

Resulting from the pressure exerted by the saturated liquid-vapor mixture in the remote bulb, stem and valve head, the bulb pressure pb acts on the top of the diaphragm to open the valve Fig.13-5.

Besides the bulb pressure, there are two other pressures under the diaphragm to move the valve toward an open or closed position: the spring pressure ps acts on the pin and the evaporator pressure pe acts on the bottom of the diaphragm to close the valve.

When the opening and closing pressures balance each other, the valve pin is in a stable fixed position.

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When the bulb pressure is greater than the sum of the spring pressure and the evaporator pressure, the valve will move to a position more open than it was and so allow more refrigerant to flow in the evaporator.

On the other hand, the valve will move toward a less open position if the closing pressures are greater than the opening pressure.

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II. 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.

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The problem can be solved by using an external equalizer thermostatic expansion valve.

The evaporator pressure pe is replaced by the refrigerant pressure pw of the evaporator outlet to balance the diaphragm.

This is accomplished by completely isolating the valve diaphragm from the evaporator inlet pressure, while at the same time permitting the evaporator outlet pressure to be exerted on the diaphragm through a small diameter tube which is connected to the evaporator outlet or the suction line 150 to 200 mm beyond the remote bulb location on the compressor side.

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4.Automatic Expansion Valves - Evaporator pressure Control

Fig.4 is a schematic diagram of an automatic expansion valve showing the principal part of the valve, which includes a needle and seat, a pressure bellows or diaphragm and a spring, the tension of which is variable by means of an adjusting screw.

Bellows or diaphragm

Needle and seat

Strainer

Inlet from receiver

Spring

Out

Spring pressure

Evaporator pressure

Adjusting screw

Fig.4, Automatic Expansion Valve

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Bellows or diaphragm

Needle and seat

Strainer

Inlet from receiver

Spring

Out

Spring pressure

Evaporator pressure

Adjusting screw

Fig.4, Automatic Expansion Valve

As in the case of the thermostatic expansion valve and all other refrigerant controls, a screen or strainer is usually installed at the liquid inlet of the valve in order to prevent the entrance of foreign materials which may cause stoppage of the valve.

in the evaporator in which it is controlling

the liquid level or it may be installed external to these units in a separate float chamber.

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5. Low-Pressure Float Control The low pressure float control (low

side float) acts to maintain a constant level of liquid in the evaporator by regulating the flow of liquid refrigerant into that unit in accordance with the rate at which the supply of liquid is being depleted by vaporization . It is responsive only to the level of liquid in the evaporator and will maintain the evaporator filled with liquid refrigerant to the desired level under all conditions of loading without regard for the evaporator temperature and pressure. The low pressure float may be installed directly

Low-Pressure Float Control

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6. High-Pressure Float Control

• The high pressure float valve is a liquid level actuated refrigerant flow control that regulates the flow of liquid to the evaporator in accordance with the rate at which the liquid is being vaporized. These device is located on the high pressure side of the system and controls the amount of liquid in the evaporator indirectly by maintaining a constant liquid level in the high side pressure float control

High-Pressure Float Control

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High-Pressure Float Control

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REFERENCES1. Roy J. Dossat, Principles of Refrigeration, 2nd Edition, John Wiley & Sons, New York, USA,

19782. Capillary tube of copper and copper alloys, GB/T 15313. Appliances Components Companies，4. http://www2.the-acc-group.com/java/X?

cgi=cubigel.infotecnica.InformacionTecnicaArticulo2.pattern&seccion=informaciontecnica#TablasGraficos

5. Stoecker W.F., Jones J.W., Refrigeration and air conditioning, 2nd Edition, McGraw-Hill Book Company, New York, USA.,1982

6. HE F.M.， Comparison between Electronic Expansion Valve and Thermostatic Expansion Valve，World Shipping， 2004, 27(5)44-45

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7. He X.D., Liu S., Asada H.H., Modeling of vapor compression cycles for multivariable feedback control of HVAC systems, ASME J. Dynamic systems, measurement, and control ,1997,119 (2) 183-191

8. Aprea C., Mastrullo R., Experimental evaluation of electronic and thermostatic expansion valves performances using R22 and R407C, Appl. Therm. Eng. 2002, 22 (2) 205-218

9. Li X.Q., Chen J.P., Chen Z.J., et al., A new method for controlling refrigerant flow in automobile air conditioning, Appl. Therm. Eng. 2004,24 (7) 1073-1085

10. Chen W., Zhou X.X., Deng S.M., Development of control method and dynamic model for multi-evaporator air conditioners, Energy Conversion and Management, 2005, 46 (3) 451-465

11. Choi J.M., Kim Y.C., The effects of improper refrigerant charge on the performance of a heat pump with an electronic expansion valve and capillary tube, Energy ,2002,27 (4) 391-401

12. Wang Shan K.， Handbook of Air Conditioning and Refrigeration, 2th, Donnelley & Sons Company, New York , McGraw-Hill, 2000