Environmental Thermal Engineering Lecture # 5 Min soo Kim Mechanical & Aerospace Engineering

Refrigeration cycle EXPANSION DEVICE Expansion Device Condenser WHERE? – IN REFRIGERATION SYSTEM Condenser Expansion Device Compressor Evaporator

Expansion Process p T Condensation… Expansion Condensation… Compression Compression Evaporation… Evaporation… h s

Expansion devices Purpose & Types Purpose – reducing the pressure of the liquid refrigerant – regulating the flow of refrigerant to the evaporator Types – manual expansion valve – capillary tube – automatic expansion valve • constant pressure expansion valve • float type expansion valve • thermostatic expansion valve (superheat controlled expansion valve) • electric expansion valve

Throttling

Throttling A throttling process occurs when a fluid flowing in a line suddenly encounters a restriction in the flow passage. Abrupt pressure drop pi > po Constant enthalpy process hi = ho Control volume pi hi po ho

Energy conservation in open system Throttling Energy conservation in open system Control volume No heat transfer No shaft work No potential energy change Velocity variation is negligible pi Vi Ti i po Vo To o

Joule-Thomson Effect Joule-Thomson coefficient hi = ho pi > po refrigeration adiabatic expansion liquefaction By throttling?

Joule-Thomson Effect Inversion curve Inversion temperature – The temperature at which Joule-Thomson cooling effect – In the left area of the inversion curve Temperature, oC Pressure, bar Inversion curve Isenthalpic curve

Manual Expansion Valve

Manual Expansion Valve Fig. Manual type expansion valve (cone type)

Manual Expansion Valve Used in ammonia refrigerator Mass flow rate is adjusted manually by controlling handle Scale plate on the handle Filter is installed in entrance to prevent scale or sand to be mixed and circulated Fig. Manual type expansion valve (cone type)

Capillary Tube

Capillary Tube Small refrigeration system – ~10kW Pressure drop due to friction and acceleration of the refrigerant. Liquid flashing into vapor The tube cannot adjust to variations in discharge pressure, suction pressure, or load Strainer Capillary tube Fig. Capillary tube and strainer

Balance Point Balance point – expansion valve feeds as much as the compressor pumps from the evaporator. The heat transfer coefficient of the evaporator must also satisfied. Compressor Capillary tube Condensing 50oC 40oC 30oC 1 2 3 Mass flow rate, kg/s Suction pressure, kPa Fig. Balance points with a reciprocating compressor and capillary tube.

Unbalanced Condition Starving_1 Heavy heat load condition  i) suction pressure rise. ii) capillary tube does not feed sufficient refrigerant. Starving the evaporator Mass flow rate, kg/s Compressor Capillary tube Balanced Starving A B Suction pressure, kPa Fig. Unbalanced conditions causing starving or flooding of the evaporator. The condensing pressure is constant.

Unbalanced Condition Starving_2 At point B (starving condition) i) the compressor draw more refrigerant out of the evaporator that the capillary tube supply. ii) the evaporator becomes short of refrigerant. Mass flow rate, kg/s Compressor Capillary tube Balanced Starving A B Suction pressure, kPa Fig. Unbalanced conditions causing starving or flooding of the evaporator. The condensing pressure is constant.

Unbalanced Condition Starving_3 Liquid back up into the condenser  reducing condensing area  raising condenser pressure  reducing compressor capacity  increasing the feed rate of capillary tube Mass flow rate, kg/s Suction pressure, kPa A Capillary tube Starving Balanced Compressor B Condensing pressure Fig. Unbalanced conditions causing starving or flooding of the evaporator. The condensing pressure is constant.

Unbalanced Condition Starving_4 Decrease of the heat transfer coefficient of the evaporator  suction pressure drops back to the pressure A. Restoring balanced flow Mass flow rate, kg/s Suction pressure, kPa A Capillary tube Starving Balanced Compressor B Condensing pressure New Balanced point Fig. Unbalanced conditions causing starving or flooding of the evaporator. The condensing pressure is constant.

Unbalanced Condition Flooding_1 Low heat load condition  suction temperature and pressure drops to point C. Flooding the evaporator Mass flow rate, kg/s Suction pressure, kPa A Capillary tube Balanced Flooding Compressor C Fig. Unbalanced conditions causing starving or flooding of the evaporator. The condensing pressure is constant.

Unbalanced Condition Flooding_2 At point C (flooding condition) i) the capillary tube feed more refrigerant to the evaporator than the compressor draw out. ii) the evaporator fills with liquid and would spill over into the compressor. Mass flow rate, kg/s Suction pressure, kPa A Capillary tube Balanced Compressor Flooding C Fig. Unbalanced conditions causing starving or flooding of the evaporator. The condensing pressure is constant.

Unbalanced Condition Flooding_3 I. Gas enters the capillary tube  high specific volume of the vapor  reducing the feed rate of capillary tube Mass flow rate, kg/s Suction pressure, kPa A Capillary tube Balanced Flooding Vapor present Compressor C New balanced point D Fig. Unbalanced conditions causing starving or flooding of the evaporator. The condensing pressure is constant.

Unbalanced Condition Flooding_4 – Point D i) Balanced condition, not a satisfactory condition ii) reduced refrigerating effect compared with that when saturated or sub-cooled liquid enters the capillary tube Pressure, kPa Enthalpy, kJ/kg D Refrigerating effect Fig. Reduction in refrigerating effect when some vapor enters the capillary tube.

Liquid line suction line HX Capillary Tube Liquid line suction line HX Pressure, kPa Enthalpy, kJ/kg Refrigerating effect 1 2 3 4 5 Condenser Evaporator Compressor Capillary tube 2 3 4 5 1 Fig. Schematic diagram of the refrigeration system with LLSL heat exchanger Fig. P-h diagram of the refrigeration system with LLSL heat exchanger

Advantages & disadvantages Capillary Tube Advantages & disadvantages Advantages Disadvantages i) simple ii) no moving parts iii) inexpensive iv) equalizes the pressures in the system during the off cycle  low starting torque i) not adjustable to changing load condition ii) susceptible to clogging by foreign matter the mass of refrigerant charge should be held in close limits

Capillary Tube “SELECTION”

Selection of a Capillary Tube Bore and the length of the capillary tube  The compressor and the tube fix a balance point at the desired evaporating temperature. Analytical method Graphical method Flow D 1 2 Fig. Incremental length of capillary tube

Analytical Method (1) . Fig. Incremental length of capillary tube Notations A = cross-sectional area of inside of tube, m2 D = inner diameter of tube, m f = friction factor L = length of increment, m p = pressure, Pa T = temperature, oC Re = Reynolds number V = velocity, m/s m = mass flow rate, kg/s G = mass flux, kg/m2s x = quality h = enthalpy, J/kg  = specific volume, m3/kg  = viscosity, Pa·s w = wall shear stress, Pa Subscript f = saturated liquid g = saturated vapor Flow D 1 2 Fig. Incremental length of capillary tube .

Analytical Method (2) Refrigerant property calculation The mass flow rate and all the conditions at point 1 are known. Select T2 Compute p2, hf2, hg2, f2, g2, f2, and g2 Flow D 1 2 Fig. Incremental length of capillary tube

Analytical Method (3) Equations for properties of saturated refrigerant 22

Analytical Method (4) Conservation of mass Conservation of energy Flow D 1 2 Fig. Incremental length of capillary tube Conservation of energy Heat transfer is negligible No work, no potential energy change

Analytical Method (5) Combination of mass & energy conservation Flow D 1 2 Fig. Incremental length of capillary tube Equations of h2 & 2

Analytical Method (6) Friction factor f – lower range of turbulent region Flow D 1 2 Fig. Incremental length of capillary tube Shear stress  w

Analytical Method (7) Conservation of momentum  the difference in forces applied to the element Flow D 1 2 Fig. Incremental length of capillary tube

Constant Pressure Expansion Valve

Schematic view Fig. Schematic view of a constant pressure expansion valve

C.P. Expansion valve Maintains a constant pressure at its outlet Sensing the evaporator pressure below the control point opens wider above the control point partially closes Less than about 30 kW Critical charge – feasible to prevent liquid from flooding Pressure limiting characteristic constant evaporating temp. control the humidity prevent freezing in water coolers protection against overload of the compressor

Balance Point Balance point – point A Compressor Mass flow rate, kg/s Suction pressure, kPa Control pressure Flooding Balance Starving Wider opening Original Narrower opening B A C Balance point – point A – expansion valve feeds as much as the compressor pumps from the evaporator. Fig. Balanced and unbalanced conditions using a constant-pressure expansion valve. The condensing pressure is constant.

Starving Condition Heavy heat load condition Compressor Mass flow rate, kg/s Suction pressure, kPa Control pressure Flooding Balance Starving Wider opening Original Narrower opening B A C Heavy heat load condition temp. & pressure attempt to rises. the valve partially closes preventing the rise in pressure decrease in the feed rate of the valve Point C : Starving of the evaporator Fig. Balanced and unbalanced conditions using a constant-pressure expansion valve. The condensing pressure is constant.

Flooding Condition Low heat load condition Compressor Mass flow rate, kg/s Suction pressure, kPa Control pressure Flooding Balance Starving Wider opening Original Narrower opening B A C Low heat load condition temp. & pressure attempt to drops. off the valve opens wider preventing the drop in pressure increase in the feed rate of the valve Point B : Flooding of the evaporator Fig. Balanced and unbalanced conditions using a constant-pressure expansion valve. The condensing pressure is constant.

Float Type Expansion Valve

F. T. Expansion Valve Maintains the liquid at a constant level in a vessel or an evaporator. Always establishes balanced conditions of flow between the compressor and itself. Float valves and float-switch-solenoid combinations are used primarily in large installation. Regulate the flow to flooded evaporators in response to the level of evaporator. Condenser outlet Evaporator inlet Float Fig. Float type expansion valve

Starving Condition Heavy heat load condition evaporating temperature & pressure rise. the compressor pumps a greater rate of flow than the valve feeding. the valve opens wider A new balance point B Compressor Mass flow rate, kg/s Suction pressure, kPa Wider opening Narrower opening Original A B C Fig. Balance points with various load conditions using a float valve. The condensing pressure is constant.

Flooding Condition Low heat load condition evaporating temperature & pressure drops. the compressor pumps a less rate of flow than the valve feeding. the valve partially closes A new balance point C Compressor Mass flow rate, kg/s Suction pressure, kPa Wider opening Narrower opening Original A B C Fig. Balance points with various load conditions using a float valve. The condensing pressure is constant.

Thermostatic Expansion Valve

Schematic view Fig. Schematic view of a thermostatic expansion valve

Thermostatic Expansion Valve Moderate-sized refrigeration system Regulating the rate of flow of liquid refrigerant in proportion to the rate of evaporation The balance of the flow rate is identical to those of float valve. Compressor Mass flow rate, kg/s Suction pressure, kPa Wider opening Narrower opening Original A B C Fig. Balance points with various load conditions using a float valve. The condensing pressure is constant.

Thermostatic Expansion Valve PS PE PC PB Evaporator Sensing bulb Thermostatic expansion valve Diaphragm Bulb pressure, PB Spring pressure, PS Evaporator pressure, PE

Thermostatic Expansion Valve Power fluid – the fluid in the bulb A certain amount of superheat is required (4K) 7K of superheat is needed to open valve completely. The operation of the valve maintains an approximately constant quantity of liquid. Valve opening, % Superheat, K 2 4 6 8 100 Fig. Throttling range of thermostatic valve.

Cross charge Pressure, kPa Temperature, oC  Superheat Evaporator Power fluid Pressure, kPa Temperature, oC -30oC 5oC 100 kPa Saturation curve Fig. Pressure-temperature characteristic of a refrigerant R22 power fluid. Fig. Evaporator and power fluid temperature in a thermostatic expansion valve with a cross charge.

Electric Expansion Valve

Electric Expansion Valve Evaporator Electric expansion valve Applied voltage Liquid sensing thermistor Fig. An electric expansion valve.

Electric Expansion Valve Sensing the presence of liquid  temperature and resistance of the thermistor  current flow through the heater  valve opening Evaporator Electric expansion valve Applied voltage Liquid sensing thermistor Fig. An electric expansion valve.