Althouse Turnquist Bracciano PowerPoint Presentation by: Associated Technical Authors Publisher The Goodheart-Willcox Company, Inc. Tinley Park, Illinois
Chapter 5
Explain the purpose and operation of refrigerant control devices. Name the six main types of controls and explain their operation. Define terms related to refrigerant control operations. Compare the various charging elements used on refrigerant controls. Explain the fast evaporation of liquid into a vapor.
Determine the proper size capillary tube to be used for specific applications. Explain the operation of special refrigerant controls. Define the purpose and function of three types of solenoid valves. Follow approved safety procedures.
5.1 Refrigerant Control Used in a refrigeration system to change the pressure of the refrigerant. Is placed between the liquid line and the evaporator. Reduces the high pressure in the liquid line to low pressure in the evaporator.
Six Types of Refrigerant Controls Automatic Expansion Valve (AEV). Thermostatic Expansion Valve (TEV). Low-Pressure Side Float (LSF). High-Pressure Side Float (HSF). Capillary Tube (Cap Tube). Thermal-Electric Expansion Valve.
5.1.1 Automatic Expansion Valve Operated by low-side pressure as the closing force and spring pressure as the opening force. Throttles liquid refrigerant in the liquid line down to a constant pressure. While compressor is running, liquid refrigerant is sprayed into the evaporator. Sometimes called a dry system, since evaporator is never filled with liquid refrigerant, but with a mist. Valve capacity should equal the evaporator capacity.
5.1.1 Automatic Expansion Valve
5.3 Bellows Automatic Expansion Valve Valve seat is softer than needle. Spring is attached at both ends. May be adjusted for either pressure or tension. Attached to the evaporator by threaded fittings or a two-bolt flange. Used mainly on domestic air conditioning units or vending machines.
5.1.1 Bellows Automatic Expansion Valve
5.1 Diaphragm Automatic Expansion Valve Has stops to prevent excessive movement of the diaphragm. Diaphragm separates the atmospheric pressure and the system pressure. Three forces control valve: Adjustable spring. Spring beneath diaphragm. Outlet pressure underneath the diaphragm.
5.1 Diaphragm Automatic Expansion Valve
5.1.2 Thermostatic Expansion Valve (TEV) Sensing bulb includes liquid-charged, gas-charged, liquid cross-charged, and gas cross-charged. Note: Liquid-charged and gas-charged use the same refrigerant in the system and bulb. Cross-charged uses a fluid in the sensing bulb different from the system refrigerant.
5.1.2 TEV Flow is through the valve into the evaporator. Low-side pressure, the temperature of the evaporator outlet, and spring pressure control flow. Provides a high flow rate as the evaporator empties (warms). Reduces flow as the evaporator fills (cools) with refrigerant.
5.1.2 Operation of TEV With unit running, temperature in sensing bulb is 10 F (5.6 C) warmer than the refrigerant temperature in the evaporator. This temperature difference produces different pressures and different forces. This is described as the superheat of the bulb over the refrigerant temperature inside the evaporator. The pressure in the sensing bulb is greater than the pressure in the evaporator. As temperature increases or decreases, the pressure will also increase or decrease.
5.1.2 Operation of TEV
5.1.2 Operation of TEV continued When compressor stops, low-side pressure and the sensing bulb pressure tend to equalize. The needle is forced into its seat. Refrigerant flow stops.
5.1.2 TEV Design The liquid and evaporator lines are connected to the brass valve body. Needle and seat are inside the body. Needle is joined to a flexible metal bellows or diaphragm. Bellows is moved by a rod connected at the other end to a sealed bellows or diaphragm and sensing element.
5.1.2 TEV Design
5.1.2 TEV Design continued The diaphragm is joined to the sensing bulb by means of a capillary tube. Fluid that charges the sensing element is labeled by color, letter, or number by manufacturer. A strainer located between the liquid line connection and orifice keep dirt from the needle and seat.
Questions Where is a refrigerant control located in the refrigeration system? Between the liquid line and the evaporator. What is the purpose of a refrigerant control in a refrigeration system? It changes the high-pressure, high-temperature liquid to a low-pressure, low-temperature liquid. Which pressures operate the automatic expansion valve? The spring pressure is the opening force and the low-side pressure is the closing force.
Questions continued Which type of motor control can be used with the automatic expansion valve system? A temperature control. Which types of thermostatic expansion valves use the same refrigerant in the system as in its sensing bulb? The liquid-charged bulb and gas-charged bulb. What are three operating pressures of a TEV? Low-side pressure, sensing bulb pressure, and spring pressure.
Questions continued What happens to the quantity of refrigerant in the evaporator of a TEV system as the temperature of the sensing bulb increases? The quantity of refrigerant increases. Do the pressures of a TEV system equalize during the off cycle? No. What is commonly located before the inlet of a TEV? A strainer.
5.1.2 Flash Gas Refrigerant that evaporates instantly (flashes) and turns into a vapor while passing through the refrigerant control orifice. The instant vaporizing of some of the liquid refrigerant cools the remaining liquid to the evaporating temperature. Amount of flash gas depends upon the temperature of the refrigerant in the liquid line and the pressure inside the evaporator. Flash gas reduces the valve capacity. The amount of flash gas can be reduced by clamping the liquid line to the suction line.
5.1.2 Superheat The difference in temperature between the vapor in the low side and in the sensing bulb. Increasing superheat starves the evaporator; only part of the evaporator is filled with liquid refrigerant.
5.1.2 Superheat
5.1.2 Liquid-charged Sensing Element Sensing element is charged with the same refrigerant as the system. Maintains a constant superheat, or compressor superheat setting, even though low-side pressures and temperatures change. Sensing element controls thermostatic valve operation.
5.1.2 Liquid-charged Sensing Element
5.1.2 Liquid Cross-charged Sensing Element Uses a liquid different from the refrigerant in the system. Some liquid is always present in the element, regardless of temperature. Valve closes quickly when the compressor stops. Load on compressor is reduced at startup. As suction pressure is reduced, superheat is reduced. Valve is more responsive to changes in suction pressure than to changes in sensing bulb temperature.
5.1.2 Liquid Cross-charged Sensing Element
5.1.2 Gas Cross-charged Sensing Element Uses the same refrigerant as the system. Charge provides that all liquid is vaporized at a predetermined temperature. Designed for temperature range from 30 F to 60 F ( 1.1 C to 15.6 C).
5.1.2 Gas Cross-charged Sensing Element
5.1.2 Gas Cross-charged Sensing Element (Adsorption) Sensing bulb is charged with a liquid different from the system refrigerant. At the desired temperature, all the liquid has vaporized. Some elements contain two substances: one is a noncondensing gas, such as carbon dioxide, that provides the pressure in the element. The other is a solid, such as carbon, silica gel, or charcoal. These substances can adsorb gas.
5.1.2 Gas Cross-charged Sensing Element (Adsorption) continued Adsorption, the ability of a substance to adsorb gas; depends upon temperature. Gas is more readily adsorbed at low temperatures. Sensing elements rely on adsorption to control the refrigerant needle valve opening in the thermostatic expansion valve. Pressure-temperature lag in operation allows for use on any refrigeration or air conditioning system.
5.1.2 Thermal-Electric (Solid State) Expansion Valve Depends on the use of thermistors, which are directly exposed to the refrigerant in the suction line, to control the expansion valve needle opening. Refrigerant flow is controlled by the temperature in the suction line. Bleed valves have a small slot in the valve seat, allowing pressure to balance during the off cycle. Allows for use of low-starting-torque compressor motors.
5.1.2 Thermal-Electric (Solid State) Expansion Valve
5.1.2 Expansion Valve with Pressure Limiters Prevents overloading the condensing unit. Designed for systems in which the evaporator pressure must not exceed a safe operating limit. Consists of a diaphragm and a spring placed between the sensing element and the needle valve. Diaphragm will collapse if low-side pressure exceeds a certain set value. May be mechanical (conventional) or pressure-limiting (MOP: maximum operating pressure).
5.1.2 Expansion Valve with Pressure Limiters
5.1.2 Sensing Bulb Mounting Location is important mount on top of the suction line. On a vertical suction line, capillary tube of the bulb should always enter from the top of the bulb. Clean suction line and bulb with steel wool prior to assembling. Must be wrapped in insulation to make sure only suction line temperature is read.
5.1.2 Sensing Bulb Mounting
Thermostatic Expansion Valve 5.1.2 Capacities vary according to: Orifice size. Undersizing can lead to starving of the evaporator; oversizing can cause hunting/surging of the valve. Pressure difference between the high side and the low side (increasing pressure difference will increase the rate of refrigerant flow). The temperature and condition of the refrigerant in the liquid line.
Questions What results from the instantaneous evaporation of a liquid? Flash gas. When flash gas is present in a liquid line, what happens to the capacity of an expansion valve? It decreases. Which component can be used to reduce flash gas? A heat exchanger.
Questions continued When measuring superheat, what two temperatures must be known? Evaporator saturation temperature and sensing bulb temperature. Which will cause a starved evaporator: an increase or a decrease in superheat? An increase in superheat. Which two TEV sensing bulbs do not use the same refrigerant that is in the system? A liquid cross-charged bulb and a gas crosscharged bulb.
5.1.3 Solenoid Valve Principles Consists of a movable armature made of an iron alloy and attached to the valve needle, all sealed into a valve body. Coil is wound around the valve housing that contains the armature. May be activated by a thermostat. Used to control the temperature of a refrigerator or room.
5.1.3 Solenoid Valve Principles
5.1.3 Types of Solenoid Valves Two-way valve Two-way valve controls flow of refrigerant through a single line.
5.1.3 Types of Solenoid Valves continued Three-way valve with an inlet that is common to two opposite openings. Controls refrigerant flow in two different lines. (Used mainly on commercial refrigerating units).
5.1.3 Types of Solenoid Valves continued Four-way reversing valve used on heat pumps.
5.1.3 Types of Solenoid Valves continued Pilot-operated solenoid Pilot-operated solenoid valve used for large commercial applications.
5.1.3 Equalizer Compensates for any pressure drop through the evaporator while the compressor is running. Used if the pressure drop between the inlet of the evaporator and the outlet is more than 4 psi (28kPa). Provides the same pressure as that in the suction line at the sensing bulb location. Equalization of pressure permits accurate superheating adjustments.
5.1.3 Equalizer
5.1.3 Hunting Sometimes referred to as surging, hunting occurs when a valve first opens too wide, then closes down too much. When a valve is hunting excessively, refrigerant flow provided to the evaporator is not uniform. The less hunting, the more effective the system will be. Liquid may reach the compressor and cause damage. Hunting can be caused by a valve that is too large for the system.
5.1.4 Low-Side Float A simple and efficient refrigerant control method. Used with large industrial systems and in some water cooling systems. Requires a means of returning excess oil to compressor. Extending the suction tube to the bottom of the float pan ensures oil return. May use a pressure-operated motor control or a thermostatic motor control.
5.1.4 Low-Side Float
5.1.5 High-Side Float Located in the high-pressure side of the system. Evaporator must be equipped with a special oil return. May use either a thermostatic or a pressure-operated motor control.
5.1.5 High-Side Float
5.1.6 Capillary Tube Acts as a constant throttle on the system. Equipped with a filter or a filter-drier to remove moisture or dirt from refrigerant. Amount of refrigerant must be carefully calculated. Must be used with a thermostatic motor control. Fittings must be leak-proof.
5.1.6 Capillary Tube
Comparing Refrigerant Controls 5.2 Note: Pressure-time cycle varies with refrigerant used.
5.3 Check Valves Limit the directional flow of refrigerant. Rotary and gear compressors have check valves in the suction line, preventing high-pressure vapor and refrigerant oil from backing up into the evaporator. May use either a disk or solid ball in construction. In multiple systems that operate at different temperatures, check valves keep refrigerant vapors in warmer evaporators from backing up into the colder evaporators.
5.3 Check Valves
5.4 Suction Pressure Valves Evaporator Pressure Regulator (EPR) Required on multiple systems in which evaporators operate at different temperatures. Required on most automobile air conditioning systems due to compressor operating at various speeds. Crankcase Pressure Regulator (CPR) Used to keep compressor suction pressure at a safe level. Prevents overloading the compressor.
Questions Which type of solenoid valve controls refrigerant flow through a single line? Two-way valve. Where is a four-way reversing valve commonly used? In a heat pump system. Where is a low-side float metering device commonly used? In large industrial systems. Which type of motor control can be used on a low-side float system? Thermostatic or pressure-operated control.
Questions continued Which type of motor control can be used on a capillary tube system? A temperature control. What is the purpose of a check valve? To allow refrigerant to flow in one direction. Where is a check valve located on a multiple-evaporator system? At the outlet of the coldest evaporator.
Questions continued What is the purpose of an EPR valve (evaporator pressure regulator)? It is used to maintain constant evaporator pressure. Where is an EPR valve located on a multiple evaporator system? At the outlet of the warmer evaporators. What is the purpose of a CPR (crankcase pressure regulator)? It prevents the compressor from overloading.
5.5 Safety Attach a 24-hour pressure-time recorder to any new installation. It will ensure the system is operating within safe pressure limits. Keep floors clear of debris. Wear safety goggles when working on refrigeration systems. When lifting, use leg muscles; do not use your back.
5.5 Safety continued Always have good ventilation and good lighting when working on systems. All electrical circuits must be well-insulated to avoid shock. All metal parts of refrigerating mechanisms should be grounded. When removing a valve from a system, use two wrenches.
Glossary adsorption The adhesion of a thin layer of molecules of a gas or liquid to a solid object. There is no chemical combination between the gas and the solid substance (adsorber). automatic expansion valve Pressure-controlled valve that reduces high-pressure liquid refrigerant to low-pressure liquid refrigerant. check valve Device that permits fluid flow in only one direction. equalizer A device used to balance pressure in a system or balance liquid levels between two containers.
Glossary flash gas Instantaneous evaporation of some liquid refrigerant in evaporator, cooling the remaining liquid refrigerant to the desired evaporation temperature. hunting The process of cycling above and below the set point. pressure limiter Device that remains closed until a certain pressure is reached, then opens and releases fluid to another part of system or breaks an electric circuit. refrigerant control Device that meters flow of refrigerant between two areas of a refrigerating system. Maintains the pressure difference between highpressure and low-pressure side of the system while unit is running.
Glossary solenoid valve Electromagnet with a moving core. It serves as a valve or operates a valve. superheat The temperature of vapor above its boiling temperature as a liquid at that pressure or the difference between the temperature at the evaporator outlet and the lower temperature of the refrigerant evaporating in the evaporator. thermistor A semiconductor with electrical resistance that varies with temperature.