Energy Efficiency Reference/Refrigeration/Technology Primer/Equipment
High-pressure, super-heated gaseous refrigerant cools and condenses giving off energy.
Assume:Zero pressure drop across the condenser
Purpose:Condense refrigerant by transferring heat to another medium, commonly air or water.
A) Air-cooled:Cross-flow finned-tube Fan draw air across finned refrigerant coils. Common on small equipment or where warm air is used in a process or for space heating. Measure condensing approach temperature difference relative to dry-bulb temperature.
B) Water-cooled:Shell and tube, double pipe, or shell and coil Refrigerant tubing submerged in cooling water. Scaling is a concern requiring water treatment and frequent cleaning. Water is usually cooled in a cooling in a cooling tower, but low-temperature heat can be recovered space, water, or process heat.
C) Evaporative Water is sprayed or cascaded on refrigerant tubes while a fan circulates air across them. Some water evaporates as it absorbs energy. Advantages are higher capacity, because water absorbs much more energy as it evaporates that air does, and the wet-bulb temperature are lower than dry-bulb temperatures.
High-pressure liquid flows through expansion device flashing to liquid-vapor mixture.
Assume:Constant enthalpy or no heat transfer across the expansion device as the pressure drops.
Purpose:Decrease liquid refrigerant pressure at constant enthaly to form liquid-vapor mixture at the suction pressure.
A) Orifice: Venturi, needle valves, and capilary tubes are commonly used to reduce pressure.
B) Flow-Control Device Throttle Valves and float valves also act as expansion devices
Low-pressure liquid evaporates absorbing heat from the cooled space or medium.
Assume: Zero pressure drop.
Purpose: Absorbs energy from a cold medium as the refrigeration evaporates, usually cools air, water, glycol, or product.
A) Direct-Expansion: a)Cross-flow, finned-tube, or shell-in-tube Liquid-gas refrigerant mix enters one end of a heat exchanger, evaporates as it passes through the device, and leaves the other end as gas only. The entire tube surface is not wetted with refrigerant, so heat transfer is not maximized.
b) Flooded-Coil Shell and Tube Refrigerant in the shell is controlled by a float valve at a level that always wets the tubes. Tubes may carry glycol, water, or other substance to be cooled. Gaseous refrigerant exits shell after evaporation.
c) Finned Tube Refrigerant is fed to coil by gravity from an accumulator. This evaporator type is often found in coolers and freezers. The refrigerant level is controlled in the accumulator by a float valve. Gaseous refrigerant flows out of the coil by buoyant forces. Air flows across tubes by natural convection or fans.
B) Liquid-Overfeed:Cross-Flow Finned-Tube Liquid overfeed is common in ammonia systems. The accumulator separates liquid gas. Liquid is pumped through the evaporator so that the coil is always flooded. As refrigerant evaporates., the liquid-gas mixture flows through the accumulator where the gas separates from the liquid. A float valve controls the liquid level.
Low-pressure, saturated gas is compressed to high-pressure, super-heated gas.
Assume: Constant entropt.
Purpose: Compress cool to hot refrigerant gas and to drive refrigerant flow.
A) Dynamic Compressors Centrifugal Liquid chillers often use centrifugal compressors. Centrifugal compressors offer unlimited capacity control because they are dynamic machines.
B) Positive Displacement CompressorsReciprocating Reciprocating compressors work much like an automobile engine, with pistons, valves and cylinders. Come compressors have unloading controls that open one or more cylinder intake valves so the not be compressed.
Helical Screw Screw compressors work by forcing the gas through a space formed by male and female rotors that compress the refrigerant. This is a common type of compressor. Most have a slide valve for part load control. The slide valve allows some refrigerant to exit the rotors without being compressed for part-load operation.
Rotary Vane Cylinder mounted eccentrically on stationary bearings reduces volume as the cylinder rotates. vanes slide in an out of the cylinder as it rotates to provide a refrigerant seal.
C) Motor-Compressor Combinations Heremetic Motor and compressor share common shaft in common housing. Housing is sealed with ony suction and discharge lines exiting. No internal seals are used. The compressor is quiet, common on small equipment, but cannot be service in the field.
Semi-Hermetic Motor and compressor on common shaft in common housing. Housing can be opened in the field for servicing. Benefits are reduced noise and no seals between compressor and motor.
Open Motor separate from the compressor. Shaft, belt, or gear drive the equipment and seals are required to keep refrigerant in the compressor.
D) Other Compressor Components Purge Units Purpose: Remove non-condensable gases from condenser to increase heat transfer coefficient and reduce condensing pressure and corrosion of the heat transfer surface.
Characteristics: A purge unit looks like a glass container with liquid refrigerant spinning in it. Bubbles of non-condensable gas rise through the liquid periodically. Purge units will be located near the top of condensers.
Defrost Purpose: remove ice that can limit heat transfer at evaporators that cool air below refreezing. 32 degrees F.
Warm Water Warm water is pumped through the evaporator, malting the ice.
Hot Refrigerant Gas Valves direct refrigerant from the hot gas line, bypassing the condenser to the evaporator. The compressor circulates the refrigerant.
Electric Heating Elements Electrical heating elements on the exterior of the coils melt the ice.
Back Pressure Regulators Purpose: to regulate different evaporator temperatures for different applications. BPR's are not efficient when they require lower suction pressure than necessary.
Oil Cooling Purpose: Cool the oil used to lubricate and seal screw compressors
Liquid injection Refrigerant liquid is expanded and injected into the compressor. To refrigerant cools the compressor as it expands to an intermediate injection pressure.
Thermosyphon High pressure liquid refrigerant is flooded through a heat exchanger to absorb heat from the oil. Buoyancy forces drive the thermosyphon system from a receiver located near the condenser exit. Liquid flows by gravity to the compressor. As liquid evaporator gas rises to the condenser. The system may use a separate condenser.
Water-cooled Oil separated from the discharged refrigerant is circulated through a water-cooled heat exchanger.
Air-cooled Oil is separated from the refrigerant after the compressor and circulated through an air-cooled heat exchanger. The oil maybe circulated by discharge pressure alone, or by a pump that raises the oil pressure 30 to 45 psig above the discharge pressure. Oil is then injected back into suction and intermediate ports on the compressor.
Economizer Purpose: Increase simple cycle efficiency by reducing the heat of compression and increasing the refrigeration effect.
Characteristics: There are two common types of economizers.
A "flash" economizer works by expanding the refrigerant in two steps rather than one. High-pressure liquid refrigerant expands once through an expansion valve to an intermediate pressure receiver where some of the liquid flashes, lowering the temperature. The gas is drawn off and flows directly to an intermediate port on the compressor where it is compressed to the discharge pressure. The liquid portion of the refrigerant at the receiver passes through a second expansion valve to the evaporator and then the compressor. The gas from the economizer decreases the compressor discharge superheat.
In a "flooded shell and tube" economizer, high-pressure liquid through an intermediate pressure receiver (shell-and-tube) where it is sub-cooled before an expansion device. The lower temperature and refrigerant flow rates through the low stage of compression reduces total compressor power. More information on economizers is available in the appendix.