Refrigeration Compressors
Inlet Vanes
- Common method of modulating capacity of centrifugal compressor where vanes are installed at the inlet of the compressor impeller - "Preswirl" refrigerant before it enters the impeller
Reciprocating Compressor
- Contains cylinders, pistons, rods, crankshaft and valves similar to an automobile engine - Refrigerant is drawn into cylinders on the downstroke of the piston and compressed on the upstroke -
Impeller
- Core component of centrifugal compressor - Center (eye) of impeller is fitted with blades that draw refrigerant vapor into radial passages that are internal to impeller body
Methods of Compressor Unloading
- Cylinder Unloaders (Reciprocating) - Cycle On and Off (Scroll) - Slide Valve (Helical-Rotary) - Inlet Valves (Centrifugal) - Variable Speed (All 4)
Cylinder Unloaders
- Fitted on most large reciprocating compressors - Remove refrigerant from cylinders in reciprocating compressor
Cycle On and Off
- Generally unloads scroll compressor by using multiple compressors and turning them on and off, as needed, to satisfy the evaporator load
Main Uses of Scroll Compressors
- Heat Pumps - Rooftop Units - Split Systems - Self-contained Units - Small Water Chillers
Discharge Phase: Scroll Compressor (Third Revolution)
- Interior edges ffeof scrolls separate, releasing compressed refrigerant through the discharge port - Completion of revolution results in volume of each pocket reduced to zero, forcing remaining refrigerant vapor out of the scrolls
Volute
- Larger space which pressure refrigerant collects around perimeter of the compressor - As volute space increases, kinetic energy/velocity of refrigerant decreases
Diffuser Packages
- Receives accelerated refrigerant vapor from impeller - Start out small and become larger as refrigerant travels through - Kinetic energy/velocity of refrigerant decreases as diffuser package size increases
Semihermetic Compressor
- Seals motor within compressor housing and is cooled by refrigerant - "Semi" because seal of motor is designed to be opened to repair or overhaul the compressor or motor
Hermetic Compressor
- Seals motor within the compressor housing - Eliminates need for shaft coupling and external shaft seals that are associate with open motors - Requires thorough cleaning compared to open compressor
Helical-Rotary Compressor
- Traps vapor and compresses it by gradually shrinking volume of the refrigerant - Rotors are meshed to fit, with very close tolerances, within the compressor housing
Slide Valve
- Unloads helical-rotary compressor - Position of valve controls volume of refrigerant vapor delivered by the compressor, by varying the amount of rotor length actually used for compression - Changing position of the slide valve enables compressor to unload to exactly match evaporator load instead of unloading in steps like reciprocating compressor
Centrifugal Compressor
- Uses centrifugal force, generated by a rotating impeller, to compress refrigerant vapor - Uses dynamic compression to convert energy from one form to another, to increase the pressure and temperature of refrigerant
Compression Phase: Scroll Compressor (Second Revolution)
- Volume of each pocket is progressively reduced, increasing pressure of the trapped refrigerant vapor - Completion of second revolution produces near maximum compression
Scroll Compressor
- Works on principle of trapping refrigerant vapor and compressing it by gradually shrinking the volume of the refrigerant - Uses two scroll configurations, mated face-to-face, to perform compression - Upper scroll = Stationary Scroll (contains discharge port) - Lower scroll = Driven Scroll (connected to a motor by a shaft and bearing assembly)
Intake Phase: Scroll Compressor (First Revolution)
-Edges of scrolls separate, allowing refrigerant vapor to enter the space between the two scrolls - At end of revolution edges of scrolls meet again, forming pockets of enclosed refrigerant
Methods of Capacity Control Conditions
1. Depends on whether the system is chilled-water or DX 2. Depends on how the airside system responds to changes in space loads
Advantage of Hot Gas Bypass (to suction line)
Amount of refrigerant piping is generally less than other method
In a constant volume chilled water system, the capacity of the compressor is typically controlled by sensing ________.
Chilled water supply temperature
Capacity Control
Commonly accomplished by unloading compressor but method used for unloading generally depends on type of compressor
Positive Displacement Compressors (Reciprocating, Scroll, Helical Rotary)
Compressors that work on the principle of trapping the refrigerant vapor and compressing it by gradually shrinking the volume of the refrigerant
What method of capacity control is commonly applied to scroll compressors?
Cycle On and Off
Assuming a constant condensing temperature, does the capacity of a compressor increase or decrease as the suction temperature decreases?
Decreases
Open Compressor
Driven by an external power source, such as electric motor, engine or turbine
What causes the suction valve to open on a reciprocating compressor?
During the intake stroke, the piston travels away from the discharge valve and creates a vacuum effect, reducing the pressure within the cylinder to below suction pressue. Since the pressure within the cylinder is less than the pressure of the refrigerant at the suction side of the compressor, the suction valve is forced open and the refrigerant vapor is drawn into the cylinder
Most commonly used compressors
Helical-rotary and scroll (Replaced reciprocating)
What is the term for the type of compressor that has the motor sealed within the compressor housing?
Hermetic or semihermetic
What method of capacity control is commonly applied to centrifugal compressors?
Inlet vanes or variable-speed drive with inlet vanes
Dynamic Compression (Centrifugal Compressor)
Involves converting energy from one form to another in order to increase the pressure and temperature of the refrigerant
Variable-Air Volume System
Maintains desired space conditions by varying the quantity of constant-temperature air
Heat of Compression
Mechanical energy used by the compressor to increase the pressure of the refrigerant vapor
Types of Classification of Compressors
Open, Hermetic, Semihermetic
Compression Stroke: Reciprocating Compressor
Piston reverses its direction and travels toward the discharge valve, compressing the refrigerant vapor and increasing the pressure within the cylinder
Intake Stroke: Reciprocating Compressor
Piston travels away from discharge valve and creates vacuum effect, reducing pressure within the cylinder to below suction pressure
Pressure-Enthalpy (P-h) Chart
Plots properties of a refrigerant: refrigerant pressure vs. enthalpy or heat content
Discharge Port: Helical-Rotary Compressor
Pockets of refrigerant are released through discharge port and rotors force remaining refrigerant from the pockets
Constant-Volume System
Provides constant quantity of variable-temperature air to maintain desired conditions
Purpose of Compressor
Raise the pressure of the refrigerant vapor from evaporator pressure to condensing pressure
Four Types of Compressors
Reciprocating, Scroll, Helical-rotary (Screw) and Centrifugal
Direct Expansion (DX) System
Refrigerant absorbs heat directly from the air used for space conditioning
Intake Port: Helical-Rotary Compressor
Refrigerant vapor enters the compressor through the intake port and fills the pockets formed by the lobes of the rotors
Disadvantage of Hot Gas Bypass (to suction line)
Refrigerant velocity in the evaporator and suction line drops very low when bypass valve is open
What are the two common methods of preventing evaporator frosting in a DX system?
Sensing suction temperature and hot gas bypass
In a constant volume DX system, the capacity of the compressor is typically controlled by sensing ____________.
Space temperature
In a VAV DX system, the capacity of the compressor is typically controlled by sensing ____________.
Supply air temperature
Adjustable-Frequency Drive (AFD) or Variable Speed Drive
This device controls capacity of a compressor by varying rotational speed of the compressor motor (Most often with centrifugal compressor)
Scroll Revolutions of Motor Shaft
Three are required to complete compression process
What is the purpose of the compressor in a refrigeration system?
To elevate the pressure, and, therefore, the temperature, of the refrigerant vapor high enough that it can reject heat to air, or some other fluid, at normally available temperatures
T or F: The intake of refrigerant vapor in a scroll compressor occurs at the outer edge of the scroll assembly and discharge occurs through the port at the center of the scroll.
True
Ported Compressor
Type of compressor where refrigerant vapor enters and exits through ports---no valves are used
Rotors
Uses in helical-rotary compressor to perform compression process (screw like part)
Chilled-water system
Uses water as the cooling media
Capacity of Compressor
Volume of evaporated refrigerant that can be compressed within a given time period
