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