Mechanical Sciences Refrigeration

Performance factor

Reduces the high pressure in the liquid line to the low pressure in the evaporator

Refrigerant-flow control

Heat added to or removed from a substance, which results in a temperature change to that substance, and no change in phase.

Sensible heat

Indirect measuring device used to monitor compressor suction line temperature and control the flow of refrigerant into the evaporator based on this temperature

Sensing bulb

The temperature of a liquid, below saturation (boiling/condensing) temperature

Subcooling

Catches liquid that leaves the evaporator through the suction line before it can reach the compressor

Suction accumulator

The temperature of a vapor, above saturation (boiling/condensing) temperature.

Superheat

Use of thermistors, exposed to the refrigerant in the suction line to control the expansion needle valve opening

Thermal-electric expansion valve

Metering device controlled by the temperature of refrigerant at the evaporator outlet.

Thermostatic expansion valve (TXV)

The temperature of a vapor, above saturation (boiling) temperature

Superheat

The temperature of a liquid, below saturation (condensing) temperature.

Subcooling

The heat added to a liquid to bring about a change in phase, with no change in temperature. Heat added to a liquid at its boiling temperature will vaporize the liquid, without changing its temperature.

Latent heat

The heat added to a substance, which results in a temperature change to that substance, and no change in phase.

Sensible heat

DESCRIBE the basic mechanical refrigeration cycle

The refrigeration cycle is divided into two pressure sections: the high pressure side and the low pressure side

The compressor raises the pressure of the fluid. The high pressure side starts when gasses
leave the compressor, and continues through to the metering device. The metering device reduces system
pressure. The low pressure side continues from the metering device through to the suction of the compressor.

Describe the two pressure sections of a refrigeration cycle.

Low pressure, superheated, refrigerant vapor is supplied to the suction of the compressor. As the compressor pistons compress the gas, the gas is heated.

The hot, high pressure gas flows from the compressor discharge through the condenser. The cooling medium (water or air) of the condenser absorbs heat from the hot, high pressure refrigerant gas.

Receiving or removing the refrigerant vapor from the evaporator, so that desired pressure and temperature can be maintained.
Increasing the pressure of the refrigerant vapor through the process of compression and simultaneously increasing the temperature of the vapor so that it will transfer its heat to the condenser cooling medium.

Compressor

Transfers the heat from the refrigerant to a medium that can absorb it and move it to a final disposal point

Condensor

Installed to collect the high pressure, low temperature liquid refrigerant as it leaves the condenser

Receivers

Sometimes call the cooling coil, it is the part of the refrigeration system where heat is removed from the product: air, water, or whatever is to be cooled.

Evaporator

A simple device that traps liquids leaving the evaporator

Accumulator

This is the most common type of compressor used in domestic, smaller commercial, and industrial condensing unit applications.

Reciprocating Compressors

In this type compressor, vapor, as it is moved rapidly in a circular path, moves outward.

Centrifugal Compressor

The low pressure refrigerant vapor from the suction line is drawn into the opening and fills the space behind the blade as it revolves.

Rotary Compressor

The rotors trap and compress vapor as they revolve inside an accurately machined compressor cylinder.

Screw-Type Compressor

This compressor uses two offset spiral disks to compress the refrigerant vapor

Scroll compressor

EXPLAIN the factors that affect compressor performance.

Capacity
Performance Factor
Balancing

Explain the operation of the following metering devices:
a. Automatic expansion valve
b. Thermostatic expansion valve
c. Thermal-electric (solid-state) expansion valve
d. Capillary tube

Automatic expansion valve (AXV) is used to reduce the high pressure of the liquid refrigerant down to a low pressure vapor-mist in the evaporator.

A thermostatic expansion valve (TXV), controls the refrigerant flow in the
evaporator by the temperature of refrigerant at the evaporator outlet.

The thermal-electric expansion valve depends upon the use of thermistors, directly
exposed to the refrigerant in the suction line, to control the expansion valve needle opening.

Capillary Tube is simply a length of tubing with a small inside diameter. It acts as a constant
throttle on the refrigerant.

DESCRIBE failure mechanisms and symptoms associated with a refrigeration system

- Insufficient cooling
- Improper refrigerant charge
- High head pressure
- Low suction pressure
- High suction pressure

DESCRIBE causes for the following refrigeration system conditions:
a. High head pressure
b. Low suction pressure
c. High suction pressure

High head pressure is caused by either restriction in the compressor discharge line or by poor heat transfer in the condenser, Also by Overcharge of refrigerant

Low Suction pressure Either a restriction in the suction line or poor heat transfer in the evaporator causes low suction pressure. Undercharge of refrigerant

High suction pressure Either heavy load conditions or improper metering of the refrigerant to the evaporator causes high suction pressure

Caused by either restriction in the compressor discharge line, poor heat transfer in the condenser or Overcharge of refrigerant.

High Head Pressure

Caused by a restriction in the suction line, poor heat transfer in the evaporator or Undercharge of regrigerant

Low Suction Pressure

Caused by Heavy Load conditions or improper metering of the refrigerant

High Suction Pressure

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Metering device used to reduce the high liquid refrigerant pressure down to a low
pressure vapor-mist in the evaporator

Automatic expansion valve (AXV)
The amount of heat required to raise the temperature of one pound of water one degree Fahrenheit

British thermal unit (Btu)
The refrigeration effect that can be achieved by the system. It is equal to the rate at which heat can be removed from the medium to be cooled.

Capacity
A length of tubing with a small inside diameter.
It acts as a constant throttle on the refrigerant

Capillary tube
Mechanical device used to receive or remove the refrigerant vapor from the evaporator, increasing the pressure and temperature of the refrigerant vapor

Compressor
Heat exchanger designed to completely evaporate the refrigerant in the coil itself, with only a vapor leaving the coil outlet.

Dry-type evaporators
Heat exchanger in the refrigeration system where heat is removed from the product: refrigerant in the evaporator absorbs heat from the material being cooled.

Evaporator
Heat exchanger completely filled with liquid refrigerant, designed so that the refrigerant level is maintained. Part of the liquid refrigerant evaporates in the coil, and this vapor goes to the accumulator

Flooded-type evaporators
A condition of liquid refrigerant returning to the compressor through the suction line before it vaporizes. It can occur when the evaporator is overfed or there is too much refrigerant in the system.

Floodback
Heat added to or removed from a substance to bring about a change in phase, with no change in temperature

Latent heat
Indicates combined operating efficiency of motor and compressor

Performance Factor (Btu/watt-hr)

=

Capacity (Btu/hr)
Power Input (watt)

Author

ereim

28845

Card Set

Mechanical Sciences Refrigeration

Description

Chap 7

Updated

8/9/2010, 10:51:05 AM

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