Refrigeration and AC

Specific Heat

ability of a substance to absorb heat or to lose it is known as the specific heat of a substance

Refrigeration Ton

cooling effect of 1 ton (2,000 lbs) of ice at 32 degrees F melting in 24 hours

General Gas Equation (Combined Gas Law)

expresses the interrelationship of the volume, the absolute pressure, and the absolute temperature of gases

Heat

form of energy (in transit)

The flow path through a basic refrigeration cycle can be described as:

from compressor to condenser to expansion valve to evaporator

Sensible Heat

heat absorbed or given off by a substance that is not in the process of changing its physical state

Latent Heat

heat absorbed or given off by a substance while it is changing its physical state

Temperature

intensity of heat

The heat absorbed by a substance undergoing a change of state is referred to as:

latent heat

Purpose of Refrigeration and Air Conditioning

maintain spaces at low temperature

Reversed Heat Engine

receives heat from a colder body and delivers heat to a hotter body (This is not in violation of the 2nd Law since work is added to the system to "pump" the heat)

1st Law of Thermodynamics

states that the net heat supplied to a system from its surrounding is equal to the net work done by the system on its surroundings Qin=Wout

Charles' Law

that the volume of a gas is directly proportional to its absolute temperature, provided the pressure is kept constant

Boyle's Law

the volume of any dry gas varies inversely with the absolute pressure provided the temperature remains constant V1P1=V2P2

Failure mechanisms that can occur in Refrigeration and AC

(Common Air Handler Problems) 1) Dirty evaporator coils 2) Air conditioner to freeze up 3) Dirty or clogged air filters 4) Evaporator coil being plugged or clogged 5) Failure of a blower motor

Important thermodynamic characteristics of a refrigerant

-Colorless and odorless in contractions of less than 20% by volume in air -BP of -21.6 at ATM pressure -liquid when under pressure of 70-75 psig -refrigerants are halocarbons (Halocarbons are organic chemical compounds containing hydrogen and one or more atoms of carbon, fluorine, bromine, chlorine, or iodine which may be present in various combinations in the compound)

Hazards associated with refrigerant

-If refrigerant concentration becomes excessive, unconsciousness or even death may result because of lack of oxygen to the brain -Frostbite

2nd Law of Thermodynamics

-states entropy must increase in a real process -This means that heat must flow from a high temperature to a low temperature to perform work -expression of the tendency that over time, differences in temperature, pressure, and chemical potential equilibrate in an isolated physical system

True or False: The purpose of the evaporator in a refrigeration cycle is to transfer heat from the refrigerant to the chilled water.

False

What is the function of a compressor in a refrigeration cycle?

Increase pressure of refrigerant so heat can be removed

Concerning characteristics of refrigerants, one having a minimum difference between the vaporizing pressure and the condensing pressure is desirable, why?

It requires less work on the compressor

Principles of Basic Refrigeration Theory

Laws of Refrigeration 1) Fluid absorbs heat while changing from a liquid state to a vapor state, and give up heat in changing from a vapor to a liquid 2) The temperature at which a change of state occurs is consistent during the change 3) Heat flows from a body at a higher temperature to a body at a lower temperature 4) Heat energy and other forms of energy are mutually convertible

Components that makeup the Refrigeration and AC cycle

Primary components: 1) Thermostatic Expansion Valve (TXV) 2) Evaporator 3) Compressor 4) Condenser 5) Receiver