chapter 51 HVAC

Typically, the trinary switch will serve three functions:

-Low-pressure protection -High-pressure cutout -Engine fan on/off

Common compressor controls include:

-Thermostatic switch -Low-pressure cutout switch -Pressure-cycling switch -Trinary switch -High-pressure cutout switch -High-pressure fan switch -Ambient temperature switch

One Btu is the amount of energy required to heat or cool

1 pound of water 1° F.

One set of switch contacts is used to signal when system pressure drops too low, about

10-15 psi (69-103 kPa).

The on/off engine fan will be engaged above pressures of_____________to create airflow across the condenser and lower high-side system pressure.

230 psi

set of contacts is used to signal high system pressure, about

385 psi

The ideal cab environment has a moderate humidity level and should reach a temperature of*

70*F to 80*F (20-27*C) within several minutes of operation.

Chlorofluorocarbons (CFCs)*

A chlorine-based composition of a fluorine hydrocarbon compound

rule 4

All refrigeration systems use two principles to move heat from one place to another.Latent heat of vaporization is used to absorb large amounts of heat inside the cab or passenger compartment.Latent heat of condensation is used to release large amounts of heat outside in the condenser.The evaporator absorbs heat; the condenser releases heat.

Any release or venting of ___________________ to the air has a powerfully destructive effect on ozone, which led to the banning of CFC refrigerants. Therefore, any new refrigerants are categorized by the effect they have on the ozone.

CFC-based refrigerants

are no longer used as refrigerants. Extensive scientific studies have proven that refrigerant made from these compounds destroys the ozone layer.

Chlorofluorocarbons (CFCs)

The thermostatic sensor prevents the compressor from operating if it detects an evaporator condition.*

Freeze-up protection

rule 2*

Heat can pass into anything that has less heat(Nothing can stop the movement of heat; the transfer of heat can only be slowed down.)

The compressor is not allowed to run if ignition voltage is greater than 16 volts

High-voltage cutout

rule 3*

If a change of state is to take place, there must be a transfer of heat.For a liquid to change to a gas, the liquid must absorb heat. Heat is removed from a liquid through vapor.For a vapor to change into a liquid, the vapor must give up heat. Heat is always transferred to a cooler surface or medium.

is the most common refrigerant additive and is often added during manufacturing

Leak-tracing dye

it contribute to the breakdown of lubricating oils.

Moisture can

itself is the central element of the entire air-conditioning process because it absorbs heat from the cab/passenger compartment air and transfers it to the atmosphere.

Refrigerant

Typically the_________ is more commonly used due to the larger capacity and physical size of commercial vehicle evaporators.

TXV system

Rule 1

To refrigerate is to remove heat. The absence of heat is cold.

freon

a refrigerant produced by reacting carbon tetrachloride commonly used as carb-cleaner with fluourine gas

A gauge that uses vacuum as a reference point is calibrated to read 14.7 psi (97 kPa) at sea level. This gauge reference is called

absolute and the units are psia for psi absolute (kPaA, kPa absolute).

pressures below atmospheric pressure are considered

absolute pressures.

For instance, if there is little or no ________________ through the condenser, heat will not be removed, and system refrigerant pressures will increase dramatically.

airflow

When a gauge is calibrated to atmospheric pressure, it will read 0 psi (0 kPa) in a room when not connected to a source of pressure. This means a gauge pressure of 0 at a room pressure of 14.7 psi (97 kPa) references to

atmospheric pressure, and the units for pressure will be psig for psi, gauge (kPaG, kPa gauge)

The condenser typically consists of___________ mounted in a series of thin cooling fins.

coiled tubing

As the refrigerant vapors are cooled and flow down through the condenser, ________ eventually occurs, at which point the gas becomes liquid refrigeran

condensation

if the evaporator is too cold, moisture will freeze on the fins and plates, preventing effective cooling. This condition where excess refrigerant floods the evaporator is called .

evaporator freezing

An _______________ restricts the high-pressure, warm liquid refrigerant line and is designed to lower the pressure of refrigerant entering the evaporator.

expansion device

An_______________ restricts the high-pressure, warm liquid refrigerant line and is designed to lower the pressure of refrigerant entering the evaporator.

expansion device

heat transfer takes place from hot to cold because

fast, hot molecules collide with cooler, slower molecules, transferring energy into the cooler molecule, thereby speeding up its movement

The condenser is mounted directly in ______________ of the radiator, where it can receive full airflow created by forward vehicle motion and the engine fan.

front

Removing heat from an object or substance is simple. Because

heat always moves toward a colder object,

heater core

heating is regulate by circulating engine coolant through an in-cab heat exchange

When the parking brake is set, there is no road speed, and the _______________ detects a high-pressure condition, the fan is requested until the parking brake is released or road speed is detected.

high-pressure switch

Operation with excessive refrigerant charges can severely stress the entire air-conditioning system, causing___________ leaks.

hose and fitting

compressor

is an engine-driven pump that increases refrigerant pressure and circulates refrigerant through the system

heat

is required for the refrigerant to change into a vapor

If the compressed gas is circulated through a condenser, large amounts of

latent heat are transferred from the gas to the atmosphere as the gas changes to a liquid.

The ____________________________ is connected in the compressor clutch electrical circuit. When a predetermined low charge is sensed, the switch stops compressor operation.

low-pressure cutout switch

In 1928, Thomas Midgley, Jr., aided by Charles Kettering invented a

miracle compound" called Freon. Freon was produced by reacting carbon tetrachloride, commonly used as "carbcleaner," with fluorine gas.

*the amount of heat an object has can br observed in how much its

molecules move or vibrate

the compressor remains off for the first 15 seconds after ignition switch engagement while battery voltage is low. This prevents low-voltage compressor clutch burnout

mpressor hold-off period

Note, however, if a leak is large enough, pressure testing with ________ is recommended instead of leak detection with dyes

nitrogen

Second, during low-temperature operation, refrigerant pressure will be low, causing the same problems of ____________, which is the ability of oil to dissolve into the refrigerant gas.

oil miscibility

Three basic types of air-conditioning compressors can be used to build pressure exceeding 350 psi

piston, rotary vane, and scroll.

The concept of temperature is related to

pressure

In__________________________, the 90-120w viscosity lubricant helps the piston rings seal tightly against their cylinder walls to prevent leakage and apply maximum pressure to the refrigerant.

reciprocating piston compressors

First, if the __________ level is too low and the system is undercharged, oil will not be circulated through the compressor

refrigerant

Outside air flows over the fins of the condenser and absorbs heat from the

refrigerant as it converts from a liquid to a gas.

*When refrigerant has evaporated in the evaporator core, its pressure___

rises

Low______________________ system voltages can cause the compressor clutch to slip and overheat and loosen electrical connections, which in turn can cause erratic compressor cycling and compressor clutch failures.

system voltages

radiant heat transfer

takes place through a medium such as gas vacuum, but the medium itself does not heat up.

Conduction

takes place when heat is transferred through a solid, such as a body panel the metal fins of a condenser or an evaporator

Using a _____________________________ located where it can respond to changes in the evaporator outlet temperature, the switch will either open or close the contacts inside the thermostatic switch .

temperature-sensitive capillary tube

British thermal unit*

the amount of energy required to raise the temperature of 1 pound of water by 1 degree Fahrenheit

Air-conditioning systems absorb heat from a cab or vehicle interior through an evaporator and transfers that heat to*

the atmosphere using a heat exchanger called a condenser

evaporator *

the cold surface of the air-conditioning system that absorbs heat from a cab or vehicle interior and transfer that heat to the atmosphere through a condenser

the spring is generally adjusted to provide for a temperature differential of 4-16°F (-15.5 to -8.8°C) between

the evaporator inlet and outlet temperatures.

However, decades later, it would be discovered that

the refrigerant's chlorinebased composition of a fluorine hydrocarbon compound commonly called chlorofluorocarbons (CFCs) damaged Earth's ozone layer.

latent heat

the the quantity of heat required to produce a change of state from a solid to a liquid or a liquid to gas

convection

the transfer of heat through a gas

Calorie*

the unit of energy that reflects the amount energy required to raise the temperature pf 1 gram of water

Newer systems use a ___________ in the evaporator that is monitored by the body control, climate control, or electrical system control module.

thermistor

The_____________________________ , also called the cold switch, is an evaporator temperature-sensing switch. It is used with either the CCOT or TXV metering systems.

thermostatic switch

It is known as a_____________________ because of its three sets of internal contacts.

trinary switch