(3) Expansion orifice
(4) Evaporator / Heater Coil
(6) Water Valve
(8) HVAC operator controls
(9) A/C Switch
(10) Condenser fan relays
(11) Condenser fan motors
(12) Blower resistor pack
(13) High-pressure switch
(14) Blower fan motor
(15) Thermostat switch
(16) Water valve actuator
(A) High temperature, high-pressure vapor
(B) High temperature, high-pressure liquid
(C) Low temperature, low-pressure liquid vapor mixture
(D) Low temperature, low-pressure vapor
(E) Coolant from engine
(F) High temperature coolant
(G) Medium temperature coolant
(H) Coolant to radiator
(J) Air from cab recirculation
(K) Air to cab louvers
The HVAC system has two modes of operation: HEAT and AIR CONDITIONING. The operator selects the mode of operation through the A/C Switch (9). InHEAT mode the operator has direct control of the blower fan and heating system, but the air conditioning system is not engaged. InAIR CONDITIONING mode the air conditioning system is engaged but the operator still has direct control over the blower fan and heating system.
Note: The HVAC system will not operate if the blower fan control is in the OFF.
The air conditioning system is activated through operator request from the A/C switch (9) in air conditioning mode.
When the air conditioning system is activated, HVAC operator controls (8) sends a signal to A/C compressor (1). High-pressure switch (13) verify that the A/C system is functioning correctly. If the switch detects pressures out of specifications, the high-pressure switch (13) prevents power from flowing through the compressor (1). Thermostat switch (15) monitors the temperature of the evaporator (4). When the temperature of evaporator coil drops below a specific point, power is no longer allowed to flow through compressor (1). Once the temperature of the evaporator coil raises back above the specific point, power is again allowed to flow through the compressor (1). The process of the air conditioning system turning on and off is called cycling.
When active, the air conditioner refrigerant begins to flow in the air conditioning system at the refrigerant compressor (1). The compressor is driven by a belt from the engine (7). The compressor is designed to change the air conditioner refrigerant from a vapor that has low pressure into a vapor that has high pressure. Also, the compressor changes the refrigerant from a vapor with low temperature to a vapor with high temperature. The increase in pressure causes the increase in temperature.
The refrigerant is sent through the condenser coil (2). In the condenser coil, heat is transferred from the refrigerant to the outside air. This changes the refrigerant to a high temperature and high-pressure liquid.
From the condenser coil (2), the refrigerant moves to the accumulator (5). the accumulator acts as a reciever/dryer. In the accumulator, moisture is removed from the refrigerant by an internal desiccant. Accumulator (5) also stores a small amount of refrigerant to adapt to changing system needs. From the accumulator the refrigerant moves to expansion orifice (3). The expansion valve restricts the flow of refrigerant and causes a pressure drop in refrigerant. The pressure drop causes the refrigerant to change from a high pressure and a high temperature liquid to a low pressure and a low temperature mixture of liquid and vapor.
The low pressure and low temperature refrigerant moves to the evaporator coil (4). While the refrigerant is in the evaporator coil, the refrigerant absorbs heat from the air flow. This changes the refrigerant to a low pressure and low temperature vapor.
The refrigerant returns to the compressor (1) as a low pressure and low temperature vapor. This marks the end of the cycle. The cycle is repeated continuously.
The heater system operates using coolant from the engine (E). The hot coolant from the engine circulates through the heating system. This coolant from the engine (E) flows from the outlet valve for engine cooling system, through water valve (6), through heater coil (4), and flows to the radiator (H). The heating system is controlled by the temperature control knob, which is located on the HVAC operator controls (10).
In Heat or A/C mode, the heater system is controlled through HVAC operator controls (8). With the temperature control knob in the maximum cold position, water valve (6) is closed. The CLOSED position prevents the flow of coolant into heater coil (4). When the control knob is moved away from the maximum cold position toward the maximum heating position, the heater control valve begins to open. As the control valve opens, high temperature coolant (F) begins to flow through the water valve (6) and into the heater coil (4). As the hot coolant flows through the heater coil, the temperature of the air that is flowing through the coil increases.
The temperature of the air that is flowing out of heater coil (4) and into the cab is controlled by the amount of coolant that circulates through the heater coil. The coolant transfers the heat into the air stream. By increasing the amount of coolant that is circulating through the heater coil, the temperature of the air that is flowing out of the coil also increases. The coolant leaves the heater coil at a reduced temperature and the medium temperature coolant (G) flows to the radiator (H).
The blower system controls how much air flow is passed through the evaporator and heater coil (4). The blower system is controlled manually through HVAC operator controls (8). When the HVAC operator controls (8), put the blower in the off position the HVAC system is not engaged. Neither the air conditioning system or the heating system will operate. When the HVAC operator controls (8) put the blower in the LOW, MEDIUM, MEDIUM-HIGH, or HIGH position, the HVAC system is activated. Depending on the position of the blower control, power is sent from HVAC operator controls (8) to blower resistor pack (12). The resistor pack (12) contains different resistors designed to adjust the power sent to blower fan motor (14). The different power levels correspond to different fan speeds in the blower system. The blower fan motor (14) pulls in air from the cab recirculation (J). This air comes from a return vent to the lower left of the operator seat. The fan pushes the air through the evaporator and heater coil (4). The air is then sent to the louvers (K).