D7E Track-Type Tractor Systems HVAC System Caterpillar


HVAC System
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1.1. HVAC System
2.2. A/C System
3.2. Heating System
4.2. Blower System

HVAC System



Illustration 1g03722239
(1) Compressor
(2) Condenser
(3) Expansion orifice
(4) Evaporator
(5) Accumulator
(6) Water Valve
(7) Heater Coil
(8) Implement ECM
(9) HVAC operator controls
(10) A/C Switch
(11) Condenser fan motors
(12) Blower resistor pack
(13) High-pressure switch
(14) Low-pressure switch
(15) Blower fan motor
(16) Ambient temperature sensor
(17) Evaporator freeze probe
(18) 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 three modes of operation: MANUAL, AUTO, and AIR CONDITIONING. The operator selects the mode of operation through the A/C Switch (11). In MANUAL mode the operator has direct control of the blower fan and heating system, but the air conditioning system is not engaged. In AUTO mode the operator sets a preferred temperature and the Machine ECM operates the heating and air conditioning system automatically. The operator still has direct control over the blower fan. In AIR 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.

A/C System

The air conditioning system is activated either through operator request from the A/C switch (10) in air conditioning mode or from the Implement ECM (8) when in auto mode and the ECM determines that the air conditioning should be activated.

When the air conditioning system is activated, the machine ECM (8) sends a signal to the A/C clutch on compressor (1). High-pressure switch (13) and low-pressure switch (14) verify that the high and low-pressure sides of the A/C system are functioning correctly. If either of the two switches detect pressures out of specifications, the ECM (8) receives a signal and shuts off the signal to the A/C clutch on compressor (1). Evaporator freeze probe (17) monitors the temperature of the evaporator (4). When the temperature of evaporator coil drops below a specific point, ECM (8) receives a signal and shuts off the signal to the A/C clutch on compressor (1). Once the temperature of the evaporator coil raises back above the specific point, ECM (8) receives a signal and re-engages the signal to the A/C clutch on compressor (1). The process of the air conditioning system turning on and off is called cycling.

When ECM (8) has engaged the air conditioning system, the air conditioner refrigerant begins to flow in the air conditioning system at the refrigerant compressor (1). The compressor is electrically driven. 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. The heat is transferred by condenser fan motors (11). The condenser fan motors (11) are activated by ECM (8) when ECM (8) sends a signal to the condenser fan motors (11). The condenser fan motors (11) are activated whenever the air conditioning system is active.

From the condenser coil (2), the refrigerant moves to the expansion valve (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.

From the evaporator coil (4), the refrigerant moves to the accumulator (5). The accumulator acts as a recieve/dryer. In the accumulator, moisture is removed from the refrigerant by an internal desiccant. The accumulator (5) also stores a small amount of refrigerant to adapt to changing system needs.

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.

Heating System

The heater system operates using coolant from the engine (E). The hot coolant from the engine circulates through the heating system. This coolant flows from the outlet valve for engine cooling system (1), through water valve (2), through heater coil (3), and back to the return valve for the engine cooling system (4). The heating system is controlled either by ECM (8) when in auto mode, or by the temperature control knob, which is located on the HVAC operator controls (9).

In auto mode the Implement ECM (8) monitors ambient temperature sensor (16) to determine how much engine coolant flow is required. The Implement ECM (8) uses that information to send a signal to water valve actuator (18) which varies the opening of water valve (6).

In manual or A/C mode, the heater system is controlled through HVAC operator controls (9). 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 (7). 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, coolant begins to flow from the outlet valve for engine cooling system (1) through the heater control valve and into the heater coil. 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 (7) 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 flows back into the return valve for the engine cooling system (4).

Blower System

The blower system controls how much air flow is passed through the evaporator (4) and heater coil (7). The blower system is controlled manually through HVAC operator controls (9). When the HVAC operator controls (9), 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 (9) 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 (9) to blower resistor pack (12). The resistor pack (12) contains different resistors designed to adjust the power sent to blower fan motor (15). The different power levels correspond to different fan speeds in the blower system. The blower fan motor (15) 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 (4) and the heater coil (7). The air is then sent to the louvers (K).

Information System:

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