York YS Service Manual download pdf (Page 17)

2.9 Evaporator

The evaporator is a shell and tube, flooded type, heat

exchanger with a distributor trough providing uniform

distribution of refrigerant over the entire shell length to

ensure optimum heat transfer. A 57 mm diameter liquid

level sight glass is located on the side of the shell to aid

in determining proper refrigerant charge. A 25 mm

refrigerant charging valve is provided.

2.10 Condenser

The condenser is a shell and tube, flooded type, heat

exchanger with a discharge gas baffle to prevent direct

high velocity impingement on the tubes. The baffle

distributes the refrigerant gas flow uniformly over the

entire shell length for optimum heat transfer. A

subcooler section is located in the bottom of the

condenser to provide effective liquid refrigerant

subcooling, improving cycle efficiency. The condenser

shell also serves as a refrigerant receiver to store the

system charge during servicing. Manually operated

isolation valves are located at the inlet and outlet of the

condenser. Valves are also provided to facilitate

removal of the refrigerant from the system.

2.11 Refrigerant Flow Control

Sub-cooled liquid refrigerant flows out of the condenser

via the liquid line into the evaporator by differential

pressure. A single or dual fixed metering orifice with no

moving parts controls refrigerant flow to the evaporator.

The orifice is selected based upon the operating

conditions of the unit.

A variable orifice arrangement is also supplied in

parallel with the metering orifice. This consists of a

solenoid valve and hand-throttling valve. The solenoid is

energized open by the differential pressure set point that

is field programmable in the control panel.

The differential pressure between condensing pressure

and evaporating pressure is compared to the set point

value. When the differential pressure is at or less than

the setpoint, the solenoid valve is energized open. The

solenoid valve is de-energized closed when the

differential pressure is equal to or greater than the

setpoint plus 10 PSIG. A hand-throttling valve is

provided to adjust the refrigerant flow rate through the

solenoid valve to match the system operating

conditions.

A manual isolation valve is located between the

condenser and the metering orifice plate. This valve, in

combination with the hand isolation valve between the

oil separator and the condenser, allows all of the

refrigerant charge to be stored in the condenser.

A liquid refrigerant supply is piped from the bottom of the

liquid line to the refrigerant cooled oil cooler. The

refrigerant gas from the oil cooler is piped directly into

the evaporator. A refrigerant charging valve is fitted into

the liquid line between the evaporator and the metering

orifice. A ¾ inch male flare connection is provided for

connecting hoses or transfer lines.

2.12 Capacity Control

Capacity control is accomplished by using differential

pressure to move the slide valve. As the slide valve is

moved axially between the compressor rotors the

volume of vapour pumped by the compressor is

changed to match the system requirements.

Leaving chilled water temperature (LCWT) is

continuously monitored by the microprocessor and

compared to the LCWT Setpoint. When the LCWT is

outside the range of the setpoint value a signal is sent

via the relay output board to energize the 4-way valve

directional solenoid valves.

When solenoid valve B is energised (Port P to Port B

and Port A to Port T) the slide valve moves in the load

direction.

High pressure oil from the oil circuit flows (Port P) flows

through the sub-plate manifold block and out to

Compressor Port SC-2 (Port B). Simultaneously, oil

flows out of Compressor Port SC-1 (Port A) through the

sub-plate manifold block and out to suction pressure

(Port T).

When the Solenoid Valve A is energised (Port P to Port

A and Port B to Port T), the slide valve moves in the

unload direction. High pressure oil flows into

Compressor Port SC-1 (Port A) and oil is relieved out of

Compressor Port SC-2 (Port B) to suction pressure.

A slide valve potentiometer is used to provide feedback

to the microprocessor to display slide valve position as a

percentage of full load.

Four manual isolation valves are incorporated into the

4-way solenoid sub-plate to isolate the 4-way directional

valve for service.

035L02381-GB0 2-7

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York YS Service Manual Page 17

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