CompAir 5280D.1.IAC Operator's Handbook Manual page 27

CompAir UK Ltd
5280D Compressor Publication 98407.1440
Page 24
9.5 Automatic Unloading and Draining
9.5.1-Continuous Running Loading / Unloading - "Topping Duty"
9.5.1.1 When the power is ON the power lamp is illuminated. A delay timer in the starter enables the
compressor to be started with oil pressure switch by-passed and the pilot solenoid valves held in unload
position.
If the compressor has been stopped via Emergency Stop button or by operation of a switching thermometer
or oil pressure switch, the starter fault circuit will have to be reset before starting.
As the air demand volume varies, so the receiver pressure will fall. The compressor output must then be
subject to automatic control to match the demand.
9.5.1.2 When the air demand continuously matches the compressor output the receiver pressure remains
steady at the normal working pressure and the receiver pressure switch remains closed to keep the pilot
solenoid valves energised. Under these conditions the compressor suction valves operate normally and the
separator drains remain closed.
The compressor continues to run on load until the demand changes.
9.5.1.3 When the air demand falls below that of the compressor output, the pressure in the receiver
increases toward the compressor maximum pressure capability. At a set point the pressure switch operates
and de-energises a pilot solenoid valve. This passes receiver air, reduced in pressure via a pressure
reducing valve, to the suction valve unloaders on all three stages of the compressor.
Simultaneously a second pilot solenoid valve is de-energised and air is vented from the diaphragm drain
valves fitted to both 2nd and 3rd stage separators.
The drain valves open to allow condensate collected from the 2nd and 3rd stage separators, together with
any released by the drain trap on the 1st stage separator during running, to be discharged from a common
drain vessel or surge tank.
9.5.1.4 A third solenoid valve, de-energised only when the compressor is stooped, continues to pass
receiver air at a reduced pressure to a water control valve keeping the valve open. This valve supplies
cooling water to the cylinders and cylinder heads whenever the compressor is running.
First stage cylinder water temperature is sensed by a thermostatic water valve to ensure the optimum flow
rate of water reaches the cylinders. (Only when the compressor STOPS does air vent from the water valve to
shut off the by-pass water flow)
9.5.1.5 The compressor will now continue to run fully unloaded with output reduced to zero and the main
water supply through the coolers shut off. By-pass water will flow around the cylinders and heads, with the
2nd and 3rd stage drains open.
9.5.1.6 An increase in demand from the main air system causes the receiver pressure to fall. The switching
sequence is reversed.
The two pilot solenoid valves are energised and air is exhausted from the suction valve unloaders in the
cylinders to re-load the compressor. First stage air is applied to close diaphragm drain valves and open the
main the water control valve. The compressor commences to run on load again and the above modes of
operation will be repeated until the compressor is stopped.
9.5.2 Automatic Stop/Start - Charging Duty
The operation is similar to the "topping duty" explained. The exception is that the pressure switch is wired to
trip the motor contactor in the starter on high receiver pressure and to re-initiate the start sequence on low
receiver pressure as well as operating the auto-drains and water valve.
A water by-pass valve is unnecessary with this operation only a water control valve is supplied.
9.5.3 Charging or "Topping Duty" with a water pump.
With a water pump, no water control valves are necessary to prevent water flow when the machine stops.
When on automatic STOP/START or charging duty, water will stop flowing as soon as the compressor stops.
However, when on continuous running loading ~ unloading or "topping duty", water will continue to flow
through the coolers and the cylinder and heads.
The thermostatic water valve will turn down the water flow to prevent overcooling and possible condensation
resulting from little heat being generated during the unloading cycle
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