MECH1300Pneumatic Power Supply

Topics

Compressor Types

Compressor Sizing

Vacuum Pumps

Chapter 11

MECH1300Pneumatic Power Supply

Cycling a Compressor – When the pressure drops to a predetermined minimum in a tank, the compressor will automatically turn on.

Cut-in pressure – The pressure at which the compressor turns on.

Cut-out pressure – The pressure at which the compressor turns off.

Cycling a compressor is controlled by a pressure switch which senses the max and min pressure.

MECH1300Pneumatic Power Supply

If starting and stopping the prime mover is not practical for the situation or causes damage.Two options –

1. Clutch – Used to disengage the motor from the compressor when the cut-out pressure is reached

2. Valve - Discharge the outlet flow to the atmosphere when the cut-out pressure is reached.

MECH1300Compressor Types

Positive displacement pump (compressor) – operate by opening up a cavity to draw in fluid, then closing the cavity to expel fluid

Gear or piston type

Produce flow against high pressures

3 Designs –

1. Reciprocating piston

2. Rotary Vane

3. Rotary Screw

MECH1300Compressor Types

Reciprocating Piston – Most common design for small to medium sized commercial compressors. Consist of one or more pistons.

As the crankshaft rotes, the pistons reciprocate, drawing in atmospheric air and pushing out compressed air.

MECH1300Compressor Types

Rotary Vane- As the rotor spins, the vanes are kept in contact with the housing by centrifugal force causing them to extent and retract. This makes the chambers between the vanes to increase in volume near the inlet, which creates a vacuum at the inlet, which lets atmospheric air push into the inlet. The volume at the outlet decreases and the air exhausts to the tank.

MECH1300Compressor Types

Rotary Screw- Compresses air between two intermeshing screws. Capable of generating pressures up to 150 psi. Most commonly used for industrial applications.

MECH1300Compressor Types

Non-Positive displacement pump(compressor) [dynamic]– imparts a velocity to a fluid

Centrifugal type, uses an impeller to spin fluid, increasing kinetic energy.

Diffuser – converts kinetic energy to pressure energy

Has single stage and multistage designs

Low pressure, high flow

MECH1300Compressor Sizing

Sized to supply sufficient pressure and flow to a system under all expected conditions, as well as allowing it to remain idle for part of the cycle.

Includes motor, compressor and receiver tank

Calculating required input power to compressor

= drive power (hp, kW)Q = compressor flow rate (scfm, ) = inlet pressure (psia, kPa abs) = outlet pressure of the air (psia, kPa abs)= overall efficiency of the compressor

MECH1300Compressor Sizing

Calculate the input power

EX: Determine the input power required to drive a compressor that delivers 200 scfm at 120 psig. The overall efficiency of the compressor is 70%

Convert to psia:

MECH1300Compressor Sizing

Calculate the input power

EX: Determine the input power required to drive a compressor that delivers 5.7 standard at 830kPa. The overall efficiency of the compressor is 75%

Convert to psia:

MECH1300Receiver Tank Sizing

Determines how much air a system can store

= volume of the receiver tank time = system demand flow (scfm, standard ) [flow out of the receiver tank] = compressor delivery flow (scfm, standard ) [flow into receiver tank] = maximum receiver tank pressure (psig, kPa gauge) = minimum receiver tank pressure (psig, kPa gauge)

MECH1300Receiver Tank Sizing

EX: A compressor unit is required to provide a flow of 50 scfm to a system for 10 minutes while the compressor sits idle. What size receiver tank is required? The system must operate between 125 and 90 psig.

Using the tank above, determine the time to recharge the receiver tank from 90 to 125 psig if the compressor delivery flow is 150 scfm. Assume the demand remains constant at 50 scfm.

𝑡=𝑉 𝑅 ∙ (𝑝𝑀𝐴𝑋−𝑝𝑀𝐼𝑁 )14.7 ∙ (𝑄𝐷𝐸𝐿−𝑄𝐷𝐸𝑀 )

=210 ∙ (125−90 )14.7 ∙ (150−50 )

=5𝑚𝑖𝑛

were switched to avoid a negative. It represents the direction of net flow.

MECH1300Vacuum Pumps

Vacuum pumps – evacuate air from one side of an object, allowing atmospheric pressure to act on the opposite side.

They are like air compressors only work in reverse, most commonly a rotary vane type.

Vacuum generators use a venturi to create a vacuum.

Venturi – a pipe with a reduced diameter section in the middle called a throat.

The throat is designed so that the pressure drops below atmospheric pressure, creating a vacuum.

Vacuum port can be connect to suction cups….ect

MECH1300Vacuum Pumps

Advantages of vacuum generators:

1. They can be power by existing pneumatic systems

2. Reliable

3. Require no lubrication

4. Virtually maintenance free