Compressor lifespan and low pressure tanks?

[iain/hsm]

To be frank IMHO the final pressure or size of compressor is irrelevant.

Work, wear and tear on any make or model of compressor,
Is all about Tribology: Wear, tear, friction and lubrication.

On the lower end, the crankshaft con rods etc is all about RPM, against gas pressure and rod load.
The loading on the bearings by the RPM

On the upper ends, work wear and tare is all on the pistons, and piston liners.
The piston speed and the piston rod stroke by the rod length and piston diameters.
coupled by the interstage compression ratio, gas velocity (speed) and gas density.

Work on the motor is the number of stages over the piston diameter (rod load)
and compression ratio by the gas density that dictates wear.
Increasing pressure just increases Kw power torque required from the motor.

Changing gas density from say air to helium lowers the Kw power required but increases
interstage temperature, P1V1T1 etc

For long life. The lowest block RPM, a balanced and low compression ratio, a balanced and equal rod load
per stage and a balanced heat dissipation coupled with a low approach interstage temperature and low fiction piston speed is the goal.

On reason why scuba compressor suppliers are reluctant to advertise there RPM with piston diameter,
piston stroke length and blow-by. Mostly only quoting the flow, max pressure and charging rate. Iain Middlebrook

 

[Danseur]

Iain,

So whatever you have, change the oil(s), run it slow and cool, no?

 

[iain/hsm]

Give you an example on a commercial divers gas transfer compressor as i'm too chicken to do it on a scuba unit for fear of upsetting.

As a simple low pressure single stage design.
Say 108psig inlet 470 psi outlet and a flow of flow 82 cfm
2 by 2-5/8" diameter pistons running at 492RPM would achieve flow and pressure
would give a rod load of 2542 lbw each piston to the con rod.
and develop 325F in gas temp
and requires 15.73 HP
At a 3.96:1 gas compression ratio

If you lowered the inlet pressure from 108psi to just 99psi
for the same flow and discharge pressure using the same diameter pistons
a tiny 9psi pressure drop on the inlet would result in:
RPM increase to 531 RPM to make flow
A small temp increase to 342F (gas temp before cooling)
Power increases now to 16.8 HP
and a deal breaking 4.28:1 gas compression ratio.

Now same compressor but engineered for balance rather than cost
using the same two 1st stages but by adding 2 by 1-3/4" second stages
Same 99psi inlet pressure same 470psi discharge and same 82 cfm flow
and same 531 RPM
The temp drops massively to
1st stage 208F and second stage 230F
Power required is lower at 15.06 HP
Rod loads are halved to 1300lbs on 1st and 1147lbs on second
Gas ratio average over two stages is 2.07:1
1st stage 2.25:1
2nd stage 1.9:1

Now if your buying on cost alone for a two stage version of the same size pump.
Cost just jacked by 30%. Your call. Iain Middlebrook

---------- Post added ----------

Iain,

So whatever you have, change the oil(s), run it slow and cool, no?

Not really.

Say you halve the speed but now you also now halve the flow.
So your now pumping for twice as long.

And although you've lowered the heat generated
but you also lower the fan cooling speed
(probably to stall at half sped)
so you could in fact overheat

Theres no real advantage this way.

Now take say a 30 cfm air compressor 4 stage running at 1300rpm in a hot climate
or one with the same flow same 4 stages but running at 570RPM

One is cheaper than the other, but one lasts longer, with less service, and less running cost
You get the picture.

Better to use a larger piston diameter and a longer rod stroke to make the flow.
Not jack the RPM up to max alone. Iain Middlebrook