COMPRESSOR SELECTION

Selecting a compressor to equal the peak flow requirement, when it is only for short

duration, obviously implies a larger capital expense as far as the compressor is

concerned, and larger electrical supply facilities.

Selecting the compressor on the assumption that every tool in the factory will be

used at the same time and at full power will have similar consequences. Power

authorities offer tariff structures to encourage reduction in peak electrical demand,

and may even have penalty tariffs if the demand exceeds what was negotiated as

the requirement, because demands that exceed what the local supply system was

designed to handle may cause voltage drops, or may open overcurrent circuit

breakers in the mains, either of which would annoy and inconvenience other

nearby electricity customers.

The manager of the factory you are designing the compressed air system for is keen

to have a low maximum electrical demand and has requested that a compressor

and receiver combination be designed to achieve this.

Mechanical Design $2$

School of Engineering $5$

RMIT Classification: Trusted

Question $2$

$[$Partner $1$ and Partner $2$ do jointly $3\mathrm{.}0$ marks$]$

a$)$ Determine the minimum capacity compressor that can cope assuming a

storage receiver will be used. Clearly explain your reasoning including

appropriate references to your graph in Question $1.$ Express your answer in

terms of required free air delivery capability.

b$)$ From the sample table of compressors provided at the e$-$reserve address

given in Chapter $2$ Appendix $2$A$,$ select a compressor that will satisfy $($a$)$

allowing for $4\%$ possible future decay in free air delivery capability as the

compressor wears. The normal working pressure required from the

compressor is $690$ kPa.g$.$ The factory manager states that no future extra

tools, or increase in load factor need be considered. Discuss the reasons for

your choice including any advantages and disadvantages and any warning

you think you should give the factory manager about the capacity of the

machine you have chosen in compliance with his$/$her aforesaid statements.

c$)$ Calculate the volumetric efficiency of the compressor you have chosen and

comment on what other efficiency figure it would be useful to have from the

maker in order to compare his product to others.

RECEIVER SIZE

The air receiver in a compressed air system has several functions namely: to reduce

pressure pulsation in the piping system; to help separate water and oil mist from

the air; to provide reserve capacity for times of peak load $($during which time a

small fall in pressure will occur$)$; and, in the event that compressor capacity exceeds

demand, to provide a buffer so that the compressor$$s unloading or stop$/$start

system does not cycle too often. Your design work will concentrate now on the

latter two functions.

The following pressure operating regime is to be designed for

kPa.g$.$ psi.g$.$

$??$ Safety valve$($s$)$ discharge $[$determined in a later

question$]$

$790115$ Compressor cycles off, or ceases to pump, if

pressure rises to this level.

$690100$ Compressor cycles on or recommences to pump

if pressure falls to this level.

$62090$ Pressure in receiver should not fall below this

level for proper functioning of tools $($given that

some frictional pressure drop will occur in the

piping$)$

$59085\mathrm{.}5$ Pressure arriving at tools should not fall below

this level. $($i$.$e$.$ maximum pipe frictional pressure

drop $=30$kPa$)$

kPa.ab$.$ psi.ab$.$

$101\mathrm{.}314\mathrm{.}7$ Atmospheric pressure

From this profile it can be seen that the compressor will normally be operating with

discharge pressures ranging from $620$ to $790$ kPa.g$.$ For the purposes of this

assignment the volumetric efficiency can be assumed, within this pressure range

$(620-790),$ to be the same as that achieved at $690$ kPa.g$.$

Mechanical Design $2$

School of Engineering $7$

RMIT Classification: Trusted

Question $3$

$[$Partner $1$ and Partner $2$ do $($a$)$ jointly $1\mathrm{.}0$ mark$]$

$[$Partner $1$ and Partner $2$ do $($b$)$ jointly $3\mathrm{.}0$ marks$]$

a$)$ Determine what volume receiver is necessary to cope with the peak caused

by the forge. Carry out this volume determination for the compressor you

have chosen in $2($b$).$ As a preliminary to this, recall the statement above that

the pressure in the receiver should not fall below $620$ kPa, and carefully

consider what the compressor might be doing when the forge peak

commences. Clearly explain the reasoning you have used in this volume

determination, including appropriate references to the graph you prepared