calculate saturated-gas loads for vacuum systems

8/11/2019 Calculate Saturated-Gas Loads for Vacuum Systems

1/3

r,

Calculate

Saturated-Cas

l,oads

for

r/acuum

Systems

Use

this

procedure

tu

find

the amount

of

noncondensable

gas

saturated

uitb

condensable gas

being

discbarged.

,r,

by

ii't'e

.::

sys

r_.SUt

,:0f(

,,,

,tO

r..nOl

r'em

::"

t

,;::::'

'

, $Yl

:i.r

l

$5tz

:,ce,

,&,+'it

COMMON

PROBLEM

in

designing

vacuum

systems

for chemical

processing applications

is the

calculation

of load

requirements. This

is es-

pecially

true

when dealing

with

ex-

isting

process

equipment. One of

the

key

factors is

an

accurate

deter-

mination

of operating conditions.

The

calculation of load is rela-

tively straightforward. To illustrate

the appropriate

procedure,

we

will

look

at a

representative case:

a

dis-

tillation

column

with

an

overhead

reflux condenser.

(The

reflux con-

denser may 0r may not be

followed

by

a

vent

condenser, see Figure l.)

The operating

parameters

needed

to size the vacuum

system

are

l.

the load

going

to

the vacuum

system

from

the

reflux

or

vent

con-

denser (if

present)

in

mass

flow

terms,

2.

the

pressure of the

load,

and

3. the

temperature of

the

load.

The

load

can be

considered as the

amount of noncondensable

gas

sat-

urated with the condensable gas be-

ing discharged from the

reflux

or

vent

condenser.

First calculate the pressure and

temperature

of

the gas

going

to

the

vacuum

svstem.

In

a

new

system,

this can

be cletermined

from

the

condenser design.

For

an

existing

system,

actual

measurements

can

be

taken.

C)nce

this

information

is

known,

the

nontondensable

gas

load

must

be

EDWARD

B.

MYERSON,

STOI(IS

VACUUM,

INI

calculated.

Unless a

reaction

occurs glands.

The procedure

is

as

follon's:

or some

carrier

gas

is injected into l. Evacuate

the systern to

about

the

system,

the amount of

noncon- 125 mm Hg

abs.

densable

gas

in the process

stram

2.

Isolate the

system

from the

vac-

can be

considered equivalent

to

the

uum

source

and

turn off the

vac-

amount

of air

leakage into

the

uum source.

equipment.

There are

several

meth- 3.

Record

the time

required for

a

ods

of determining

air leakage. change

in

pressure

(the

pressure

They

include

change

must be large enough

to

al-

l. a

rate-of-rise test

to

find

the air low

for

an accurate

time reading);

leakage

of an

existing

system

(1),

do

not

allow

the system pressur

to

2.

measurement

using an

air-leak- rise

above

380 mm

Hg

abs.

age meter

(1),

4.

Given the system

volume

,

the

3.

determination

via standard pressure change, and the elapsed

charts

based on system

size and op- time, calculate the air-leakage

rate

erating

pressure

(1)

or,

(at

70"F') via:

4.

calculations

based

on system

size,

fittings,

openings, and operat- L

--

O.39'lVLPlt

(1)

ing pressure

(2).

The

rate-of-rise

method

yields

a

reasonable

representation

of

the

air

leakage

of

a

system.

The

test,

how-

ever,

can

only

be

done on

a

system

that

can

be taken off

line, emPtied

of

process

fluids, and

then evacu-

atecl

to

a pressure

of

125

mm Hg

abs.

or

less. The test should

be

per-

f


2/3

noncondensable-gas

florv

rate

are

empirical

and.

thus,

not

as

rigorous'

ir-leakage

curves

are

Published

bv

the

Heat

Exchange

lnstitute,

see

Figure

2.

To

use

thse

t'urves.

both

system

volumc

and

operattng

I)res-

sure

data

are

reqtrired.

\\'hcn

using

ordinary

shaft

sels,

add

up

tn 5

lbih

to

allon'

for

additional

leakage'

The

final

method

is

basecl

on

both

system

size

and

the

number

and

sizes

of

flanges,

ports,

etc.

The

pro-

cedure

coniists

of

two

stePs

(2)'

First,

estimate

the

air-leakap;e

rate

according

to

vesset

size

using

the

fol-

lowing

equations:

Lr

=

l'2ArDL.P'26

(for

100

to

l0

tcrr)

be

understood;

the

concePt

states

that

the

total

pressure

is equal

to

the

sum

ofthe

paitial

pressures'

and

the

total

pressure

times

the

mole

frac-

tion

of

a gas

is

equal

to

its

Partial

Dressure.

'

T., .alculate

the

saturation

of

a

noncondensable

with

two

condensa-

bles,

first

calculate

the

partial

pres-

sure

of

the

noncondensable:

NOMENCLATURE

D :

seal

diarneter

(in.);

S

:

flow

of

saturated

vapor

(lb/h);

L

:

air

leakage

into

s1'stem

(lb/h):

L,

-

specific

leak

rate

tlb/h/in.):

Ll,

=

leaLage

rate

ol

fittirrgs.

ralre:'

seals,

etc.,

(lb/h/in

)

M

:

molesl

Mu,

:

molecular

weightl

N

:

low of

noncondensables

(lb/h):

P

:

pressure

(torr,

lnm

Hg

abs

):

=

rinre

(mlll)r

l'

:

sl'stem

vrtlume

(tr):

W

:

mass

flow

(Ib,,,/h)l

avg

=

average

c

=

condensablest

i

=

comportent

ii

n :

n()Ircotrdertsablcs:

p

=

partial:

t

:

t{)tal:

\'

:

\'ap()r

(cottclerrsables

at

tcllPer-

ature

].).

(6)

a

If

Lr

:

1i'DLsPosa

(for

10

to

I

torr)

(7)

The

total

air

leakage

is:

L,:L+[Lr

(8)

Once

the

air-leakage

rate

and non-

conclensable-gas

flow

have

been

de-

termined,

the

saturation

can

be

eas-

ilv

calculated.

For

a

simple

problem

o

air

or

other

dry

gas

saturated

with

Vstuum

Syslem

Its*,

tJ.,*m

ryrlem

lor

dirtillotion

column

wilh

ovetheod

telux

condenser'

L

:

O.l

06pl^,06

(2)

a

single

condensable.gas,

the

satura-

(forTti0tol00torr)tionamountlS(.al(.ulateovla

S

:

'Mzu,P,lMw"(P

-

P')

(9)

L

:

0.072po.o26yoorr

(3)

(for 100

to

l0

torr) When

there

is more

than

one

non-

condensable,

the

average

molecular

f

:

0.026P{'{r34y006

(4)

weight

of

the

noncondensables

(for

r0 ro

I

rorr)

(+)

"Hfii:

*

:1^l:

i::iil:

weight

is

t alculaterl

b1

'fht'

nt'xt

step

is to

determittc

the

air

leakace

frt.'ln

atl

valves,

'ittings,

u',,*

:

ll(14',lLIu/1\t'

,eals,

t..

'I-his

is

done

for

each

item

+

...

+ \4t,lN1trt,lW,)

(10)

usilrg

thc

frrllorn

ing

equati('n\

and

f

inrling

the

sllctific

leakage

rate.

L.,

For

applicati.ns

in'ol'ing

rnore

frorn

-lablc

1 :

than

one'condensable

and

at

least

one

n()n('ontlensablc,

the

technique

L,

:

3.98ri'Dls

(5)

is

sorncwhat

clif{erent'

lt

reqtrires

(for

760

to

100

torr;

that

the

concept

of partial

prejsut'c

Toble

l.

Eslimoles

or

speciic

leok

roles.

ComPonenl

Stotir

Seols

Threoded

(onnedions

(onvenlional

Gosket

Seols

0-tings

Thermolly

(ycled

Goskels

r

100"

Rolary

Seols

Potking

Glonds

lilethonicol

Seols

lsolotion

lolves

Plug

(ock

Bolt

Globe

Gole

Throllling

Yolves

Aaess

Porls

Yiew

Vtlindows

Specillt

leol

tole

(lb/h/in.)

0.015

0.005

0.002

0.005

0.018

0.03?

0.25

0.t0

0.01

0.02

0.02

0.01

0.2s

0.02

0.015

\l\R(

ll

l )l)l


3/3

,iii#

-r.l

$q

\

.^.sV

s9

t,/.1

,I

r

rL\

:1:

:i::

'::" :

;l'l

1E

-1

6(

llN

z\>,/

s

/

{

I

ra

i

t:-l

>:)

.,

,.,.:'

j

(:'

ar::-ii,:

.:i

-

.15'

{5

t:. it:

.i;:.il.:,;,,;,;

tigure

2.

ir-leokoge volues

or

commerciolly

lighl

syslems'

P,:

Prn

*

P*r

* P*:

-l-he

partiaf

pressure

of

a

condensrtbLe

when

saturated

is equal

to the

vaPor

pressure

of the

condensable

at

the

saturation

temperature

in a

mixture

of multiple

condensables.

So,

P,",

:

P,*r

(12)

the

n,

Fu,,

-

P,

-

P,,*,

- P,sr

(13)

Next.

calculatc

the

number

of rnolcs

of noncondensablcs:

(

I I

)

Third,

calculte

the

total

number

of

moles

Present:

Pr"lP,:

M,IM,

(15)

M,:

M"(P,lPu,,)

(16)

vent

condenser

to

the

vacuum

sls-

tern

because

both

the air

leakagc

and the saturatcd

\aPor

loads

arc

knon'n.

along

r+ith

thc prcssrrre

arrd

temperature.

To

fi

nish

vacuuln-svs-

tem-sizine

sim pl,v

involves

determin-

ing the

nutnber, tvpe,

and

size

o

lhe

required pumps.

r

LtTERATtTRE

CITEI)

l.

"Starrdarrls

ir

Steanr

.fct

Vat'rtrtttt

Systcnrs."

,1tlr

Etl..

Hcat

Exchange

II'rstitrrtc,

(.lcr

cllrrd

1

I

{)8t11.

2.

Rvans,

J.

L., and

D,

L. RoPer'

"Prrxtss

\';rt'ttunr

Systettt

l)esigtt

ancl

()pcrirtiorr."

l\'lc(irau-Hill^

Ncu

\irrk

(ll)tl(i).

then,

Fourth,

calculate

the

number

of

moles

of

each

condensable

present:

h,ilM,

:

P,;1P,

...

(17)

Finallr,

calt

ulate the

salttrali()ll

am()unt

of each condensable:

l{"'

: t14.75t''"

(18)

'I'his

procedure

applies

regardlcss

o{'the

quantity

of condensables

present.

It

allows

for

the

dett'rmitra-

iion

o'

the

load

frorn the

refltrr

or

lr|,,:

W,,lhLu"

(

l4)

calculate saturated-gas loads for vacuum systems

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