Brooks, Model sla5840 – Brooks Instrument SLA5840 User Manual

Page 27

4-5

Brooks

Model SLA5840

Section 4 Maintenance

& Troubleshooting

Installation and Operation Manual
X-PR-SLA5800-RT-eng
Part Number: 541B120AAG
August, 2009

It is generally accepted that the mass flow rate derived from this equation
is only accurate to ±5%. The sensor conversion factors given in Table 4-1
are calculated based on a gas temperature of 80° C and a pressure of one
atmosphere. The specific heat of most gases is not strongly pressure,
and/or temperature, dependent. However, gas conditions that vary widely
from these reference conditions may cause an additional error due to the
change in specific heat caused by pressure and/or temperature.

4-4 Orifice Sizing

The Orifice Sizing Nomograph, Figure 4-2, is used to calculate the control
valve's orifice size when changing any or all of the following factors from
the original factory calibration:
gas
operating pressure (inlet and outlet)
flow range
The flow controller's orifice is factory-sized to a preselected gas, operating
pressure and flow range. Note that the orifice is marked with its size in
thousandths of an inch corresponding with Table 4-2 showing the orifice
capacities for each standard size. When changing the aforementioned
factors, calculate the new orifice size by following the procedure and
example outlined in the following paragraphs.

Example: Determine the orifice size for the following conditions:
Gas:

Hydrogen

Flow Rate:

2,000 sccm

Outlet Pressure: 30 psig
Inlet Pressure:

50 psig

1. Determine the orifice Nitrogen equivalent flow rate (refer to Table 4-1).

Where:
Q

NITROGEN

Orifice Nitrogen equivalent flow rate (sccm)

GAS

Orifice Desired flow rate of the gas (sccm)

NITROGEN

Density of Nitrogen at 0°C

GAS

Density of the gas (taken at customer temperature)

GAS

Specific gravity of the gas (taken at customer temperature)

NITROGEN

Specific gravity of the Nitrogen at 0°C

Refer to Table 4-1 for densities.

NITROGEN

GAS

NITROGEN

GAS

NITROGEN