Rerating The Appliance - Blodgett MT3255 Series Service And Repair Manual

MT3255 and MT3270

RERATING THE APPLIANCE

Due to the lack of oxygen at higher elevations, the
unit may need to be rerated. (The orifice size may
need to be adjusted to accommodate different air
pressures at higher elevations.) If not rerated, in-
complete combustion may occur releasing Alde-
hydes and CO or Carbon Monoxide. Any of these
are unacceptable and may be hazardous to the
health of the operator.
To choose the correct orifice for different altitudes
several factors must be known:
1. Altitude
2. BTUs per burner
3. Manifold pressure
4. Correct orifice size at sea level
5. BTU value of the gas
The following are generally accepted heating
values:
A.) Natural Gas --- 1000 BTU/Cu Ft
B.) Propane --- 2550 BTU/Cu Ft
C.) Butane --- 3000 BTU/Cu Ft
6. Specific gravity
The following are generally accepted values
(Air = 1.0):
A.) Natural Gas --- 0.63
B.) Propane --- 1.50
C.) Butane --- 2.00
NOTE: For other gases contact your local gas sup-
plier for values.
Use the following formulas to calculate the correct
orifice:
Firing rate
1.
# of burners
BTU per burner
2.
Heating value of Gas
CuFt/Hr
3.
Specific Gravity Multiplier
4. Use TABLE F-1 from the National Fuel Gas
Code Handbook to determine the proper ori-
fice size at sea level.
NOTE: The sea level orifice size is needed to
determine the proper orifice at any
elevation.
5. Use TABLE F-4 from the National Fuel Gas
Code Handbook to determine the correct ori-
fice for the applicable elevation.
6. Use TABLE F-3 from the National Fuel Gas
Code Handbook to determine the specific
gravity multiplier.
EXAMPLE
Known factors:
1. Altitude = 5000 ft.
2. BTUs per appliance = 55,000
3. Number of burners = 2
4. BTU value of the gas = 900
5. Specific gravity = .50
Calculations:
55,000
1.
= 27,500 BTU per burner
2
27,500
2.
= 30.55 CuFt/hr
900
30.55
3.
= 27.77 Equiv. CuFt/hr
1.10
Using the tables in the National Fuel Gas Code
Handbook we can determine that:
1. Correct orifice size at sea level = #40
2. Correct orifice size at 5000 ft = #42
4---15
= BTU per burner
= CuFt/hr
= Equiv. CuFt/hr