Heat Loss Calculations and Heater Selection Air and Gas

8/13/2019 Heat Loss Calculations and Heater Selection Air and Gas

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Technical

I-16

Low Temperature Heater Selection A

ypical heater selection for the previousexample might be a type CAB heater withfinstrip elements. Available 15 kW stockheaters include a CAB-1511 with chromesteel elements or a CAB-152 with iron sheathelements, both rated at 26 W/in2. From theproduct page, the face area of a 15 kW CABheater is 1.19 ft2:

Velocity (fps) = 450 ACFM =6.3 fps 1.19 ft2x 60 sec.

Estima ting Sheath Operating Tempe rature The maximum operating sheath tempera-tures for finstrips are 750F for iron and 950Ffor chrome steel. Using graph G-107S for iron

sheath finstrips, a 150F outlet temperatureand a watt density of 26 W/in2requires avelocity in excess of 9 ft/sec to keep sheathtemperatures below maximum permissiblelevels. With only 6.3 fps in the application, aCAB-152 heater with iron sheath elements isnot suitable. Using graph G-108S for chromesheath finstrips, approximately 3 ft/sec. airvelocity results in a maximum of 900F sheathtemperature. Since this is lower than theactual velocity of 6.3 fps, a CAB-1511 withchrome steel finstrips is an acceptable heaterselection. (Use graphs G-100S, G-105S,G-106S and G-132S for air heating with regularstrip and finstrip heaters.)

High Temperature Heater Selection TypeTDH and ADHT heaters with tubular elementsare recommended for high temperature ap-plications. Steel sheath tubulars may be usedwhere the sheath temperature will not exceed750F. Finned tubulars can be used in applica-tions up to a maximum sheath temperature of1050F. INCOLOYsheath tubulars may beused for applications with sheath temperaturesup to 1600F. Allowable watt densities for tu-bulars and finned tubulars can be determinedby reference to graphs G-136S and G-151-1through G-156-1.

Estima ting Sheath Operating Tempe rature

Select a heater for a high temperatureapplication with an inlet air temperature of975F and a velocity of 4 ft/sec. Since thetemperature is above 750F, an INCOLOYsheath must be used. Using graph G-152-1the allowable watt density is 11 W/in2forsheath temperatures of 1200F or 22 W/in2fortemperatures of 1400F. In this application,a stock ADHT heater2with a standard wattdensity of 20 W/in2can be used.

Note 2 Special ADHT duct heaters, deratedto the required watt density, can be suppliedwhen element ratings less than the standard20 W/in2are needed.

Technical InformationDetermining Energy Requirements - Air & Gas HeatingAir & Gas Heating

Air and gas heating applications can be dividedinto two conditions, air or gas at normalatmospheric pressure and air or gas under lowto high pressure. Applications at atmosphericpressure include process air, re-circulation andoven heating using duct or high temperatureinsert air heaters. Pressurized applicationsinclude pressurized duct heating and otherprocesses using high pressures and circula-tion heaters. Procedures for determining heatenergy requirements for either condition aresimilar except the density of the compressedgas and the mass velocity of the flow must beconsidered in pressurized applications. Selec-tion of equipment in both conditions is critical

due to potentially high sheath temperaturesthat may occur.

Determi ni ng Heat Requi rements for

Atmospheri c Pressure Gas Heating

The following formulas can be used to deter-mine kW required to heat air or gas:

Equation A

kW =CFM x lbs/ft3x 60 min x CpxT x SF3412 Btu/kW

Where:CFM =Volume in cubic feet per minute

Lbs/ft3

= Density of air or gas at initial temperatureCp=Specific heat of air or gas at initial

temperatureT=Temperature rise in FSF =Suggested Safety Factor

For quick estimates of air heating require-ments for inlet temperatures up to 120F, thefollowing formula can be used.

kW =SCFM x Tx 1.2 SF 3,000

Where:SCFM =Volume of air in cubic feet per minute

at standard conditions1(70 F atstandard atmospheric pressure)

3,000 =Conversion factor for units, time andBtu/lb/F

1.2 SF =Suggested safety factor of 20%

Graph G-176S When airflow (ft3/min) andtemperature rise are known, kW requirementscan be read directly from graph G-176S.Note Safety factors are not included.

Note 1 Based on an average density of 0.08lbs/ft3and a specific heat of 0.24 Btu/lb/F. Forgreater accuracy, use Equation A and valuesfrom the Properties of Air Chart in this section.

Process Air Hea ting Calculation Example A drying process requires heating 450ACFM of air1from 70F to 150F. The existingduct- work measures 2 ft wide by 1 ft high andis insulated (negligible losses). To find heatingcapacity required, use Equation A:

kW =450 ACFM x 0.08 x 60 x 0.24 x 80 x 1.2 SF 3412 Btu/kW

kW =14.58

Heater Selection

Finstrip(CAB heaters), Fintube(DH heaters)or tubular elements (TDH, ADH and ADHTheaters) will all work satisfactorily in lowtemperature applications. Finstrips or finnedtubular elements are usually the most cost

effective. Tubular elements are recommendedfor high temperatures. Once the desired typeof heating element is selected, the next stepis to calculate the air velocity and estimatesheath temperatures to verify that maximumoperating temperatures are not exceeded.Calculate the air velocity over the elementsand refer to allowable watt density graphs forestimated operating temperature.

Calculating Air Velocity Air velocity can becalculated from the following formula:

Velocity (fps) = Flow (ACFM) Area of Heater (ft2) x 60 sec.

Graph G-176SAir Heating

600

F

Rise

550

F

Rise

500F

Rise

450

FRise

400F

Ris

e

350F

Ris

e

300

FRise

250F

Rise

200

FRise

150F

Rise

100FR

ise

75FRise

50FRise

Based on Air Density of0.08 Lbs/Ft3 and a SpecificHeat of 0. 237 Btu/Lb/F

0

100 300 500 700 900 1100

Air Volume (Cubic Feet Per Mi nute)

17 0

15 0

13 0

11 0

90

70

50

30

10

10

Kilowatts

Chromalox Technical Documents


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Technical

I-17

Air & Gas Heating wi th Str ip and

Finstrip

HeatersCustom De signs Strip and finstrip heatersare frequently mounted in banks by the enduser. Graphs G-105S and G-106S on this pagecan be used in conjunction with other graphsto determine maximum watt density forvirtually any custom design low temperatureheating application.

Graph G-105S Strip HeatersTo use this graph:

1. Selectmaximum desired outlet airtemperature on line A.

2. Chooseeither chrome steel sheath or rustresisting iron sheath (points B) on thebasis of operating conditions.

3. Selectminimum anticipated air velocityon B. Note natural circulation is equalto approximately one foot per second.

4. Draw a straight line through points A and

B to a reading on C. Read maximumallowable watts per square inch from line C.

5. Selectdesired length heater with anequivalent watt density or less from theproduct page in this catalog.

Graph G-106S FinstripHeatersTo use this graph:

1. Selectmaximum desired outlet airtemperature on line D.

2. Chooseeither chrome steel sheath or rustresisting iron sheath (points E) on thebasis of operating conditions.

3. Selectminimum anticipated air velocityon B. Note natural circulation is equalto approximately one foot per second.

4. Drawa straight line through points D andE to a reading on F. Read maximum

allowable watts per square inch fromline F.

5. Select desired length heater with anequivalent watt density or less from theproduct page in this catalog.

Recomm endations for Custom Installations Strip heaters should always be mountedsideways in the ductwork with the narrowedges facing the air stream. The total numberof elements installed should be divisible by 3so that the heater load will be balanced on athree phase circuit.

Technical InformationAllo wab le Watt Den sity & Heater Selec tio n - Air He ating

Graph G-105S Str i p Heater Ai r Heati ng-Selecti on of Watt D ensi ty

Graph G-106S Finstr ipHeater Air Heati ng-Selecti on of Watt D ensi ty

W/In2

20

15

10

5

0A B C

OutletAirTemperature(F)

70 0

60 0

50 0

40 0

30 0

20 0

10 0

0

16

9 41

169

4

1

AirVelocity(F.P.S

.)

AirVelocity(F.P.S.)Iron Sheath

Chrome Steel Sheath

W/In2

25

20

15

10

5D E F

Outl

etAirTemperature(F)

70 0

60 0

50 0

40 0

30 0

20 0

10 0

0

169

4

1

AirV

elocity(F.P.S.)Iron Sheath

Chrome Steel Sheath

30

16

9

4

1

AirV

elocity(F.P.S.)


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Technical

I-18

1F.P.SorFreeAir

4F.P.S.

9F. P.S.

16F.P.S. DistributedAirVelocity

4F.P.S.

9F.P.S.

16F.P.S. DistributedAir

Velocity

1F. P.S. orFreeAir

1F.P.S. orFreeAir

4F.P.S.

9F.P.S.

16F. P.S. DistributedAirVelocity

16

F.P.

S.

9F.P.S.

4F.P.S

.

1F.P.

S.

Still

Air

Graph G-100S Str ip Heater (Chrome) Ai r Heati ng

Allowable Watt Densit ies for 1000F Sheath Temp.

4F. P. S.

9 F.P.S.

1F.P.S. orFreeAir

16F.P.S. DistributedAirVelocity

26

24

22

20

18

16

14

12

10

8

6

4

2

0

WattsPerSquareInch(W/In2)

0 100 200 300 400 500 600 700 800 900 1000Outlet Air Temperature (F)

When Calculating Heater Capacity, Use theMaxim um Outlet Temperature and the LowestAir Velocity. For Close Grouping of Heaters,

Use 80% of the Calculated Value.

NotesStrip Heaters


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Technical

I-19

800F

1000F

600F

640F

400F

200F

1200F

1000F

800F

640F

600F400F

200F

200F

400F

600F

700F

800F

1000F

1200F

1400F

200F

400F

600F

800F

840F

1200F1400F

1000F

200F

400F

800F

840F

1000F

1200F

1400F

600F

400F200

F

600F

800F

910F

1000F

1200F

1400F

Graph G-154-1 Fint ube& Tubular H eaters

Sheath Temperatures wi th 16 FPS Distr ibuted

Air Veloci ty

80706050403020

100

W

attDensityinW/In2o

fEffectiveLength

FinnedTubularUnits

0 200 400 600 800 1000 1200 1400

Outlet Air Temperature (F)

TubularUnits

Limit for FinnedTubular for Normal

Life Expectancy

40

35

30

25

20

15

10

5



Air Veloci ty

8070605040302010

0

WattDensityinW

/In2o

fEffectiveLength

FinnedT

ubularUnits

0 200 400 600 800 1000 1200 1400


T u b u

l a r U n i t s


Life Expectancy

40

35

30

25

20

15

10

5



Air Veloci ty

8070605040302010

0WattDensityinW/In2o

fEffectiveL

ength

FinnedTubularUnits

0 200 400 600 800 1000 1200 1400


TubularUnits

40

35

30

25

20

15

10

5


Life Expectancy

0 200 400 600 800 1000 1200 1400

Technical InformationAllo wab le Watt Den sity & Heater Selec tio n - Air He ating

Graph G-151-1 Fint ube& Tubular Heaters


Air Veloci ty

80706050403020

100

W

attDensityinW/In2o

fEffectiveLength

FinnedTubularUnits

0 200 400 600 800 1000 1200 1400


TubularUnits

40

35

30

25

20

15

10

5


Life Expectancy

Graph G-152-1 Fint ube& Tubular Heaters Sheath

Temperatures wi th 4 FPS D istr ibuted Air Veloci ty

8070

605040302010

0

WattDensityinW/In2o

fEffectiveLength

FinnedTubularUnits


TubularUnits

40

35

30

25

20

15

10

5


Life Expectancy



Air Veloci ty

8070605040302010

0

WattDensityinW/In2o

fEffec

tiveLength

FinnedTubularUnits

0 200 400 600 800 1000 1200 1400


TubularUnits

40

35

30

25

20

15

10

5


Life Expectancy


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Technical

I-20

Air & Gas Heati ng Cryogeni cs

Industrial gases are usually stored in a liquidstate with heat being added to vaporize andboil off the gas as usage requires. Generalheat equations apply except that pipes, tubesand vessels containing the cryogenic fluid orgas frequently represent a heat source ratherthan a heat loss. If the size and materials ofthe tanks or vessels are known, then heatcalculations for the temperature rise can beperformed as in standard vessel heating orboiler problems. The following example istypical of a cryogenic heating application.

Problem Vaporize and preheat 30,000SCFH of liquid Nitrogen (N

2) from -345F to

70F at atmospheric conditions. The proper-ties of N2from Cryogenic Gas Tables are:

Boiling point, -320F Specific heat Btu/lb/F =0.474 (liq.), 0.248 (gas) Latent heat of vapor-ization =85.7 Btu/lb Atm. density of N

2at 32F

=0.0784 lb/ft3.

Solution Amount of liquid N2to be vaporized

30,000 SCFH x 0.0784 lb/ft3=2,352 lbs/hr

1. Raiseliquid from -345F to -320F (boilingpoint) T=25F.

kW =Wt xCpxTx SF 3412 Btu/kW

Where:Wt =Weight of material in lbsCp=Specific heat of the liquid N

2T=Temperature rise in FSF =Suggested safety factor of 20%

kW =2,352 lbs x 0.474 x 25 x 1.2 =9.8 kW 3412 Btu/kW

2. Vaporizethe liquid N2

kW =2,352 lbs x 85.7 x 1.2 =70.9 kW 3412 Btu/kW

3. Raisethe temperature of the N2from

boiling point -320F to 70F T=390F.

kW =2,352 lbs x 0.248 x 390 x 1.2 =80 kW 3412 Btu/kW

Total kW/hr required =9.8 +70.9 +80 =169.7

Equipment Recommendations Generally,cryogenic applications utilize both a vaporizerunit and a gas preheater. High watt densityheaters immersed in the cryogenic fluid canbe used for the vaporizer. Standard circulationheaters and watt densities are recommendedfor gas preheating. Protect the heater termi-nals from frost and moisture with elementseals and liquid tight terminal covers.

45Max.

30Max.

30Max.

45Max.

Air Flow

4. TotalkW =2.86 +0.47 +0.70 =4.03 kW

5. For Oven Applications, add 30% to coverdoor losses and other contingencies. kWhrequired (including safety factor) is

kWh =kW =4.03 kW =5.37 kW x 1.3 =6.98 kW t 0.75 hrs

Equipment Recommendations Severalprocess air heaters, including strip heaters, fin-strips, bare tubulars or type OV oven heaters,are suitable for oven heating applications.

Pressure Drop for Process Air Heaters

The pressure drop through TDH and ADHprocess air heaters with bare tubular or finned

tubular elements, CAB heaters with finstripelements, and ADH and DH air heaters withfinned tubular elements will vary considerablydepending on product design and construc-tion. Chromalox sales engineering can providepressure drop calculations for virtually anyduct heater (or circulation heater) application.Graphs G-112S3, G-189S1, G-227-2, andG-227ADH on the following page provideguidance for estimating the pressure dropfor many Chromalox process air heaters1.Graph G-189S1 can be used for most finnedtubular applications providing the elements aremounted in a three or six row configuration.

Transitions in Ducts In some air distributionsystems, the duct heater may be considerablylarger or smaller than the associated ductwork.The duct heater can be adapted to different sizeductwork by installing a sheet metal transition.The transition must be designed so that theslope on the upstream side of the equipment islimited to 30 (see below). On the leaving side,the slope should not be more than 45.

Note 1 Contact the factory for pressuredrop calculations for duct heaters mountedlengthwise or in series and for GCH gas circula-tion heaters. These applications require specialcalculations for proper application and air

handler sizing.

Technical InformationDetermining Energy Requirements - Air & Gas Heating

Ma terial Recommendations Ordinarycarbon steel is subject to brittle fractureat temperatures below -20F and is gener-ally not recommended. Stainless steel, highnickel bearing alloys or aluminum alloys maybe used. Use Teflonfor gaskets asTeflonremains pliable at low temperatures.

Air & Gas Heati ng Batch Ovens

Most oven applications consist of heating workproduct inside an insulated enclosure. Heatloss calculations involve the determination ofthe heat requirements to heat the enclosureand work product using heated air circulatedby natural or forced convection. Any make upor ventilation air must also be considered. Thefollowing example outlines the calculation of

the heat required for a typical oven heatingapplication.

Problem An oven with inside dimensions of2 ft H x 3 ft W x 4 ft D is maintained at 350F.

The oven has sheet steel walls with 2 inches ofinsulation and is ventilated with 400 cfh (ft3/hr)of 70F air which exhausts to the outside toremove fumes. The oven is charged with 250lbs of coated steel parts on a steel tray weigh-ing 40 lbs. The process requires the parts to beheated from 70F to 350F in 3/4 hour.

Weight of steel =290 lbsSpecific heat of steel 0.12 Btu/lb/FWeight of air =0.080 lbs/ft3at 70FSpecific heat of air =0.24 Btu/lb/FTemperature rise =280FSurface losses with 2 inch insulation =18 W/ft2/hr at 280F temperature difference (GraphG-126S)Surface area of oven =52 ft2Time =3/4 hr (0.75)Airflow rate =400 ft3/hr

Solution

1. CalculatekWh required to heat metal.

kW =290 lbs x 0.12 Btu/lb/F x 280F =2.86 kW 3412 Btu/kW

2. CalculatekWh required to heat ventilated air

kW=400 cfh x 0.080 Lbs x 0.24 Cpx 280 T x 0.75 t=0.47kW3412 Btu/kW

Where:cfh =Air flow rate (400)Lbs/ft3=Density of air (0.080)Cp=Specific heat of air (0.24)T=Temperature rise (280)=Time in hours (0.75)

3. Calculatesurface losses. Since the oven isalready at temperature, losses are at fullvalue.

kW =18 W/ft2/hr x 52 ft2area x 0.75 hr=0.70 kW 1,000 W/kW

Recommended D imensions

for D uct Transi ti ons


6/7

Technical

I-21

ADHT

M=9.5

"ADHM

=18.375

"

ADHM=9

.5"

ADHTAir

Heaters

ADH

AirH

eater

s

Pr

essureDropI

nchesofWater

(Psi=

InchesofWaterx0.0

361)

2. 0

1. 0

0. 7

0. 5

0. 3

0. 1

0.07

0.05

0.03

0.01

Thre

eRow

sC

rosswis

e

AnyR

ati

ng

Lengthwis

eT

DH

-24

Length

wis

eT

DH

-18

Leng

thwis

eTDH

-12

Lengthw

iseTDH-6

TDH Mounted Lengthwisein Duct

AirFlow

TDH-6TDH-12TDH-18TDH-24TDH Mounted Crosswisein DuctAirFlow

0.20

0.18

0.16

0.14

0.12

0.10

0.08

0.06

0.04

0.02

0

Thre

eRows

M=9.5"Six

Rows

M=18.

375"

0. 7

0. 5

0. 3

0. 1

0.07

0.05

0.03

0.01

PressureDropI

nchesofWater

(Psi=

InchesofWaterx0.0

361)

Graph G-189S1 Pressure Drop Vs. Veloci ty

FintubeElements and Ai r Heaters

0 5 fps 10 fps 15 fps 20 fps 25 fps 30 fps 35 fps 0 300 fpm 600 fpm 900 fpm 1200 fpm 1500 fpm 1800 fpm 2100 fpm

Air Velocity (Std. Air)

Note Contact factory for pressure drop calculations for finned tubular element airheaters mounted lengthwise in duct.

PressureDropI

nchesofWater

(Psi=

InchesofWaterx0.0

361)

Graph G-227ADH Pressure Drop Vs. Veloci ty ADH

and ADHT Tubular Element Ai r Heaters



ADH or ADHT MountedCrosswise i n Duct

AirFlow

Note Contact factory for pressure drop calculations for ADH/ADHT air heatersmounted lengthwise in duct and ADHT heaters where M is greater than 9.5"

Technical InformationDeter m ining Pressur e Dr op - Air and Gas Heat ing

600

500

400

300

200

100

0

Approximate

TerminalBoxTemperature(F)

Graph ADHTB ADH/ADHT Terminal Box Tempera-

tures Field Wi r ing Selecti on Guide

0 200 400 600 800 1000 1200


Data only valid for ADH or ADHT air heatersinstalled in bottom or sides of duct

Graph G-112S3 Pressure Drop Vs. Veloci ty Finstr ip

and CAB Ai r Heaters

1. 0

0. 5

0. 3

0. 1

0.07

0.05

0.03

0.01

0.007

0.005

0.003

0.001



Based on 1-5/8" spacingand 4.7 fins per linear i nch

Triple RowDouble RowSingle Row

PressureDropI

nchesofWater

(Psi=

InchesofWaterx0.0

361)

0 5 fps 10 fps 15 fps 20 fps 25 fps 30 fps 35 fps

0 300 fpm 600 fpm 900 fpm 1200 fpm 1500 fpm 1800 fpm 2100 fpm


Finned Tubulars MountedCrosswise in Duct

AirFlow

Graph G-227-2 Pressure Drop Vs. Veloci ty

TDH Tubular Element Air Heaters


7/7

Technical

I-22

Technical InformationDetermining Energy Requirements - Air & Gas Heating

Air & Gas Heating wi th

Cir culat ion Heaters

To calculate the heat energy requirements forheating compressed air or gases, the first stepis to determine the flow rate in pounds perhour. If the density of the air or gas under theactual pressure is known, the kW require-ments can be calculated directly. The followingexample illustrates this procedure.

Example Heat 20 ACFM of air at 30 psig from60F to 210F. From the Properties of Air Chart,the density of air at 60F and 30 psig is 0.232lb/ft3with a specific heat of 0.24 Btu/lb/F. ThekW required can be calculated from the formula:

kW =ACFM x lbs/ft3x 60 min x CpxT x SF3412 Btu/kW

Where:ACFM =Actual flow in ft3/min at inlet

temperature and gauge pressure (psig)Lbs/ft3= Actual density at inlet temperature

and gauge pressure (psig)Cp=Specific heat of air or gas at inlet

temperature and gauge pressure (psig)T=Temperature rise in FSF =Suggested Safety Factor

kW =20 x 0.232 x 60 x 0.24 x (210 - 60F) x 1.2

3412kW =278.4 lbs/hr x 24 x 150 x 1.2 =3.52 kW 3412

When the density and specific heat of a gas ata specific temperature and pressure are un-known, the actual flow rate can be con-verted to a known pressure and temperatureusing the physical laws of gases.

Example Heat 45 ACFM of Nitrogen (N2) at

35 psig from 50F to 300F. From the Physicaland Thermodynamic Properties of CommonGases Chart, the density of Nitrogen at 70F is

0.073 lb/ft3

with a specific heat of 0.2438Btu/lb/F. Convert 45 ACFM at 35 psig and50F to SCFM of Nitrogen at 70F using thefollowing formula:

SCFM =ACFM x Actual psia x Standard T 14.7 psia Actual T

SCFM =Std. ft3/min at 14.7 psia and 70FACFM =Actual flow in ft3/min at inlet

temperature and gauge pressure (psig)Actual psia =gauge pressure in lb/in2+14.7 psia14.7 psia =absolute pressure in lb/in2

T =Rankine (F +460)

SCFM =45 x (35 +14.7) x (70 +460) 14.7 psia (50 +460)

SFCM =158.1 ft3/min

Using the calculated SCFM in place of ACFM inequation A, the kW required is:

kW =158.1 x 0.073 x 60 x 0.2438 x (300 - 50) x 1.2 3412kW =14.8 kW

Determining Maximum Sheath

& Chamber Temperatures

When heating air or gases in insulated pipechambers or circulation heaters, the pipe wall

temperature will normally exceed the outletgas temperature. Excessively high wall and/orsheath temperatures can create an unsafe ordangerous condition. Maximum sheath andchamber temperatures can be estimated usingthe mass velocity of the gas and Graph G-237.In the above air heating example, assume a4.5 kW Series 3 heater rated 23 W/in2hasbeen selected. From Chart 236, the free crosssectional area of a Series 3 (3 inch) heater is0.044 ft2. Calculate mass velocity from thefollowing equation:

Mass Velocity =Flow lbs/hr 3,600 sec

(lbs/ft

2

/sec) Free area ft

2

hrMass Velocity =(278 lbs/hr)3,600 sec 0.044 ft2 hr

Mass Velocity =1.75 lbs/ft2/sec

On Graph G-237, locate the mass veloc-ity (1.75) on the horizontal axis. From thatpoint, locate a 23 W/in2curve. Read acrossto the vertical axis (sheath temperature riseabove outlet temperature) to 880F. Adding880F +210F (outlet temp.) =1090F sheathtemperature. Averaging the sheath and outlettemperatures (1090F +210F 2), yields amaximum chamber temperature of 650F.

Since the maximum chamber wall temperatureis less than 750F, a stock GCH heater with acarbon steel vessel and INCOLOYelementsrated 23 W/in2can be used.

3W/In2

6W/In2

9W/In2

12W/In2

15W/In2

18W/In2

23W

/In2

28W/In2

30W/In2

20W/In2

25W/In2

Graph G-237 Sheath Temperature Vs. Mass Veloci ty

Maximum

SheathTem

peratureRise

AboveOutletGasTemp.

(F)

0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

1400

1200

1000

800

600

400

0

200

Mass Vel ocity (Lbs/Ft2/Sec)

NotRecommended

Chart 236 Circulati on Heaters

Free Internal Cross Sectional AreaPipe BodyNom. IPS

(Std.)

TotalArea(Ft 2)

FreeArea(Ft2)

No.0.475"

Elements

2

3

0.023

0.051

0.018

0.044

2

3

5

8

0.139

0.355

0.124

0.303

6

18

10

12

0.566

0.785

0.481

0.696

27

36

14

16

18

0.957

1.268

1.622

0.847

1.091

1.357

45

72

108

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