Introduction
Given the tube geometry and coil dimensions ,the "COILS" program predicts the thermal performance of various types of convective heat transfer surfaces with bare or finned tubes in inline or staggered arrangement and in parallel or counter flow configuration. By convective surfaces is meant is that external radiation from a furnace or flame is not considered,though non-luminous heat transfer coefficient is evaluated. Hence the program is suitable for predicting the performance of induvidual coils such as:
1.water tube boiler evaporator /HRSG evaporator
2.superheater
3.economizer
4.tubular air heater(single, multiple pass)
5.steam-air heater /air-cooled condenser
6.coil heating industrial heat transfer fluids such as therminol/glycol

Note:This is NOT a heat exchanger program. The fluid outside the tubes should be either air or flue gas and not a liquid. Fluid inside can be steam, air or flue gas, water, steam-water mixture or heat transfer fluid whose thermal, transport properties are known.
The program may be used to arrive at the geometry for a given duty or may be used to predict the performance of an existing coil. By playing with variables such as tube size,spacing,fin geometry,lengths,one can arrive at the optimum size for a new application which matches the duty and gas/fluid pressure drops desired.This is called the design mode. In the performance mode,one checks the thermal performance for a given coil,whose geometry and dimensions are known. For example the plant engineer may use it to see how the superheater in the HRSG behaves under different conditions of steam flow, steam pressure, gas flow or gas analysis. One can study if the economizer will “steam” under new or different operating modes or compare the actual performance with predicted and see if there is a possibility of fouling.Hence this program may be used for a variety of purposes.
The cooling or heating gas/air flows outside the bare or finned tubes while the fluid, such as steam/water/liquid/gas/air flows inside the tubes. All the tubes of the coil should have the same tube/fin configuration. If there are different configurations within the coil such as two different fin geometries or tube sizes, then the program may be run as many times as required, taking the results from each run and inputting them as data for the next run.
The "COILS" program computes several useful data for the coil as can be seen from the DEMO. In addition to the duty and exit fluid temperatures for a wide variety of bare/finned tube configurations, one can evaluate gas side pressure drop, tube wall and fin tip temperatures at inlet and exit of the coil,fluid velocities and heat transfer coefficients. With an economizer or superheater,for example, the tube wall temperature at both inlet and exit of the coil are computed so that concerns about acid dew point corrosion/material selection may be addressed. Steaming if present in an economizer is also computed. A wide variety of coils used in waste heat boilers, heat recovery equipment and boiler plants can be evaluated with this program.
The program runs on both British and Metric units. It runs on any Windows platform such as Windows 95,98, NT or later versions with as little as 16 MB RAM memory and on any processor. Hard disk space required is less than a MB.The DEMO has two examples in it. In order to run the DEMO,do the following:
1.Create a directory called VG
2.Download and save the file "coilpgmd.exe" to the VG directory.(click below to download file)
3.on clicking this file,all the files will expand.
4.run using the command,"c:\vg\coild"
5.There are two examples in the DEMO-Examples a and b. One for a superheater and one for an economizer.Dont change any data and run these examples.
Example a
Data:A HRSG superheater operates under the following conditions.
Flue gas flow=280000 lb/h
gas inlet temperature=950 F(analysis:% volume Co2=3,H2o=7,N2=75,O2=15)
gas inlet pressure=14.5 psia
fouling factors on gas/steam side=0.001 ft2hF/Btu
heat loss=2 %
steam flow=100,000 lb/h
Inlet pressure=500 psia
inlet temperature=saturation
Coil data: 2 x1.738 in tube,22 tubes/row,6 deep,10 ft long; fins:3 x0.625x0.05 solid. 4 in square inline arrangement, counter flow. Use 22 streams, correction factor=1.Fin conductivity is typically 20-25 Btu/fthF.
Solution:Enter the data as shown in the three forms .Select 1 for type in fluid data sheet. Click Run and results appear as shown. {bml exama.bmp} It is seen that:
Duty=9.94 MM Btu/h
steam temperature in=467 F,out=610 F
gas temperature out=819 F
steam pr drop=8.3 psi(coil only)
gas pr drop=0.83 in wc(tubes only)
surface area=2894 ft2
overall heat transfer coefficient=9.92 Btu/ft2hF
Exit tube wall temperature=703 F,fin tip temperature=766 F
Note that these results have no margins. One may use a duty of  say 9.8 MM Btu/h,steam pressure drop=10 psi, gas pressure drop=1.25 in wc etc, depending on guarantees, piping involved and additional gas/steam pressure drop etc.
By clicking the menu item Report,one can get a summary of results,which is also saved with extension .vgp.
Example b
This is an economizer; Metric units are used.
Data:!00,000 kg/h of flue gas at 300 C enters a coil.Tubes are 51x45 mm,30 wide,12 deep,4 m long with bare tubes. Tube spacing is 76 mm inline.70,000 kg/h of Feed water at 23 kg/cm2a and at 111 C enters the unit. Fouling factors are 0.0002 m2hC/Kcal. Predict the performance.
Solution
Enter the data as shown. Select 2 for fluid type.Enter the flow,temperature,pressure and fouling factor.Click Run .It is seen that the exit gas temperature=220 C,water exit temperature=141 C,duty=2.11 MM Kcal/h. Gas pressure drop=26 mm wc and water side pressure drop=.77 kg/cm2.Add margins to the results as required.
The program also estimates the % weight of steam formed in case the economizer steams.This situation is likely in gas turbine economizers at low loads.The program also prints out the results in the format shown below,which is useful for reporting.{bml report.bmp}
The file is saved in a text format with the extension “vgp” to the project name. For example “examplea” project would be saved as examplea.vgp. One can then import in into any document or report.

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