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Estimate Overall Liquid Hydrocarbon Heat Transfer Rate(U) in S & T

In the preliminary design of shell and tube heat exchangers you need an estimate of the overall heat transfer coefficient (U). Process simulator programs give you a UA from which you can estimate the surface if you have a U value. An estimate for a all liquid hydrocarbon U value can be made from the following:

          

      Rt = Fouling + Sqrt(avg. tube viscosity)/150 
             +((avg. shell viscosity)0.27)/140
      U = 1/Rt

Fouling is the total for both sides. The above is limited to a maximum viscosity of 3 Cp. For the tube side. There is no limit on the shell viscosity. This is also limited to bare tube surface with no internal turbulation devices.

 

   


Mist Flow Boiling Inside Tubes

This is a flow pattern to avoid in heat transfer. The mist flow region is dependant upon velocity, % vapor and stratification effects. In this type of flow the tube wall is mostly dry and the liquid droplets are carried along in a vapor core. Therefore the heat transfer is much lower because the much higher thermal conductivity of the liquid is in very little contact with the tube wall. The higher the % vaporization, the lower the velocity needs to be to avoid mist flow. For example in a vertical tube where the vaporization is 50 % and the vapor density is 1.0 lb/cu ft, the velocity needs to be below approximately 80 ft/sec. If the vaporization is 75 %, the maximum velocity is approximately 30 ft/sec. This comes from the Fair equation. In a horizontal tube where there can be stratification, these maximum velocities are much lower. If the mist flow region can not be avoided, then twisted tape turbulators can be used to increase the heat transfer. They will throw the liquid in the vapor core toward the tube wall.

 

   


When to Consider a Long Baffle in the Shell

The cost curve for a shell and tube heat exchanger decreases with increasing surface. The curve flattens at about 6000 square feet of bare surface. If the first selection has multiple shells that are not countercurrent flow and each shell has less than 6000 square feet, consider using a long baffle for cost savings. This is especially true if the exchanger is of a type where the long baffle can be welded to the shell (less likely to bypass fluid).

 

   


Estimated Tube Length That Lowers Tube Pressure Drop

When the calculated tube side pressure drop exceeds the allowable, there are several design options. One option is to design with shorter tubing when the number of tube passes is one. To estimate the new tube length use the following:
           New Lg. = Lg. (Allowed Dp/Calc. Dp)1/3
           Where Lg = Length
           Dp = Tube pressure drop

The final tube length needs to be slightly longer than calculated because the calculated surface will be larger due to a lower tube velocity which gives lower heat transfer.

 

   


Avoid Small Baffle Cuts in S & T Condensers

There will be a theoretical liquid level when there is condensation in a heat exchanger. The condensing heat transfer coefficient decreases as its' liquid film increases. For best heat transfer the liquid level should be low as possible. Small baffle cuts in a shell and tube exchanger will hold a higher liquid level than large cuts. Use a separated flow model equation system to determine the theoretical liquid level. Unless you want subcooling, don't use a baffle cut that would hold a liquid level higher than the theoretical one.

 

   


Maximum Condensing Rate Inside Tubes

Following is a close estimate of the maximum heat transfer rate for total condensation. It is based on the maximum for the average hydrocarbon is 750 BTU/hr-ft2-F. It is good for other types of chemical compounds.

            Hi = 750 (Kliq / 0.07) (0.9)

            Where Kliq is the liquid thermal conductivity of the condensate
            For example this gives a maximum heat transfer rate for steam
            to be 3600.

 

   


Condenser Baffle Cuts

There will be a theoretical liquid level when there is condensation in a heat exchanger. The condensing heat transfer coefficient decreases as its' liquid film increases. For best heat transfer the liquid level should be low as possible. Small baffle cuts in a shell and tube exchanger will hold a higher liquid level than large cuts. Use a separated flow model equation system to determine the theoretical liquid level. Unless you want subcooling, don't use a baffle cut that would hold a liquid level higher than the theoretical one.

 

   


Reflux(Knockback) Condenser

Don't design this like the usual vertically condensing heat exchanger where both gas and liquid flow in one direction. In this type condenser the coldest condensate will be in contact with the entering hot vapor. Nearly everything about this type condenser is different. It is both difficult to design and difficult to control. The flow patterns, pressure drop and heat transfer calculations are different. Be sure the heat transfer calculations are zoned.

 

   


Horizontal vs. Vertical Baffle Cut in S & T Exchangers

In shell and tube heat exchangers it is safer from a thermal design standpoint to use vertical baffle cuts but horizontal cuts have an advantage in certain situations. Horizontal cuts are best if the shell side stream is clean and single phase. There will be less of the shell side stream by-passing through the tube pass lanes. Since in a multi-tube pass exchanger there will be more horizontal tube pass lanes than vertical pass lanes, you need to flow perpendicular to these pass lanes for minimum by-passing of the shell stream. This means horizontal cut. Where you do not want to use horizontal cut is when there is either condensing or where there is the possibility of foreign material being in the flowing stream It is suggested to use a maximum fouling factor of .002 for horizontal baffle cut. It may be possible to use horizontal cut in certain boiling applications.

 

 


Face Area for HRSG Units

The starting point in the design of a heat-recovery steam generator(HRSG) is the face area. This will determine the preliminary duct dimensions and starting face areas of any economizers and superheaters.

It is:

     Face area = #/hr exhaust gas / 2500.

     Where face area is in square feet.
This is based on using 2 inch O.D. tubing with 1 inch high fins The tubing is arranged on 4 1/8 inch triangular pitch.

 

 


Purchasing Shell & Tube Exchangers

It is to the benefit of purchasers of shell and tube heat exchangers, to not insist on their design. If the heat exchanger is to be built to TEMA requirements, it will void the guarantee. The last line of paragraph G5.2 says " The thermal guarantee shall not be applicable to exchangers where the thermal performance was made by the purchaser."

 

 


Vertical Thermosyphon - Check for Liquid Preheat Zone

When designing vertical thermosyphons with boiling at low operating pressures, check for the presence of a liquid preheat zone. Back pressure raises the boiling point at the interface of liquid preheat zone and subcooled boiling. This boiling point rise creates a liquid zone with relatively low heat transfer and it reduces the temperature driving force(MTD). If the operating pressure is below approximately 25 PSIA, there should be a liquid preheat zone. The lower the operating pressure, the more likely there is liquid preheat. If there is not liquid preheat there may be an input error.

 

   


Entrance and Exit Space for Shell Nozzles

There have been cases where not enough space was under the shell nozzles. This can be critical for applications like horizontal thermosyphons or other pressure drop sensitive applications. Check the distance from the nozzle I.D. to the nearest tube row or impingement plate. If there is an impingement plate this distance should be or more of the nozzle I.D. If there is no impingement plate this distance should be 1/6 or more of the nozzle I.D. If pressure drop is not a consideration and TEMA requirements are met and vibration is not a problem then the above calculated distance could be reduced. This criteria naturally doesn't apply to shells with distributor belts or where the nozzle is beyond the back of a U-tube bundle.


For information on calculating shell nozzle pressure drops refer to "Calculate Shell Nozzle Pressure Drop" in the calculation section.

 


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