Caution When Using a Longitudinal Baffle in the Shell Side
The following are potential problems when considereing using a longitudinal baffle in a new S & T heat exchanger:
1. The largest temperature drop across the long baffle is more than 250 F. Then the thermal efficiency is lost due to conduction across the long baffle. Check and make sure this has been taken into consideration
2. If the long baffle is not welded to the shell, the pressure drop across the long baffle is more than 7 to 8 psi. This will also lose thermal efficiency. The seal on the long baffle should be tested in the shop after fabrication.
Optimun Number of Tube Rows
The optimum number of tube rows is a function of the maximum acceptable temperature rise of th air side. There are three limitations and the smallest air rise of the three should be used.
The limitations are: 1. Limit the LMTD correction factor to a minimum of 0.9 for one tube pass - maximum air outlet temperature to be the same as the process side outlet temperature. Minimum temperature difference at the hot end to be 8 to 10 F. 3. Maximum air outlet temperature to be 300 F. if tension wound fins are used.
What Coil Diameter to Use to Start Design
When starting to design a coil or other single continuous tube heat exchanger, the diameter is unknown.
An example of this is an economizer in a heat recovery system. In this case it is desirable to have a single
flow path rather than using parallel paths where headers are required. The following gives guide lines for
liquids on a diameter selection:
| 1" tube |
3000-5000 # / tube / hr |
| 1 ¼" pipe |
5000-10,000 # / tube / hr |
| 1 ½" pipe |
10,000-17,000 # / pipe / hr |
| 2" pipe |
17,000-35,000 # / pipe / hr |
| 3" pipe |
35,000-70,000 # / pipe / hr |
| 4" pipe |
70,000-130,000 # / pipe /hr |
Excess Heat Exchanger Surface Problems
Excess surface doesn't always mean being safe. It can lead to control problems, pulsations
or freezing of condensate. Vaporization services and reboilers can particularly be a problem.
Provide a way to control the flow of the heat medium in a new plant. In an existing installation
without control, the boiling temperature difference may be so high, there is a complete flash of
the vapor. Then the liquid feed rushes in to replace it which results in pulsations that may give
downstream problems. The quickest solution is to either plug the tubes or put an orifice in the outlet
vapor line to restrict the flow.
Tube Count Calculation for S & T
If you don't have a tube count table for a shell and tube exchanger, the tube count can be calculated.
The following equation is good for any size tube on any tube pitch. It is primarily for situations where
there is not a need for allowance for bundle entrance and exit area.
Count = F [0.7854 x TC2 -(PLw +Do -P)(TC x Npl)]
P2
Where:
Do Tube o.d.
F 1.0 for square pitch
1.15 for triangle pitch
Npl Number of tube pass lanes(1 for two pass)
PLw Tube pass lane width(typical is 0.625 inches)
P Tube pitch
TC Bundle diameter - tube o.d.
For tube pass lane width for square rotated tube pitch use 1.414P -Do. The decrease in the number of tubes
due to bundle entrance and exit area could be allowed for by using a larger PLw.
Check Piping Connections When There is Under-perforance
When a heat exchanger is installed and it is not achieving the desired heat duty, the first thing to check
is the piping. Is the piping connected to the right sides? It may be piped up backwards. The worst case is
when the shell side has a viscosity more than approximately 3 cp. and there is no extra heat transfer
enhancement inside the tubing. This could cause the fluid, when piped to the tube side, to be in laminar
flow with its low heat transfer coefficient.
Choosing Fin Spacing
In waste heat applications, the fin spacing depends not only on the heat transfer but the cleanliness
of the exhaust gas. If the gas is fouled from soot or other fine particulates, use a maximum of 5 fins
per inch. For very dirty gases the fin spacing can be as low as 2 fins per inch. Usually there will be soot
if fuels heavier than diesel fuel are fired. The designer needs to know the source of the waste heat gas so
that he can make a decision on what fin spacing to use.
Suggestions for Low-Fins and Potential S & T bundleVibration
Tube bundles are more likely to vibrate if there is not a close clearance between the tubes and baffles.
Low-fins are more susceptible to vibration because of the valleys between the fins. Another factor that
makes them susceptible is that some low-fins are manufactured with the fin O.D. smaller than the bare ends.
Some suggestions if the design software shows that the bundle may vibrate are:
- Specify the low-fin tubing be bare where it passes through baffling.
- Specify a tight tube hole tolerance.
- Purchase tubing that has a fin O.D. the same as the bare ends.
HRSG Nozzle Size
For an estimate of the nozzle size entering and leaving a HRSG unit use:
D = 0.14 x SQRT (flow)
Where:
D = diameter of nozzle in inches
Flow = Gas flow in lbs/hr
This is based on a total of 0.8 inches of water
Allowable Pressure Drop Suggestions
If you are at a loss as to what allowable pressure drop to specify, here are some suggestions:
Gas 3 - 5 psi
Liquid 8 - 10 psi
Change of phase
Boiling
0.5 to 1.0 psi for greater than 10 % vapor
1.0 to 5.0 psi for less than 10 % vapor
Condensing operating pressure
Less than atmospheric 0.5 psi
Atmospheric to 25 psi 1.0 psi
25 to 50 psi 2.0 psi
50 to 150 psi 3.0 psi
150 psi + 3.0 to 5.0 psi
Hydrocarbon U Estimate (Air-Coolers)
In the preliminary design or checking of process air-coolers you need an estimate of the overall heat
transfer coefficient (U). An estimate that is based on fin surface can be made from the following:
Liquids
Rt = 0.165 x Sqrt (avg. tube viscosity) + 0.145
U = 1/Rt
Where: viscosity is less than 3 Cp.
Gases
Rt = 0.29 x Sqrt (100/OP) + 0.145
U = 1/Rt
Where: OP is operating pressure in PSIA
When to Slope Single Tube Pass Tubes in Condensing Service
At low vapor velocities it has been proven that even a slight downward slope of tubes gives a
significant increase in heat transfer. But this does not mean the larger the slope the higher
the heat transfer. The benefit of sloping stops at an angle of approximately 10o. A common case
of a condenser needing to have the tubes sloped is when they are operating near atmospheric pressure
and there is one tube pass. An example of this is a sulfur condenser. It has a low pressure drop usually
less than 0.5 psi. They typically are designed with a slope of 1/8 inch per foot of tubing.
Copyright © 2002-2011 Gulley Computer Associates
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