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Air Cooler Using Wind

Where cooling water is not available and the outlet temperature is not critical, an air cooler can be built that depends only on the wind for cooling. It will have the best performance when the tubes have high fins and the tubes are perpendicular to the wind direction. In areas where the wind does not have a prevailing direction, arrange the tubes in a bird cage type pattern. Then there is cooling no matter which way the wind blows. If there is a prevailing wind direction, use an air cooler bundle that sets on a stand that faces the wind.

 

 


Check Liquid Thermal Conductivity at High Reduced Temperatures

There have been instances where process simulators have given results where the liquid thermal conductivity was nearly the same as the vapor thermal conductivity when the reduced temperature was still significantly lower than the critical temperature. Examine carefully the liquid thermal conductivity when it's reduced temperature is above approximately 0.70. You may be able to justify a higher conductivity value and thus a higher heat transfer coefficient by using an independent and reliable correlation for the calculation.

 

 


Kettle Reboiler - Location of Vapor Outlet Nozzles

When it is necessary to have dry vapor leaving the kettle side, the location of the nozzles is important. The inlet nozzle should not be located directly under the vapor outlet. This probably results in some liquid carryover. When there is a single vapor outlet, it is usually centered over the bundle with the inlet nozzle located some distance away. There have been cases where someone other than the thermal designer changed the location of this vapor nozzle without the thermal designers OK. In one case the vapor outlet was moved to the back of the kettle resulting in appreciable liquid carryover

 


Evaluating a Shell & Tube Exchanger For a New Service

The best information to have for a shell and tube heat exchanger is a specification sheet and a full set of drawings. If b oth are not available, it is better to have the drawings. This is because they are more accurate on the mechanical details and they have tube layout details and seal bar information that the specification sheet does not have. What is most often missing on older heat exchangers is the bundle drawings. In this case, you need the original specification sheet. Then you can use it's data and simulate the shell side heat transfer and pressure drop by running a thermal design Program to get a baffle configuration. Then this is used with the new process data to evaluate the new service. This procedure will not be as accurate as having the exact baffling but it is the best you can do if this is all you have to work with.

 


When to Add Shells in Series

Usually you design for the least number of shells for an item. But there are times when it is more economical to add a shell in series to the minimum configuration. This will be when there is a relatively low flow in the shell side and the shell stream has the lowest heat transfer coefficient. This happens when the baffle spacing is close to the minimum. The minimum for TEMA is (Shell I.D./5). Then adding a shell in series gives a higher velocity and heat transfer because of the smaller flow area in the smaller exchangers that are required.

 


Fixed Tube Sheet Exchanger and High Shell Side Pressure Drop

When there is a design problem meeting the allowable shell side pressure drop, reverse the stream sides. Since it is a fixed tube sheet exchanger, the unit can be designed with one(1) tube pass. Other types of heat exchangers can be esigned with a single tube pass but they can have more operating problems. The pressure drop can be further reduced by using axial nozzles that are on the exchanger centerline. This eliminates large turning pressure drop losses.

 


Temperature Limit of Wrap-On Fins for Aircoolers

Above a certain temperature, it will be too hot for wrap-on fins. Due to thermal expansion, the aluminum fins will lose good contact with the tubing. In this case an integral type fin tube should be used. The summer time air outlet temperature is a very rough approximation. To be more exact the tube wall temperature needs to be calculated for the hottest tube row. Then:

     Twall = Ta +(Th1 - Ta) x Ro X Uc

 Where  Twall = temperature of tube wall
              Ta     = air outlet temperature
              Th1   = temperature inside tube
              Ro    = thermal resistance of air
              Uc    = clean overall heat transfer coefficient

Example: Steam condensing at 488 F. Assume Uc is 7.5 and Ro is 0.12.
                If the air ourlet temperature is 200F, then:
                Twall = 200 + (488 - 200) x 0.12 x 7.5
                Twall = 459 F

As you can see, the problem is more severe at high heat transfer rates. Not even the aircooled manufacturers agree exactly what this maximum tube wall temperature should be. The ASME code for allowable stress of aluminum has a maximum temperature of 400 F. I believe this is the upper limit. Then the above example is operating too hot for wrap-on fins.

 


Vibration Cure When Designing Shell & Tube Bundles

The cure depends upon whether it is flow induced or acoustical type vibration. Both types can be cured by using a lower cross flow velocity across the bundle. To do this, use double or triple segmental baffles. This can not only lower the velocity but the closer resulting baffle spacing increases the natural frequency of the bundle. Another possibility is to use a " No Tubes in Baffle Window" design. Then you can use as many baffle supports as necessary with very little effect on shell pressure drop.

If the vibration is the acoustical type, use either 30 degree triangular pitch or square rotated pitch. The former is the best. Another cure is to use a deresonating baffle. In a few cases, putting the problem stream inside the tubes would be better.

 


Specifying Pressure Drop For Heavy Liquids Inside Tubes

Frequently process engineers specify 5 or 10 psi for allowable pressure drop inside heat exchanger tubing. For heavy liquids that have fouling characteristics, this is usually not enough. There are cases where the fouling excludes using turbulators and using more than the customary tube pressure drop is cost effective. This is especially true if there is a relatively higher heat transfer coefficient on the outside of the tubing. The following example illustrates how Allowable pressure drop can have a big effect on the surface calculation. A propane chiller was cooling a gas treating liquid that had an average viscosity Of 7.5 cp. The effect on the calculated surface was as follows:


   Allowable tube pressure drop     Exchanger surface

   ----------------------------     -----------------

           5 psi                        4012  Sq. Ft.

          25 psi                       2104  Sq. Ft.

          50 psi                       1419  Sq. Ft.

You can see that using 25 psi pressure drop reduced the surface by nearly one-half. This would result in a price reduction for the heat exchanger of approximately 40%. This savings offset the cost of the pumping power.
 


Air flow accessories - don't overlook louvers and screens when calculatinh fan HP

Air static pressure loss is used to calculate the horsepower required for fans used in process air coolers. Charts and equations in the literature are usually for the tube bundle only. Frequently air coolers have accessories like louvers and fan guards. They may also have hail,bug, or lint screens. Don't overlook the accessory pressure drop because they can increase the static pressure as much as 25%.

 


Estimate Liquid Hydrocarbon Heat Transfer Coefficient In Tubes

Use the following equation to estimate the heat transfer coefficient when liquid is flowing inside 3/4 inch tubing:


       Hio = 150./sqrt(avg. viscosity)

    Where:

           Hio - BTU/Ft2-hr-F

           Viscosity - cp.

This is limited to a maximum viscosity of 3 cp
 


Calculate Tube Bundle Diameter

Following are equations for one tube pass bundle diameter when the tube count is known or desired:


30 Deg.   DS =  1.052 x pitch x SQRT(count) + tube O.D.

90 Deg.   DS =   1.13  x pitch x SQRT(count) + tube O.D.



Where:

      Count = Number of tubes

      DS    =  Bundle diameter in inches

      Pitch = Tube spacing in inches

	  

 


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