Heat transfer between two materials requires one material to be at a different temperature than the other. This temperature differential is the driving force behind heat transfer. When transferring heat between materials through a barrier (metal tube, etc.) the rate of heat transfer is affected by the thermal conductivity of the barrier. The barrier includes the vessel or tube wall and stagnant fluid films at the surface of the wall. When two fluids are flowing on either sides of this barrier, the surface area required to transfer the required quantity of heat is defined by the rate at which this barrier will transmit heat. These relationships are summarized in the heat transfer equation: Q = U A dT where Q, the rate of heat transfer, is expressed in terms of the overall heat transfer coefficient (U), the heat transfer area (A), and the temperature difference (dT).
The Shell and Tube heat exchangers are sized based on performance, pressure drop, and dimensional constraints. It can be used for any combination of liquid, gas, condensing, or boiling on either the shell or tube side (except phase change on both the sides and superheating or sub-cooling of either fluid). Accurate sizing can be done without supplying the thermal conductivity or heat transfer coefficient data. The program provides an easy way to size a custom heat exchanger to meet your specific needs (so that it can be more accurately specified to exchanger vendors for their sizing and process guarantee) or for rating existing heat exchangers for a new service.Empirical equations for film heat transfer coefficients are combined with heat balance equations and equations defining exchanger mechanical characteristics. The number of tubes in the heat exchanger are then increased until this combination of equations represents a valid heat exchanger design based on the available temperature driving force. This method allows you to perform heat exchanger sizing without knowing the heat transfer coefficient. All that is required are the incoming flows and fluid physical properties. For sensible heat transfer, the user must provide the flow rate of the secondary stream so that its temperature drop can be determined. Existing exchangers can be analyzed by entering the physical data, required flows, required heat transfer, and the existing heat exchanger physical specifications. If the number of tubes calculated by the program is less than the actual number of tubes available then the heat exchanger should meet the required demand. The amount of excess capacity can be estimated by increasing the flow rates until the calculated number of tubes equals the corresponding actual value.
Enter the required physical and mechanical data in the input data fields provided. The Properties button allows the user to import the file containing the properties of the fluid present on the primary side selected in the Option menu. The properties have to be determined in the property window accessible from the Smartdraw screen and then saved in a file before closing that window. Clicking on the column for the desired fluid (either shellside or tubeside) will also direct the properties to be imported into that column. Repeat this process for the other side of the exchanger (two property files required). Alternatively, the user may manually enter the properties in the respective input fields. Clicking the Calculate button initiates the computation and brings up the Heat Exchanger Capacity window (next section). The Convert button will load the Conversion Calculator, and the Specify button will transfer all pertinent sizing data directly to the Specification Writer.
In this window, enter the outlet conditions of the primary stream as the target values for the program. The program will also prompt the user for the flow rate or the latent heat to determine the total heat to be transferred by the heat exchanger. If the “Data Out of Range” error occurs, check to verify that enough energy is available in the secondary stream to provide the necessary heat transfer. The iterative indicator will appear after selecting the Proceed button. When the indicator equals 1.0, the correct number of tubes has been determined. If this value doesn't appear to be approaching 1.0 fairly quickly, it is likely that the limitations are too great on the exchanger to accomplish the required transfer in a reasonably sized heat exchanger. Select the Exit button to return to the data input/output screen so that the input can be modified (data will remain as entered).
When the Boiling Liquid option is selected from the Option menu, the window shown above appears when a computation is initiated. Enter the required data and select the boiling surface material (for boiling liquid option only) before clicking the Proceed button.