As an approximation, use pressure loss coefficients of 1.0 for expansion and 0.5 for contraction in calculating nozzle pressure drops. The nozzle pressure drops are usually 15 to 25% Of the total pressure drop. Low allowable pressure drops use a higher percentage of the total pressure drop.
The velocity head term to use with the pressure loss coefficient is not a constant in determining nozzle sizes. The tube side uses higher allowable velocity heads.
The nozzle pressure drop for the shell side is difficult to calculate accurately. The type of tube pitch and the distance of the tube bundle from the entrance and exit have an effect. The presence of an impingment plate adds to the pressure drop. You need to refer to TEMA, as metengr in his post has mentioned, to find out the limits on velocity heads for various situations. TEMA also has equations for shell entrance and exit areas. They have a maximum velocity head of 1500 unless there is an impingement plate.
A starting point for tube side nozzles is a velocity head of 3000. There will be cases where higher velocity heads are used.
The normal way that heat exchanger nozzles are sized is that the end user (purchaser) or engineering contractor specifies the type, and size on the appropriate heat exchanger Specification Sheet that should ALWAYS accompanies a request for quotation and a purchase order.
The nozzles have to function in concert with the process conditions of temperature and pressure at both the inlet and the outlet – among other things. The nozzles also represent an entrance and exit pressure loss as well as a fluid distribution or drain tool. Only the user/owner – never the designer/fabricator – knows what pressure drop restraints he can tolerate across the exchanger – on both the shell and tube side.
If you are the design engineering contractor you know the fluid pressure you can deliver up to the exchanger nozzle – as well as the size of the piping that delivers the fluids to the exchanger. You also know what pressure drop you can tolerate or desire. You furnish the information to the fabricator with all the rest of your specifications and the fabricator will propose a method and possibly options that you can select as the proper exchanger for your application. The designer/fabricator of the exchanger can suggest/recommend nozzle sizes – but it is the end user who will decide whether or not to accept the proposal in accordance with what is needed. This is often the case when fluid distribution problems are foreseen due to the mechanical design and the fabricator needs to have over-sized nozzles to allow liberal distribution around the tube bundle. I’ve been involved in one case where the shell side pressure drop was very critical in a high vacuum condenser and the fabricator required over-sized nozzles in order to achieve an even and efficient vapor distribution around the tube bundle. Although infrequent, these cases do show up from time-to-time.
There are other cases where fabricators assume a nozzle size and build a “standard”, off-the-shelf heat exchanger. Usually, these are relatively small units and usually serve the utilities applications. Under these conditions the user accepts what could be higher pressure drops across the exchanger in light of quick availability and price of exchanger. Many times, as in waste steam condensers, it doesn’t matter that much.
Shell side nozzles are sized for both overall pressure drop and material erosion. A high entrance velocity would need a impigment baffle plate. This usually has a significant increase in the pressure drop. Purchase specifications may limit entrance velocity based on tube material. Normally, entrance velocity should not exceed 10 ft/sec
Tube side nozzles and inlet water box should be sized not to cause localized jetting, where tube velocities are not equal to the assumed average. This can impact calculated heat exchanger performance.
