Floating head tube-side fluid, flow

Tube-Side Passes Most exchangers have an even number of tube-side passes. The fixed-tube-sheet exchanger (which has no shell cover) usually has a return cover without any flow nozzles as shown in Fig. 11-35M Types L and N are also used. All removable-bundle designs (except for the U tube) have a floating-head cover directing the flow of tube-side fluid at the floating tube eet. 

The tube-side fluid now flows into the floating head, which acts as a return header for the tubes. The tube-side flow makes a 180° turn and flows back through the top hah of the floating-head tubesheet. The floating head is firmiy attached to the floating-head tubesheet. But why is it that one end of the tubes must be left free to float The reason is thermal expansion—or, more precisely, the differential rate of thermal expansion between the tubes and the shell. 

But this is not the case with a floating-head exchanger. The tube-side fluid reverses direction in the floating head. It has to. There is no way to attach the tube-side outlet nozzle to the floating head. It is a mechanical impossibility. So we bring the tube-side fluid back to the top half of the channel head. So, half of the tubes are in countercurrent flow with the shell-side flow. And that is good. But the other half of the tubes are in concurrent flow with the shell-side flow. And that is bad. 

Referring to Fig. 19.1, we can see how a floating-head exchanger works. The tube-side flow enters the bottom of the channel head. This assumes the cold fluid to be on the tube side. The cold fluid may be on the shell side or the tube side of an exchanger. The convention is to put the cold fluid nozzle on the bottom of the exchanger. Sometimes this is necessary. Sometimes it does not matter, but it is still the convention. 

Variable-Area Flow Meters. In variable-head flow meters, the pressure differential varies with flow rate across a constant restriction. In variable-area meters, the differential is maintained constant and the restriction area allowed to change in proportion to the flow rate. A variable-area meter is thus essentially a form of variable orifice. In its most common form, a variable-area meter consists of a tapered tube mounted vertically and containing a float that is free to move in the tube. When flow is introduced into the small diameter bottom end, the float rises to a point of dynamic equiHbrium at which the pressure differential across the float balances the weight of the float less its buoyancy. The shape and weight of the float, the relative diameters of tube and float, and the variation of the tube diameter with elevation all determine the performance characteristics of the meter for a specific set of fluid conditions. A ball float in a conical constant-taper glass tube is the most common design it is widely used in the measurement of low flow rates at essentially constant viscosity. The flow rate is normally deterrnined visually by float position relative to an etched scale on the side of the tube. Such a meter is simple and inexpensive but, with care in manufacture and caHbration, can provide rea dings accurate to within several percent of full-scale flow for either Hquid or gas. 

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