Thursday 10 March 2011

WORKING WITH HEAT EXCHANGER


Introduction:-


Shell-and-tube heat exchangers can handle a variety of services in a vast range of allowable design pressures and temperatures, from full vacuum to 6000 psig (41.5 MPa) and from cryogenic temperature to 2000°F (1100°C). Hence, these are the most versatile heat transfer equipment used in the process, power, and refrigeration industries.
In order to understand the design and operation of the shell-and-tube heat exchangers, it is important to know the nomenclature and terminology used to describe them and the various parts that go into their making. Only then can we understand without any anomaly, the design, communications, and reports on these units put out by re­searchers, designers, fabricators, and users.

Nomenclature And Parts: -

TEMA Standards (Tubular Exchanger Manufacturers Assoc., 1978) and HEI Standards (Heat Exchange Institute, 1980) have specified the nomenclature for the size and type of a shell-and-tube heat exchanger. Those according to the TEMA Standards are given below, while the HEI Standards' nomenclature is discussed in the next question.
The nominal shell diameter and the tube length designate size. In the British system of units, the nominal shell diameter is the inside diameter of the shell in inches rounded off to the nearest integer. However, for the kettle-type reboiler, the nominal port diameter precedes the nominal shell diameter. The tube length for a straight tube unit is the distance between the outermost tube sheet faces. For a U-tube unit, it is the distance between the outermost face of the tube sheet and the bend tangent, i.e., only the straight leg. In the Sl and the metric system, all the dimensions are given in millimeters (mm). The type of the exchanger is designated by a set of letters associated with the front end, the shell, and the rear end as given in Fig. 1.1 and illustrated by the following examples.

1.      Split-ring floating head (S) exchanger with removable channel and cover (A), single-pass shell (E), 23g in (590 mm) inside diameter with tubes 16 ft (4880 mm) long. Size 23-192 (590-4880), Type AES.

2.      U-tube exchanger (U) with bonnet-type stationary head (B), split flow shell (G), 19 in (480 mm) inside diameter with tubes 7 ft (2130 mm) straight length. Size 19-84 (480-2130), Type BGU.

3.      Fixed tube sheet exchanger with removable channel and cover (A), bonnet-type rear head (M), two-pass shell (F), 33g in (840 mm) inside diameter with tubes 8 ft (2440 mm) long. Size 33-96 (840-2440), Type AFM.

Special designs may be specified as best suits the manufacturer.

• In the discussion above, terms such as single-pass and two-pass shell have been used. These, respectively, mean that the shell-side fluid travels only once through the shell (single-pass) or twice, i.e., it enters at one end, travels to the other, and then returns to the end where it entered the shell (two-pass). Similarly, one can have multiple tube passes also. The number of tube passes is equal to or greater than the number of shell passes. In general, the multi-shell and tube passes are usually designated by two numerals separated by a hyphen, with the first numeral indicating the number of shell passes and the second numeral indicating the number of tube passes. Thus, a one-shell pass and two-tube pass AEL exchanger will be written as 1-2 AEL. It should be emphasized that this is not according to the TEMA Standards. They require the number of shell and tube passes to be spelled out as in the above examples.

The limits keep changing with larger and larger units being constructed. TEMA Standards (1978) have firm guidelines up to 60 in (1500 mm) shell diameter, while the "Recommended Good Practice" section in the TEMA Standards suggests guidelines up to 100 in (2500 mm) shell diameter. People are already talking about 120 in (3000 mm) shell diameter (Taborek, 1979), and soon the size may be limited only by the capability to transport the units.

Types Of Tube, Tube Thickness, Pitch, Layout Angle, And Tube Count: -

They are the following:
1. Plain tubes
2. Finned tubes
3. Duplex or bimetallic tubes
4. Enhanced surface tubes
Plain tubes are the cheapest per unit length and are the easiest to handle. Hence they are the first choice unless the special requirements of a particular situation prove them to be unsuitable or inefficient.
1. Straight tubes
2. U-tubes with a U-bend
3. Coiled tubes
The welded tubes are rolled into cylindrical shape from strip and material welded automatically under precisely controlled conditions. The seamless tubes are either extruded or hot pierced and drawn over a mandrill.
As the name implies, finned tubes have fins attached to the tubular surface. Fins can be longitudinal, radial, or helical and may be on the outside or inside or on both sides of the tubes (some are shown in Figs. 1.4 and 1.5). These are generally used when at least one of the two fluids is a gas.
Duplex or bimetallic tubes are in reality two tubes of different metals, one closely fitting over the other with no gap in between the two tubes. These are made by drawing the outer tube onto the inner one or by shrink fitting or by other mechanical means.
These are used where the corrosive nature of the tube-side and shell-side fluids is such that no one metal or alloy is compatible with the fluids. Thus, while the tube-side fluid touches one metal, the shell-side fluid is in contact with the other metal. The metals selected should be such that they will corrode at similar rates. Over 100 material combinations are available. An example is the use of steel outside and admirality or cupro-nickel inside the ammonia condensers using water in the tubes as a coolant.
These are generally used in condensers where the condensate drains into valleys (or troughs) leaving only a very thin film on the ridges (or peaks) resulting in an increased heat transfer coefficient. Some such tubes are also used in double-phase-change situations, where vapor phase condenses on one
The surface is prepared with a special coating to provide a large number of nucleation sites for use in boiling operations. The bubbles form at a low superheat.
Tubes should be able to withstand the following:
1. Pressure on the inside and the outside
2. Temperature on both the sides
3. Thermal stresses due to the differential expansion of the shell and the tube bundle
4. Corrosive nature of both the shell-side and the tube-side fluids

The thermal conductivity (or thermal resistance) of the tube plays an important role in equipment involving phase change (condensers, reboilers, etc.). Hence a tube material with large thermal conductivity and compatible with the above four points should be selected. The thickness should be specified. This is to take care of the thinning at the U-bend. The length and the number of tubes should be so selected as to give the required heat transfer area (based on the tube outer diameter) clear off the tubesheet, since the tube length in the tubesheet does not contribute to the heat transfer. 

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