## Thursday, 10 March 2011

### TUBE PITCH

The center-to-center distance between any two adjacent tubes is called the tube pitch. The ratio of the tube pitch to the tube outer diameter is called the pitch ratio. The value of the pitch ratio normally varies from 1.1 to 1.5, with the most common value being around 1.25. To avoid any ambiguity in the specification of the tube pitch, generally, a sketch should be provided along with the tube layout.

On What Factors Does The Number Of Tubes In A Shell Depend?

It depends upon the tube diameter, layout angle, pitch, type of tube bundle  (fixed, U-tube, floating head), outer tube limit, type of shell, umber of shell and tube passes, whether impingement baffle is provided, locations of entry and»exit nozzles and pass partition plates and the adjacent tubes, location of tie-rods, etc. The number of tubes in a particular case can be estimated from standard tables (Perry and Chilton, 1973; Kern, 1950).

Why Are Baffles Used?

Baffles are used to direct the shell-side and tube-side flows so that the fluid velocity is increased sufficiently (within the limits imposed by the allowable pressure drop) to obtain high heat transfer coefficients and to reduce fouling deposits. In horizontal units, the baffles also provide support to the tubes against sagging and vibration damage.

Overall Heat Transfer Coefficient

What Is The Relation For The Amount Of Heat Transferred In An Exchanger?

The relation is

Q = Ud Atlmtd Ft

Where
Q = amount of heat transferred per unit time
Ud = overall design heat transfer coefficient
A = heat transfer area based upon tube O.D
∆.lmtd = log mean temperature difference
Ft = correction factor to ∆Tlmtd FT multi-pass arrangements

Give the relation for Ud.
Ud is given by

Where
ho = outside heat transfer coefficient (1/ho is the resistance per unit area to heat transfer offered by the outside fluid film)
Rfo = fouling resistance on the outside of the tube
∆x = tube wall thickness
kw = tube wall thermal conductivity (∆x/kw is the resistance to heat transfer
offered by the tube wall)
Rfi = fouling resistance on the inside of the tube
Ao = outside tube surface area per unit tube length
Ai = inside tube surface area per unit tube length (Ao/Ai transfers the inside resistance to the common area based on the outside of the tube)
hi = inside heat transfer coefficient

For plain tubes, Ao/Ai = do/d

What Materials Of Construction Are Used In Heat Exchangers And Pressure Vessels?
Any component or the entire unit can be made of carbon-steels, alloy steels, copper and its alloys, nickel and its alloys, titanium, aluminum, tungsten, or other special alloys. Sometimes, nonmetallic materials, such as glass, Teflon, graphite, or composite materials are also used.

Discuss The Use Of Some Of The Important Materials?
Stainless steels, carbon- and low-alloy steels, nickel-chromium alloys, nickel-copper alloys, and admirality brass are among the important materials used in the heat exchanger and pressure vessel fabrication.

Stainless steels: Austenitic (300-series) stainless steels are the most frequently used materials in nuclear heat exchangers,, and are widely used in other heat ex­changers for chemical process industries as well Their corrosion rate is very low, and these can be produced in controlled composition. When carbon or low-alloy steels are also used along with austenitic stainless steel in the same equipment the stress-.relieving^ temperature of the veldt of the former lies in the sensitization zone for the latter, resulting in the redistribution of carbon and deterioration of the 'properties' of the stainless steel (Q.9.148). In such a case, one should use extra low carbon austenitic stainless steel Type 304L or stabilized grade such as Type 321. As it turns out, most exchangers of stainless steel do have carbon-steel or low-alloy steel com­ponents. Hence, generally Type 3Q4L, is used instead of Type 304.
Carbon and low-alloy steels:
These are very widely used in the shells, heads tube sheets and channels as well as in other parts, such as the baffles and tie-rods, In cases where," due to corrosion, stainless steel or some other expensive material has to be used, cost can be reduced significantly by using carbon- or low-alloy steels clad with stainless steel or other materials (see Q. 10.47 for cladding). In such a case, the strength is provided by the carbon- and low-alloy steels, and the cladding provides the resistance against corrosion. Thus, one does not need to purchase a thick sheet or plate of the costlier material. This practice is adopted both in the nuclear and nonnuclear usage of the heat exchangers.
Nickel-chromium-ion alloys (Inconel):
These have a very high percentage of nickel (76 Ni, 16 Cr, 8 Fe) and are used where the austenitic stainless steel tubes fail due to stress corrosion cracking or creep. This phenomena, due to the combined action of stress an corrosion, affects the stainless steel in caustic and chloride environments,
Copper-nickel alloys:
These are very frequently used for the heat exchanger tubes in seawater service due to their low corrosion rate and overall good performance.
There are some other materials, such as titanium, aluminum, and admirality brass that are used as the heat exchanger tubing depending upon the situation at ham.