Temperature fluid

In petrochemical plants, fans are most commonly used ia air-cooled heat exchangers that can be described as overgrown automobile radiators (see HeaT-EXCHANGEtechnology). Process fluid ia the finned tubes is cooled usually by two fans, either forced draft (fans below the bundle) or iaduced draft (fans above the bundles). Normally, one fan is a fixed pitch and one is variable pitch to control the process outlet temperature within a closely controlled set poiat. A temperature iadicating controller (TIC) measures the outlet fluid temperature and controls the variable pitch fan to maintain the set poiat temperature to within a few degrees. 

Fig. 2. Fluid temperature profiles in (a) a parallel flow heat exchanger and (b) a counterflow heat exchanger. Terms are defined in text.

For heat exchangers other than the parallel and counterflow types, the basic heat-transfer equations, and particularly the effective fluid-to-fluid temperature differences, become very complex (5). For simplicity, however, the basic heat-transfer equation for general flow arrangement may be written as... 

Syltherm XLT is a polydimethylsiloxane intended for Hquid-phase systems which operate at low temperatures. Syltherm 800 is a modified dimethylsiloxane polymer intended for Hquid-phase systems. The recommended maximum fluid temperature is greater than the autoignition temperature. 

Fluid Temperature, °C Minimum velocity, m/s Pumping rate factor Pressure drop factor Outside tubes Inside tubes... 

When the heat duty requirement, is specified and the fluid temperature change, AT, is fixed, as a result of operating or equipment limitations, the required volumetric pumping rate from the heat balance is... 

The equations presented herein do not include any viscosity correction to reflect the difference between the viscosity at the wall temperature and the bulk fluid temperature. This effect is generally negligible, except at low temperatures for organic fluids having viscosities that are strongly temperature dependent. For such conditions, the values tabulated in Table 2 should be appropriately modified. 

There are also many empirical formulas used for calculatiag the friction head loss in piping systems. These must be used carefuUy because many are based on the properties of specific fluids and are not appHcable over a broad range of fluids, temperatures, and pressures. For example, the Ha2en and Wdhams formula widely used for water flow ... 

Ash Fusibility. A molded cone of ash is heated in a mildly reducing atmosphere and observed using an optical pyrometer during heating. The initial deformation temperature is reached when the cone tip becomes rounded the softening temperature is evidenced when the height of the cone is equal to twice its width the hemispherical temperature occurs when the cone becomes a hemispherical lump and the fluid temperature is reached when no lump remains (D1857) (18). 

The definition of the heat-transfer coefficient is arbitrary, depending on whether bulk-fluid temperature, centerline temperature, or some other reference temperature is used for ti or t-. Equation (5-24) is an expression of Newtons law of cooling and incorporates all the complexities involved in the solution of Eq. (5-23). The temperature gradients in both the fluid and the adjacent solid at the fluid-solid interface may also be related to the heat-transfer coefficient ... 

Equation (5-47b) is based on the work of Bays and McAdams [Jnd. Eng. Chem., 29, 1240 (1937)]. The significance of the term L is not clear. When L = 0, the coefficient is definitely not infinite. When E is large and the fluid temperature has not yet closely approached the wall temperature, it does not appear that the coefficient should necessarily decrease. Within the finite limits of 0.12 to 1.8 m (0.4 to 6 ft), this equation should give results of the proper order of magnitude. 

Design Temperature The design temperature is the material temperature representing the most severe condition of coincident pressure and temperature. For uninsulated metallic pipe with fluid below 38°C (100°F), the metal temperature is taken as the fluid temperature. 

With external insulation, the metal temperature is taken as the fluid temperature unless service data, tests, or calculations justify lower values. For internally insulated pipe, the design metal temperature shall be calculated or obtained from tests. 

Ash fusion temperatures, including the spread between initial deformation temperature and fluid temperature... 

The galvanic potential of metals can vary in response to environmental changes such as changes in fluid chemistry, fluid-flow rate, and fluid temperature. For example, at ambient temperatures steel is noble to zinc (as in galvanized steel). In waters of certain chemistries, however, a potential reversal may occur at temperatures above 140°F (60°C), and the zinc becomes noble to the steel. 

Hvp = the vapor pressure of the fluid expressed in feet of head. The vapor pressure is tied to the fluid temperature. 

The Hvp, vapor head, is calculated by ob.serving the fluid temperature, and then consulting the water properties graph in this chapter. Let s say we re pumping water at 50° F (10° C). The Hvp is 0.411 feet. If the water is 212° F (100° C) then the Hvp is 35.35 feet. The vapor head is subtracted because it robs energy from the fluid in the suction pipe. Remember that as the temperature rises, more energy is being robbed from the fluid. Next, we mu.st subtract the Hf... 

Here R=r is the parameter for radiative heat transfer in K units, p is a heat of reaction term, in K/atm units tj is the fluid temperature in the j-th axial position e is the particle emissivity 1 is the celt dimension in m 6 is the clock time in minutes... 

Common seal designs may handle fluid temperatures in the 0°F to +200 °F (—17°C—93°C) range. When temperatures are above the +200 °F... 

Approach temperature differences between the oudet process fluid temperature and the ambient air temperamre are generally in the range of 10 to 15 K. Normally, water cooled heat exchangers can be designed for closer approaches of 3 to 5 °K. Of course, closer approaches for air cooled heat exchangers can be designed, but generally these are not justified on an economic basis. 

In many eases, the heat flow (Q) to the reaetor is given in terms of the overall heat transfer eoeffieient U, the heat exehange area A, and the differenee between the ambient temperature, T, and the reaetion temperature, T. For a eontinuous flow stirred tank reaetor (CFSTR) in whieh both fluid temperatures (i.e., inside and outside the exehanger) are eonstant (e.g., eondensing steam), Q is expressed as... 

---