Heat transfer pipe flow

Pressure drops from Dowtherm A heat transfer media flowing in pipes may be calculated from Figure 10-137. The effective lengths of fittings, etc., are shown in Chapter 2 of Volume 1. The vapor flow can be determined from the latent heat data and the condensate flow. With a liquid system, the liquid flow can be determined using the specific heat data. 

Dead time is also called transportation lag, because it is the time required for fresh heat transfer fluid to displace the contents of the exchanger and its associated piping. The dead time is the worst enemy of control, because until it has expired, a change in the heat transfer fluid flow (or temperature) will not even begin to have an observable effect. For a heat exchanger, the dead time is usually between 1 and 30 seconds. When the equipment is correctly designed, the dead time is much less than the time constant. 

Standard heat pipe is shown on Figure 1. Basic phenomena and equations are related with liquid-vapour interface, heat transport between the outside and the interface ( radial heat transfer), vapour flow and liquid flow. 

Y. He, Y. Jin, H. Chen, Y. Ding, D. Cang and H. Lu, Heat transfer and flow behaviour of aqueous suspensions of TiOa nanoparticles (nanofluids) flowing upward through a vertical pipe, Int. J. Heat Mass Transfer, 50, 2272-2281 (2007). 

Excess-liquid heat pipes operate in much the same manner as gas-loaded heat pipes, but utilize excess working fluid to block portions of the pipe and control the condenser size or prevent reversal of heat transfer. Vapor-flow-modulated heat pipes utilize a throttling valve to control the amount of vapor leaving the evaporator. In this type of control scheme, increased evaporator temperatures result in an expansion of the bellows chamber containing the control fluid. This in turn closes down the throttling valve and reduces the flow of vapor to the condenser. This type of device is typically applied in situations where the evaporator temperature varies and a constant condenser temperature is desired. 

Tube-Type Collector - A type of solar thermal collector that has tubes (pipes) that the heat transfer fluid flows through that are connected to a flat absorber plate. 

The heat-transfer phenomena for forced convection over exterior surfaces are closely related to the nature of the flow. The heat transfer in flow over tube bundles depends largely on the flow pattern and the degree of turbulence, which in turn are functions of the velocity of the fluid and the size and arrangement of the tubes. The equations available for the calculation of heat transfer coefficients in flow over tube banks are based entirely on experimental data because the flow Is too complex to be treated analytically. Experiments have shown that, in flow over staggered tube banks, the transition from laminar to turbulent flow Is more gradual than in flow through a pipe, whereas for in-line tube bundles the transition phenomena resemble those observed in pipe flow. In either case the transition from laminar to turbulent flow begins at a Reynolds number based on the velocity in the minimum flow area of about 100, and the flow becomes fully turbulent at a Reynolds number of about 3,000. The equation below can be used to predict heat transfer for flow across ideal tube banks. 

Low-Speed Units Low-speed units usually consist of jacketed pipes connected in series (by special return bends), where the heat-transfer medium flows in the annulus between the two pipes. The scraper assembly consists of a series of blades attached to springs (which hold the blades against the inner pipe wall) that in turn are attached to a central shaft driven at 15 to 50 rpm. Figure 11-22 illustrates this type of evaporator. 

Heat conduction, convection, boiling heat transfer, radiation, transient heat transfer, forced flow in pipes and packed beds, mass transfer by diffusion, and diffusion in porous solids. 

Several wick stmctures are in common use. First is a fine-pore (0.14—0.25 mm (100-60 mesh) wire spacing) woven screen which is roUed into an annular stmcture consisting of one or more wraps inserted into the heat pipe bore. The mesh wick is a satisfactory compromise, in many cases, between cost and performance. Where high heat transfer in a given diameter is of paramount importance, a fine-pore screen is placed over longitudinal slots in the vessel wall. Such a composite stmcture provides low viscous drag for Hquid flow in the channels and a small pore size in the screen for maximum pumping pressure. 

The cross-sectional area of the wick is deterrnined by the required Hquid flow rate and the specific properties of capillary pressure and viscous drag. The mass flow rate is equal to the desired heat-transfer rate divided by the latent heat of vaporization of the fluid. Thus the transfer of 2260 W requires a Hquid (H2O) flow of 1 cm /s at 100°C. Because of porous character, wicks are relatively poor thermal conductors. Radial heat flow through the wick is often the dominant source of temperature loss in a heat pipe therefore, the wick thickness tends to be constrained and rarely exceeds 3 mm. 

Heat transfer in static mixers is intensified by turbulence causing inserts. For the Kenics mixer, the heat-transfer coefficient b is two to three times greater, whereas for Sulzer mixers it is five times greater, and for polymer appHcations it is 15 times greater than the coefficient for low viscosity flow in an open pipe. The heat-transfer coefficient is expressed in the form of Nusselt number Nu = hD /k as a function of system properties and flow conditions. 

P). Otherwise the baffles should be located iaside the cod helix. A conventional jacket consists of a vessel outside the main vessel with a gap for the flow of heat-transfer fluid. Half-pipe jackets are usefld for high pressures up to 4 MPa (600 psi). They are better for Hquid than for vapor service fluids and can be easdy 2oned. Dimple jackets are suitable for larger vessels and process conditions up to 2 MPa (300 psi) and 370°C. Internal cods can be either hehcal or baffle cods (Fig. 34). 

The hydrocarbon gas feedstock and Hquid sulfur are separately preheated in an externally fired tubular heater. When the gas reaches 480—650°C, it joins the vaporized sulfur. A special venturi nozzle can be used for mixing the two streams (81). The mixed stream flows through a radiantly-heated pipe cod, where some reaction takes place, before entering an adiabatic catalytic reactor. In the adiabatic reactor, the reaction goes to over 90% completion at a temperature of 580—635°C and a pressure of approximately 250—500 kPa (2.5—5.0 atm). Heater tubes are constmcted from high alloy stainless steel and reportedly must be replaced every 2—3 years (79,82—84). Furnaces are generally fired with natural gas or refinery gas, and heat transfer to the tube coil occurs primarily by radiation with no direct contact of the flames on the tubes. Design of the furnace is critical to achieve uniform heat around the tubes to avoid rapid corrosion at "hot spots."... 

Economic Pipe Diameter, Laminar Flow Pipehnes for the transport of high-viscosity liquids are seldom designed purely on the basis of economics. More often, the size is dictated oy operability considerations such as available pressure drop, shear rate, or residence time distribution. Peters and Timmerhaus (ibid.. Chap. 10) provide an economic pipe diameter chart for laminar flow. For non-Newtouiau fluids, see SkeUand Non-Newtonian Flow and Heat Transfer, Chap. 7, Wiley, New York, 1967). 

Isothermal Gas Flow in Pipes and Channels Isothermal compressible flow is often encountered in long transport lines, where there is sufficient heat transfer to maintain constant temperature. Velocities and Mach numbers are usually small, yet compressibihty effects are important when the total pressure drop is a large fraction of the absolute pressure. For an ideal gas with p = pM. JKT, integration of the differential form of the momentum or mechanical energy balance equations, assuming a constant fric tion factor/over a length L of a channel of constant cross section and hydraulic diameter D, yields,... 

FIG. 6-31 Friction factors for condensing liqiiid/gas flow downward in vertical pipe. In this correlation F/pL is in fr/h. To convert fr/h to mvs, multiply by 0.00155. (From Bergelin, et al., Proc. Heat Transfer Fluid Mech. Inst., ASMF, 1949, p. 19. )... 

The half-pipe jacket is used when high jacket pressures are required. The flow pattern of a liquid heat-transfer fluid can be controlled and designed for effective heat transfer. The dimple jacket offers structural advantages and is the most economical for high jacket pressures. The low volumetric capacity produces a fast response to temperature changes. 

Vibrating-conveyor dryers are suitable for free-flowing solids containing mainly surface moisture. Retention is limited by conveying speeds which range from 0.02 to 0.12 m/s. Bed depth rarely exceeds 7 cm, although units are fabricated to carry 30- to 46-cm-deep beds these also employ plate and pipe coils suspended in the bed to provide additional heat-transfer area. Vibrating dryers are not suit le for fibrous materials which mat or for sticky sohds which may ball or adhere to the deck. 

The pipe has also been used for the transfer of heat between two immiscible liquids in cocurrent flow. For hydrocarbon oil-water, the heat-transfer coefficient is given by... 

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