Conductance of piping

Ai - inner surface area of the pipe, m2 d0 and di - outside and inside diameters of the pipe, m t - thickness of the insulation, m kp - thermal conductivity of pipe (usually steel), W/m.°C... 

K = thermal conductivity of pipe, BTU-in/hr/ft /°F Lr = minimum required length of coil, ft La = developed length of coil, ft LTD = least temperature difference, °F M = mass flow rate, Ib/hr N = number of turns in coil NFS = nominal pipe size of coil, in. 

Thermal conductivity of pipe material, kW/(m-°C) Thermal conductivity of surrounding, kW /(m-°C)... 

Measurement by Electromagnetic Effects. The magnetic flow meter is a device that measures the potential developed when an electrically conductive flow moves through an imposed magnetic field. The voltage developed is proportional to the volumetric flow rate of the fluid and the magnetic field strength. The process fluid sees only an empty pipe so that the device has a very low pressure drop. The device is useful for the measurement of slurries and other fluid systems where an accumulation of another phase could interfere with flow measurement by other devices. The meter must be installed in a section of pipe that is much less conductive than the fluid. This limits its appHcabiHty in many industrial situations. 

The effective thermal conductivity of a Hquid—soHd suspension has been reported to be (46) larger than that of a pure Hquid. The phenomenon was attributed to the microconvection around soHd particles, resulting in an increased convective heat-transfer coefficient. For example, a 30-fold increase in the effective thermal conductivity and a 10-fold increase in the heat-transfer coefficient were predicted for a 30% suspension of 1-mm particles in a 10-mm diameter pipe at an average velocity of 10 m/s (45). 

The third characteristic of interest grows directly from the first, ie, the high thermal conductance of the heat pipe can make possible the physical separation of the heat source and the heat consumer (heat sink). Heat pipes >100 m in length have been constmcted and shown to behave predictably (3). Separation of source and sink is especially important in those appHcations in which chemical incompatibilities exist. For example, it may be necessary to inject heat into a reaction vessel. The lowest cost source of heat may be combustion of hydrocarbon fuels. However, contact with an open flame or with the combustion products might jeopardize the desired reaction process. In such a case it might be feasible to carry heat from the flame through the wall of the reaction vessel by use of a heat pipe. 

The use of fixed plastic tanks is especially problematic for nonconductive flammable liquids because the efficiency of grounding aids (such as a conductive dip pipe or a submerged grounded metal plate) decreases as liquid... 

The overall heat transfer coefficient, U, is a measure of the conductivity of all the materials between the hot and cold streams. For steady state heat transfer through the convective film on the outside of the exchanger pipe, across the pipe wall and through the convective film on the inside of the convective pipe, the overall heat transfer coefficient may be stated as ... 

Heat transfer through a pipe wall. A pipeline parr 15m long carries water. Its internal diameter d, is 34 mm and its external diameter is 42 mm. The thermal conductivity of the pipe X is 40 W m K". The pipeline is located outdoors, where the outdoor temperature Oao is -8 C. Determine the minimum flow velocity necessary in the pipe to prevent the pipe from freezing. The heat transfer coefficient inside the pipe is = 1000 W m K and outside the pipe = 5 W m" K aiid = 4 W m -K . The specific heat ca-... 

It is very difficult to estimate the magnitude of the contact conductance G. Normally the total conductance of the heat exchanger is determined, and G - is calculated from Eq. (9.48). Only in the case that rhe plate fins are welded to the pipes with a metallurgical contact is the contact conductance infinite, leading to zero contact resistance, that is 1 /G,. = 0. 

The transfer of heat from one molecule to an adjaeent molecule while the particles remain in fixed positions relative to each other is conduction. For example, if a piece of pipe has a hot fluid on the inside and a eold fluid on the outside, heat is transferred through the wall of the pipe by conduc tion. This is illustrated in Figure 2-1. The molecules stay intact, relative to each other, but the heat is transferred from molecule to molecule by the process of conduetion. This type of heat transfer occurs in solids or, to a much lesser extent, within fluids that are relatively stagnant. 

A fire tube contains a flame burning inside a piece of pipe which is in turn surrounded by the process fluid. In this situation, there is radiant and convective heat transfer from the flame to the inside surface of the fire tube, conductive heat transfer through the wall thickness of the tube, and convective heat transfer from the outside surface of that tube to the oil being treated. It would be difficult in such a simation to solve for the heat transfer in terms of an overall heat transfer coefficient. Rather, what is most often done is to size the fire tube by using a heat flux rate. The heat flux rate represents the amount of heat that can be transferred from the fire tube to the process per unit area of outside surface of the fire tube. Common heat flux rates are given in Table 2-11. 

The team should be realistic about the time required to see improvements in end-of-pipe measures in most cases the pilot project success will be measured on efficiency improvements and other in-process measures alone. In this case it is important to demonstrate that all PSM and ESH issues are being managed. You should consider having a management systems audit (validation) conducted by a group independent of the integration project team. This may be done in conjunction with the next scheduled audit. This may be a corporate or divisional audit function or a consultant engaged specifically for this task. 

Figure 10-166B. Cross-sectional view of pipe and tracer with thermal conducting cement. (Used by permission Foo, K. W. Hydrocarbon Processing, V. 73, No. 1, Part. 1, 1994. Gulf Publishing Company.)...

L = wind velocity factor, Btu/hr-ft -°F ho = convective heat transfer coefficient, Btu/hr-ft °F hj = steam, heat transfer coefficient, Btu/hr-ft °F ko = thermal conductivity of insulation, Btu/hr-ft-°F L = length of pipe, ft n = number of tracers... 

Tjjj = temperature of ambient still air (°C) da = outside diameter of pipe (mm) d = outside diameter of inner layer of insulation (mm) d2 = outside diameter of next layer of insulation (mm) d = outside diameter of n th layer of insulation (mm) X = thermal conductivity of insulating material (W/mK)... 

As an alternative option, the insulation should meet the recommendations of BS 5422 1977. This Standard tabulates thicknesses of insulation too numerous to mention here, according to whether (1) the pipes carry central heating or domestic hot water, (2) the system is heated by gas and oil or solid fuel, (3) the water temperature is 75°C, 100°C or 150°C and (4) the thermal conductivity of the insulant is 0.04, 0.55 or 0.70 W/mK at the appropriate mean temperature. 

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