Heat pipes design

J. E. Kemme, Heat Pipe Design Considerations, T-4221 -MS, Los Alamos Scientific Laboratory, University of California, Los Alamos, N.M., 1969. 

Heat Pipe Design Handbook, Dynatherm Corp., CockeysviUe, Md., 1972. 

Although worldwide production of heat pipes designed for applications involving the thermal control of electronic components or devices was in excess of 1,000,000 per year in 1992, it is difficult to calculate a mean time to failure (MTTF) for heat pipes, thermosyphons, and other similar devices due to the relatively small amount of data that exists on actual products in operation. Experience with a wide variety of applications ranging from consumer electronics to industrial equipment has demonstrated that mechanical cleaning of the case and wick-... 

HEAT PIPE DESIGN OF NPC THREE-LEVEL CONVERTER... 

Figure 9-61 H20 Heat Pipe Performance Capability for NGST Heat Pipe Design.159...

The accelerated heat pipe life test project was cancelled due to a shift in program direction with the selection of a gas cooled reactor concept to support nuclear electric propulsion. During execution of the project, a heat pipe design was established, a majority of the laboratory test equipment systems were specified, and operating and test procedures were developed. Procurements for the heat pipe units and all major test components were undenway at the time the stop work order was issued No technical Issues had been identified which would have prevented testing as planned. The final MSFC close-out report is provided in Reference 13-16... 

In many appHcations, especially in the chemical and semiconductor fields, the closest possible approach to isothermal operation may be desired. Under these conditions, the effects of vapor velocity must be considered if the velocity of the vapor exceeds about Mach 0.1, when a noticeable temperature differential shows itself in the heat pipe. If near isothermal operation is desired, designers restrict the vapor velocity to lower levels. 

Although there are several limits which apply to heat pipe operation, these generally lend themselves to specific design solutions or occur at sufficiently high levels of performance to permit a wide latitude of practical appHcations. The envelope of these limits is shown generically in Figure 3. 

Heat/Solvent Recovery. The primary appHcation of heat pipes in the chemical industry is for combustion air preheat on various types of process furnaces which simultaneously increases furnace efficiency and throughput and conserves fuel. Advantages include modular design, isothermal tube temperature eliminating cold corner corrosion, high thermal effectiveness, high reHabiHty and options for removable tubes, alternative materials and arrangements, and replacement or add-on sections for increased performance (see Furnaces, fuel-FIREd). 

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."... 

The lift pipe design was tapered to a larger diameter at the top. This minimized the effects of erosion and catalyst attrition, and also prevented the instantaneous total collapse of circulations when the saltation concentration, or velocity, of solids is experienced (i.e. the slump veloeity-that velocity helow which particles drop out of the flowing gas stream). In a typical operation, 2 % to 4 % eoke can he deposited on the catalyst in the reactor and burned in the regenerator. Catalyst circulation is generally not sufficient to remove all the heat of eombustion. This facilitated the need for steam or pressurized water coils to be located in the regeneration zone to remove exeess heat. 

Goodfellow, H. D. Hood Design for Ventilation Systems. Heating/Piping/Air Conditioning 59 (February 1987) no. 2, pp. 60-66. 

Compressible fluid flow occurs between the two extremes of isothermal and adiabatic conditions. For adiabatic flow the temperature decreases (normally) for decreases in pressure, and the condition is represented by p V (k) = constant. Adiabatic flow is often assumed in short and well-insulated pipe, supporting the assumption that no heat is transferred to or from the pipe contents, except for the small heat generated by fricdon during flow. Isothermal pVa = constant temperature, and is the mechanism usually (not always) assumed for most process piping design. This is in reality close to actual conditions for many process and utility service applications. 

Cooper, K W. and R. W. Erth, Centrifugal Water Chilling Systems Focus on Off-Design Performance, Heating/Piping/Air Conditioning, p. 63, Jan. (1978). 

Entrainment limit, heat pipe, 13 230 Entrainment rate, calculating, 11 814-816 Entrance span areas, in thermal design, 13 258... 

Reservoir designs, heat pipe, 73 236 Reservoir drug delivery systems, 9 77 Reservoir insulin delivery systems, 9 69 Reservoirs... 

The heat pipe has properties of interest to equipment designers. One is the tendency to assume a nearly isothermal condition while carrying useful quantities of thermal power. A typical heat pipe may require as little as one thousandth the temperature differential needed by a copper rod to transfer a given amount of power between two points. For example, when a heat pipe and a copper rod of the same diameter and length are heated to the same input temperature (ca 750°C) and allowed to dissipate the power in the air by radiation and natural convection, the temperature differential along the rod is 27°C and the power flow is 75 W. The heat pipe temperature differential was less than 1°C the power was 300 W. That is, the ratio of effective thermal conductance is ca 1200 1. 

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