The Armfield HT10XC is a computer-controlled service unit, which supports several accessories. These provide a wide range of demonstrations into the modes of heat transfer, factors that affect heat transfer, and some of the associated problems.
The Armfield Computer Controlled Water Cooling Tower has been designed to give students an appreciation of the construction, design and operational characteristics of a modern evaporative cooling system. The unit is also an excellent example of a specialized heat exchanger, where two streams of fluid flow (water and air) are brought into direct contact and in which there is a mass transfer from one stream to the other.
This Armfield accessory has been designed to demonstrate the laws of radiant heat transfer and radiant heat exchange using light radiation to complement the heat demonstrations where the use of thermal radiation would be impractical. The equipment supplied comprises an arrangement of energy sources, measuring instruments, aperture plates, filter plates and target plates which are mounted on a linear track, in different combinations, to suit the particular laboratory teaching exercise chosen.
A long horizontal rod, which is heated at one end, provides an extended surface (pin) for heat transfer measurements. Thermocouples at regular intervals along the rod allow the surface temperature profile to be measured. By making the diameter of the rod small in relation to its length, thermal conduction along the rod can be assumed to be one-dimensional and heat loss from the tip can be ignored.
Analytical solutions are available for temperature distribution and heat flow as a function of time and position for simple solid shapes which are suddenly subjected to convection with a fluid at a constant temperature. Simple shapes are provided together with appropriate classical transient-temperature/heat flow charts which allow a fast analysis of the response from actual transient measurements.
The shell and tube heat exchanger is commonly used in the food and chemical process industries. This type of exchanger consists of a number of tubes in parallel enclosed in a cylindrical shell. Heat is transferred between one fluid flowing through the tubes and another fluid flowing through the cylindrical shell around the tubes.
The tubular heat exchanger is the simplest form of heat exchanger and consists of two concentric (coaxial) tubes carrying the hot and cold fluids. In these miniature versions the tubes are separated into sections to reduce the overall length and to allow the temperature at points along both fluid streams to be measured.
The plate heat exchanger is extremely versatile and commonly used in the food and chemical processing industries. Different combinations of plates and gaskets can be arranged to suit a particular application. The miniature exchanger supplied consists of a pack of plates with sealing gaskets held together in a frame between end plates. Hot and cold fluids flow between channels on alternate sides of the plates to promote heat transfer.
The Armfield Radial Heat Conduction accessories have been designed to demonstrate the application of the Fourier Rate equation to simple steady-state conduction radially through the wall of a tube. The arrangement, using a solid metal disk with temperature measurements at different radii and heat flow radially outwards from the centre to the periphery, allows the temperature distribution and flow of heat by radial conduction to be investigated.
A hot surface loses heat (heat is transferred) to its surroundings by the combined modes of convection and radiation. In practice these modes are difficult to isolate and therefore an analysis of the combined effects at varying surface temperature and air velocity past the surface provides a meaningful teaching exercise.
Radiative heat transfer between a thermometer and its surroundings may significantly affect the temperature reading obtained from the thermometer, especially when the temperature of a gas is to be measured while the thermometer ‘sees’ surrounding surfaces at a higher or lower temperature than the gas. The error in the reading from the thermometer is also affected by other factors such as the gas velocity over the thermometer, the physical size of the thermometer and the emissivity of the thermometer body.