Building Services Engineering Research & Technology

Heat transfer calculation across the external envelope of a building is, generally, made by assuming a hypothetical temperature for the outside environment called ‘sol-air temperature’. The sol-air temperature model has obvious limitations since it neglects the impact of factors of great significance such as the emissivity and configuration factors of the surrounding obstructions, the ground and the sky. Attempts made to include long-wave radiation exchange between building surfaces and the surrounding environment, have been over-simplified by assuming the surroundings to be at air temperature. However, without detailed information concerning the nature of such surroundings this assumption will remain unjustifiable and may lead to unreliable calculations of heat transfer. The thermal performance of a wall or a roof will vary with the effect upon its exposure to the sun, the ground, the sky and the proximity of obstructions and the air movement, as these may influence long-wave radiation and convection heat transfers. A new formulation for sol-air temperature prediction which allows for these factors is introduced and is presented in this paper. Numerical calculations were performed using monthly mean hourly data of the outside air temperature, wind speed and solar radiation for El-Oued in southern Algeria and for Bracknell in southern England. Results showed little difference between the present model and the sol-air temperature model for high wind speeds, whereas for low wind speeds the difference is significant.

The cooling load component of building-integrated photovoltaic (PV) walls has been investigated by numerical simulation of heat transfer across PV-walls. The cooling load component across the PV façades is predicted by the room transfer function method, based on the predicted heat gains for three cases at different locations (Beijing, Shanghai and Hong Kong). The simulation results are compared with the cooling load components of conventional walls that are directly exposed to the sun. The comparison indicates that the cooling load component reduction ratio due to photovoltaic integration ranges from 33% to 52% for a typical day in a year.

In compiling a revision to the CIBSE Guideon fluid flow in pipes and ducts, a survey was undertaken of available data on pressure loss factors of fittings. This paper details some of the contradictory information concerning components of ductwork. The primary source used by engineering institutions of several countries is that compiled by the Russian, Idelchik. His information is frequently far too detailed to be of use for day-to-day duct system design and so requires some interpretation and simplification. This the author has endeavoured to achieve whilst also using other sources where possible. Nevertheless this source was found to contain several errors. In the case of some of these errors, the true interpretation could readily be guessed at, but not always. Some other more recent research by others has also been found to contain contradictions. Even the use of a single preferred source does not guarantee uniformity of results. In one instance, two guides and the author provide three different sets of figures, each resulting from interpretations of the same primary source. In most published data there has been an assumption that friction effects of ductwork can always be presented in a non-dimensional manner. The implication is that size does not matter. Recent research shows that this is not so and confirms that as with pipework, size cannot be ignored for values of Ζ for elbows. Both size, and Reynolds Number are shown to have a large effect on the value of Ζ for elbows but neither have any significant effect on Ζ for tees. As to the question of variation in pressure loss due to minor manufacturing details, this has been tested only recently. Tests carried out on seemingly identical components but each obtained from a different manufacturer, showed a remarkable scatter. Although this was largely due to the results from the item supplied by just one manufacturer, this does illustrate the problem of predicting the pressure drop in any real ductwork system. This paper makes no attempt to include all the comparisons which were made. Rather, it highlights a few of the glaring contradictions and possible errors in hitherto accepted data. In trying to assemble data which is reliable and simple to use in day-to-day design work, the author has been presented with several dilemmas. The more contentious of these, and the final decisions taken, are explained.

The variation in the heat output of panel radiators obtained by altering the emissivity of the wall behind them has been examined. This work was conducted through experiments and Computational Fluid Dynamics (CFD). The results indicate that the presence of a high emissivity (black, such as the usual painted or wallpapered) surface to the wall increases the mass flow rate and air velocity behind the heat source compared to a reflective material. This is due to the radiation heat transfer to the wall creating an additional convecting surface behind the radiator. The results imply that the heat transfer rate can be increased by 20% through the use of a black instead of a reflective wall. The work concentrated on the air-gap behind the radiator, so these results will not be directly applicable to a normal radiator. An extrapolation indicates that the output of single bank (plate) radiator will be increased by 10% and a double radiator by 5%. Wall surface temperature results indicate that a reflective wall does indeed decrease the heat loss through the wall. The trend shown in the data obtained from the CFD analysis agreed well with the experimental results. The flow and temperature plots obtained from the CFD work help to explain the heat exchange and fluid flow processes that take place between the radiator and the wall. This understanding should lead the engineer to a better consideration of radiator placement and design.

This article provides an overview of energy loads associated with the application of powder coated aluminium profiles on the exterior wooden surface of double glazed windows. The entire powder coating process has been analysed to measure its energy and environmental contribution towards window cladding. Materials involved in aluminium clad windows have been analysed for their energy impacts. The energy embodied in a standard double glazed 1.2 m × 1.2 m aluminium clad window has been calculated, and is quantified as 1459 MJ, 1967 MJ, and 5.96 GJ respectively for argon, krypton and xenon infill gases. The energy consumed in powder coating the window cladding has been estimated to be 27 MJ and the total embodied energy of adding aluminium cladding to the window has been evaluated as 724 MJ.