A novel low pressure technique for measuring the air tightness of buildings.

点击翻译按钮获取中文摘要

The importance of energy efficiency of buildings has been acknowledged in the UK in recent years by the Code for Sustainable Homes and substantial improvements to the Building Regulations. The adventitious leakage of buildings has received particular attention as standards in the UK have for many years been relatively poor. Most new buildings now have to show satisfactory levels of airtightness to comply with the Regulations. The airtightness of a building envelope is allied directly to the efficiency of the space conditioning, which is typically the main energy use. Therefore, reducing adventitious leakage can make a significant reduction in energy consumption. The method used presently to ascertain compliance with the Regulations is called the steady pressurisation technique. This technique has been used globally for many years, but does have a number of deficiencies; namely the difficulty and expense of testing large buildings, uncertainties due to wind and buoyancy factors, uncertainties due to experimental and measurement errors, and its inability to obtain results in the range of naturally occurring pressure differences.A novel technique is being developed at the University of Nottingham which accurately relates a known volume pulse to flow through the building envelope at pressures typically found in infiltration, hence significantly improving on the current steady technique. The underlying principle of the technique is to subject the building envelope to a known volume change in a short period of time (i.e. 1.5 s). This generates a flow rate through the adventitious openings that in turn creates a pulse in the internal pressure, characteristic of the building's leakage. The basic pulse concept has been tried previously by others, but has never before been made to work. There are three key novel features that have led to its success this time. The first is to measure the pressure difference across the building envelope a short time before and after the pulse, thereby allowing wind and buoyancy effects to be largely eliminated. The second is to generate a pressure pulse in such a way that a period of quasi-steady flow is obtained, thereby eliminating inertia effects associated with unsteady flow. The third is to minimise the variation of the pressure difference during the quasi-steady period such that flexing of the building envelope is not a problem. The quasi-steady period gives directly the leakage characteristic at low pressure and after adjustment to still air conditions and a small correction for the effective flow rate arising from compressibility of the air it can be plotted in the same way as the steady technique. Work to date has focused primarily on using a piston and cylinder to generate the volume pulse. This has worked well but preliminary tests have shown that a simpler, more portable design using a nozzle may also be possible. The first aim of the proposed project will be to research the piston technique further, as testing and analysis of specific factors is still needed to remove uncertainties. The second aim will be to explore the concept of a nozzle pulse technique. This has the most potential for marketing as a low pressure leakage tester, as it would make the system even more compact and portable, hence saving transport costs and space. To date, however, it has not been possible to measure the volume change accurately. It is intended to conduct a theoretical investigation of how this may be done and then, if the theory suggests it is possible, test it experimentally. The increased demand for leakage measurement coupled with the deficiencies of the conventional method means there is a genuine need for the described technique. The technique will be quicker, cheaper, more portable and more accurate than the conventional method. As such, the potential beneficiaries will be regulators, builders/developers, and leakage testing companies.