Assessing the performance of chilled ceilings

With the environment in which they are installed having a significant effect on the performance of chilled ceilings and beams, WILLIAM BOOTH explains the benefits of mock-up testing.In the last 10 years there has been a substantial uptake in the use of chilled ceilings to provide comfort cooling in new and refurbished buildings. When combined with appropriate ventilation, chilled ceilings can create a very controllable and thermally stable environment. Proving that a system performs as the manufacturer claims is a basic requirement for all projects. However, the efficiency of the identical system in a specific environment is difficult to determine. The cooling performance of chilled ceilings depends on the installation site. While manufacturers can give a prediction of their product performance, based on their own standard testing, this will not have covered every option. So while the manufacturer provides the proof, if you are going to install a considerable number of chilled beams the validation of the specific performance for the job becomes an issue. Benchmark tests There are two main benchmark tests that can be used to assess thermal performance of chilled beams, the NORD and the DIN test. While Scandinavian countries generally use the NORD test, the rest of Europe uses DIN 4715-1, which measures the performance of chilled ceilings in the presence of natural convection. Both tests will produce a standard result to compare the thermal performance of equipment from different manufacturers. However, neither will give any results or information on a number of key factors that would deem the design to be successful in the real world. BSRIA’s MicroClimate Centre has tested a variety of prototype and standard chilled ceilings for manufacturers and building owners and operators, in accordance to DIN 4715-1. However, when specifying a chilled ceiling for a particular project, the mock-up testing that provides assessment of a representative chilled ceiling in a realistic environment should be considered. Choosing a cooling system without assessing its suitability can be proven to be a risky and costly exercise. Knowing in advance by testing a mock-up is good news for everyone involved in a project. One of the key benefits of any mock-up (physical model) is the opportunity to experience the resulting environment prior to construction. This involves creating an accurate and detailed look and feel of what the end result would be. As a result, optimum operational, control and commissioning settings and potential installation difficulties can be established in advance. In the mock-up, various scenarios that could occur in the real world can be explored — but in a controlled environment. One of the most important aspects of mock-up testing is resolving problems when individual components or the system do not work well together. A mock-up typically incorporates the fully operational chilled ceiling system — including the BMS controls, fresh-air-supply grilles and extracts, and luminaires. Including office furniture, office equipment and people simulators helps create a more realistic setting where building designers, owners and future occupiers can experience the internal environment prior to the final construction. There are five good reasons for asking for a mock-up to be carried out either on site or in the laboratory. • Verifying a design or system concept. • Trouble-shooting an installed system. • Optimising a refit. • Rehearse installation techniques. • Independent verification of a manufacturer’s test results. Mock-up testing Standard DIN or NORD testing creates a rather artificial environment, as the measurements are performed under steady-state conditions. The tests make no allowance for the thermal storage of the building fabric and take no account of air movement generated by the ventilation system. Therefore, to ensure the installation will meet the required cooling and comfort criteria and to minimise the impact of not getting it right first time on site, a full-size mock-up test should be undertaken using the proposed HVAC system. This will reveal the interaction between the individual components. One of the most important aspects of mock-up testing is resolving problems when individual components or the system do not work well together. This can happen when control responses are too quick or to slow, chilled ceilings produce too much or too little cooling, and air supply devices are producing the ‘wrong type of air’. These problems can be assessed and resolved quickly and easily at the mock-up stage. In the mock-up, various scenarios that could occur in the real world can be explored in a controlled environment. These scenarios could include the interaction of the system with adjacent modules, the effect of changing external conditions (summer/winter), change in the controls strategy and change in furniture layout and occupancy levels.   A mock-up typically incorporates the fully operational chilled ceiling system including the BMS controls, fresh air supply grilles and extracts, and luminaires. The inclusion of office furniture, office equipment and people simulators help create a more realistic setting. In this setting, building designers, owners and future occupiers can experience the internal environment prior to the final construction. Key parameters are recorded to quantify the cooling performance. Parameters that affect the thermal comfort of the occupants, such as air temperatures and air velocities, can also be measured. Chilled beam on site mock-up One of the applications commonly mocked-up is chilled beams or ceilings to handle IT cooling loads in offices. BSRIA MicroClimate was recently contracted to design an on-site mock-up to investigate an under-performing chilled-beam system. We constructed a 6 m-wide test chamber on a floor that was between tenancies. Special instrumentation was used to offset heat losses through the façade, as well as simulating full design solar gain. Problems reported elsewhere in the building were replicated in the mock-up, but under controlled and known conditions. Quantitative surveys of room air movement, coupled with flow visualisation and thermal imaging, characterised the shortfalls in performance and enabled various contributory factors to be identified. Various potential remedial measures were also identified. It was particularly convenient for representatives of the various interested parties to experience the internal conditions. William Booth is Head of Physical Modelling and Site Investigations at the BSRIA MicroClimate, Tel 01344 465600, email: microclimate@bsria.co.uk

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