Proving the performance of fan-coil units
Carrying out tests of the performance and noise characteristics of fan-coil units that are likely to represent reality requires sophisticated test facilities. This is part of Carrier’s extensive European facilities at Montluel in France.
There are many ways of testing the thermal and acoustic performance of fan-coil units for air-conditioning systems. JULIAN JOWITT compares them.'There are many ways to skin a cat,’ is the phrase that comes to mind when discussing the almost infinite number of ways to balance and then measure the output of a hydronic fan-coil unit (FCU) — the delivery end of many chiller-based air-conditioning systems. Some ways are better than others, however. Firstly, it is important not to be swayed by some company claims of having the only test facility in the industry or the only one that meets particular standards. There are several suitable laboratories within the industry. Performance guarantees
It is difficult to design a fan-coil unit with all the associated performance guarantees that are required without this high-quality testing. All major European manufacturers agree that the only way for a client to guarantee the validity of the manufacturer’s catalogue data is to test it. The principal specifications that consultants have in recent years required to be proven are thermal output and generated noise. These parameters need to be tested at the same time, as it may be easy to ‘fudge’ the figures by say reducing the thermal output to achieve the noise specification or vice versa. Carrier, for example, tests to both British Standards and Eurovent standards and has recently invested over €500 000 in its facilities at its European manufacturing plant in Montluel near Lyon in France. One would of, course, like to test an FCU in the actual environment that it will be installed in. However, in situ it would be impossible to balance so many parameters to the demanding tolerances required by the standards. Before you can even think of measuring the thermal output and noise you must first balance the dry-bulb temperature of the incoming air, re-create the moisture content (relative humidity), balance the water flow, obtain the correct entering-water flow temperatures, followed by measurement of the airflow against the actual grilles and ductwork and, finally, the resultant external static resistance. Only when all the above are balanced can you think about testing the output duty and measure the noise and air flow pattern correctly. While there are several laboratories around, there are very few test cells capable of testing while simultaneously balancing all the above parameters. Actual environment
Most test facilities, when testing, can do very little else but simply bolt the FCUs to a hole in the wall and blast air into a reverberation chamber to measure the noise. The reverberation characteristics are then indeed known and become part of the formula for sound power level (SWL) formula. However, such testing in no way represents the actual environment the FCU will operate in on site. The room is the wrong shape and size, windows are not usually taken into account, and wall, ceiling and floor fabrics are different — not to mention the lack of typical room furniture. All this makes the measured airflow pattern, noise (NR) level etc. totally unrealistic. We are not saying that it is impossible to give the client a guarantee of performance under this sort of test circumstance. The standards currently in operation regard the results of these sorts of tests as valid. However, quite sensibly, many customers out there are more critical. Although valid test results are achievable with this limited type of laboratory facility, it is now becoming possible to provide clients with a much greater degree of reassurance that systems will provide exactly the performance that they would want. It is possible to devise test environments, such as that at Carrier’s factory, in which all of the conditions found at a particular installation that might affect energy or sound can be simulated. The walls can be made to move, the windows moved and changed in size, a wide variety of fabrics can be applied, typical office furniture can be installed and the FCU installed above a suitable suspended ceiling attached to the actual grille and ductwork. A video set-up of a smoke test proves the air-flow characteristics. Realistic conditions
Importantly, not only can FCUs be tested under realistic conditions but the grille and ductwork are also subject to analysis. This provides an ideal opportunity to establish the true characteristics of grilles and ducting. It is not unusual for the industry to overestimate the external resistance imposed by these items, which often means that many installed FCUs could possibly be turned down, with resultant noise and energy reductions. At Carrier, not only is the noise tested but it is tested to the ultimate standard of ISO9614 Pt 1. This is latest type of testing standard. It is called the ‘intensity’ method and, in simple terms, consists of two microphones pointing towards each other. This has the effect of omitting reflective, reinforced and compound noise. The big benefit of measuring noise from FCUs with this method is that you measure what you point at, so you can measure breakout separately from the return duct. In other versions of the process, a combined reading is taken of the return-air and breakout noise, and this reading is extrapolated. In some test proce- dures, further generalised inaccuracy can be introduced if the supply-air measurement was taken in the reverberation room with its known characteristics and then the combined figure was taken in the transmission room, which has totally different noise characteristics. Spirit
There are many places within this complex testing regime where parameters can be altered and judgements made, so it seems that the fairest assessment is one that can be made under conditions that most closely match those in which a system will be installed. In an environment governed by standards and regulation, it is a case of playing to the spirit of the law rather than merely playing to the rules. Julian Jowitt is integrated systems manager with Carrier UK, United Technologies House, Guildford Road, Leatherhead, Surrey KT22 9UT.