Peter Winterbottom (right) , HVAC manager, ABB, discusses how variable drives bring better energy efficiency with no loss of air quality
Hospitals demand a lot from their HVAC systems. A quality air supply is vital. And, in areas such as operating theatres, there is no substitute for air of the highest quality.
A modern building management system (BMS) has a major role to play in managing air quality, responding to demand and changing conditions to keep the air clean and at the correct temperature and humidity. The latest systems have begun incorporating electronic variable-speed drives as an integral component, helping ensure that fans supplying air are driven at the appropriate speed.
A traditional mechanical control system for ventilation plant typically uses a mechanical arm on a jockey motor raising and lowering a damper to control air flow. This is more costly and less efficient. It is also more delicate and prone to disturbances, so requires more maintenance, and is less flexible.
The variable-speed drive, by contrast, is robust, energy-saving, easy to install and low on maintenance. Drives also have the flexibility to communicate with each other, with other devices on the network and with an overriding control system. A mechanical system can do none of this; apart from receive an analogue input.
With onboard microprocessors, modern variable-speed drives can form an important part of a building control system. Many incorporate sophisticated control features that allow them to take the place of external controllers. These programming and communications options help designers achieve more efficient systems.
Adjust conditions
In hospitals with a BMS, variable-speed drives can help adjust conditions accurately. For example, demand controlled ventilation could become the norm and substantial energy savings could be achieved in hospitals with variable occupancy and fluctuations in heat gain. Alternatively, they may be programmed to reduce consumption at peak times, which can be a requirement of electricity supply contracts.
The UK's largest acute teaching hospital project has installed variable-speed drives to maintain a clean air supply in its operating theatres. The £210M Edinburgh Royal Infirmary uses more than 100 AC drives from ABB for air-handling applications throughout the hospital. In the 20 operating theatres, the drives help supply a filtered airflow with a maintained air volume.
As static pressure builds up over the air filters, the drives need to compensate for the reduction in flow rate to maintain the air volume.
The BMS monitors the system for any reduction in performance and signals the drive to increase the airflow accordingly. The drives operate in standby mode when the theatres are not in use, maintaining a reduced airflow to save energy.
Drives are relatively straightforward to connect to a BMS, either by hardwiring or via a fieldbus system. A fieldbus has the advantage of requiring only two wires rather than individual hard-wired connections for each drive.
Fieldbus can also support an unlimited number of control points - systems incorporating hundreds of drives are not uncommon. This gives BMS designers more options, allowing them to use more of the drives' features.
If a drive is intended for use on a fieldbus system, it should have interfaces to all the major protocols, such as LonWorks, Profibus, BACnet, Trend IQ and Modbus. Trend IQ is one of the most popular BMS protocols on the UK market, but few drives use it and so an interface may be needed.
BACnet is a new standard gaining popularity. BACnet was developed by the American Society of Heating, Refrigeration and Air Conditioning Engineers specifically for building controls. One of its great strengths is that it caters for different levels of sophistication using different conformance classes that cover everything from smart sensors to head-end computers.
Unlike LONWorks, BACnet can also run over a variety of different types of cabling, this offers another potential way to keep costs down.
Better insulation
In terms of HVAC, different technologies suit different levels of control, because the amount of data and the speed of communication required are different in each case.
Improved building insulation seems to be the future. The new Building Regulations Part L, have tightened standards of air permeability of buildings. This requires more effective ventilation, giving variable-speed drives an increasingly important role for designers and users.
By increasing air tightness and using mechanical ventilation with heat recovery, the energy consumption for space heating can be reduced by up to 30%. The savings tend to be greater in commercial buildings than in domestic ones, as commercial buildings are more influenced by outside air movements because of their size.
Better-sealed buildings mean higher energy efficiency but, unless effective ventilation is provided, it also means trapping stale air and humidity inside buildings, potentially leading to the so-called sick building syndrome.