Hospital indoor air quality is critical. It cannot be too humid, too dry, too cold or too hot. Pressure must be higher in some rooms, yet doors must open easily. Laboratories and surgery rooms must be sterile and patients with certain conditions isolated. It takes experts to arrange all this in a cost effective way.
Pumps, cooling towers, and humidification comprise 40% of a building’s energy costs. Further expenses may derive from clean room functions and environmental regulations. Proper solutions reduce energy expenses from 10 to 25 %.
Energy savings are mainly achieved through electronic motor control. Traditional air handling solutions have AC motors connected to the main power supply – with speed fixed to the frequency and voltage of the AC supply. These are designed for maximum capacity with valves and dampers to regulate flow.
Since energy consumption is proportional to flow cubed, it is extremely important that fans and pumps run as slowly as possible while maintaining indoor air quality.
In 1968, Danfoss became the first company in the world to mass produce electronic variable frequency motor controls, making it possible to disconnect the motor speed from the mains frequency and vary the speed of the motor. Later technology made it possible to regulate the torque as well.
Since the beginning, Danfoss has had a dedicated organisation to manufacture, sell, service and develop solutions for heating, ventilation and air conditioning in buildings. Dedicated and reliable products, people and solutions make Danfoss one of the leading global providers of electronic motor controls.
All the tricks
The basic components within a typical large central plant HVAC system include some or all of the following:
Chillers, Condenser Water pumps, Cooling Tower fans, Primary Chilled Water pumps, Secondary Chilled Water pumps, Air Handling Units, electronic motor controls etc.
In pump and fan applications some form of flow control is used. This may be just a flow setting device (e.g. throttling valve or damper) to set the flow rates to system design point during commissioning and balancing of the system. However, all HVAC systems are designed for worst case conditions. They must be able to provide the necessary cooling load for the hottest, most humid days of the year. The majority of time, there is an excess capacity.
In efficient buildings some form of active flow control is used (e.g. inlet guide vanes or variable speed drives in VAV systems) to match the capacity of the system to the actual requirements of the building. Matching the capacity to the requirements increases the comfort of building’s occupants and saves energy.
Whether the control is just a flow setting device or an active flow control device, using the most energy efficient means of controlling flow will result in lower energy consumption and reduce building utility costs.
Condenser water, primary and secondary chilled water pumps in HVAC systems are centrifugal type pumps. Examples of the types of fans used in AHUs (Air Handling Units), frequency converters (FCUs) and Cooling Towers include forward or backward curved centrifugal fans, plug fans, vane axial fans or variable pitch vane axial fans.
Distributed control
It is common practice in many countries today to install the variable speed drive near the motor/pump/fan it operates instead of mounting it in a central control panel. The main advantages of this are that it reduces the size of the main switchboard/control panel, results in simpler installation and easier commissioning. However to ensure long term reliability of the variable speed drive and protection of personnel in the plant room, for such an installation it is important to ensure that the “enclosure protection” of the variable speed drive is not compromised.
Avoid radio frequency interference
Variable speed drives control the speed of AC motors by varying the frequency (and voltage) of the supply to the motor. Very basically they do this by switching transistors on and off at a fast rate (typically at least 4500 times per second). Therefore it is important to pay attention to radio frequency interference (RFI) or electro-magnetic in-compatibility, (EMC).
For reliable operation of other electrical/electronic components in the building it is important that the switching of the variable speed drives transistors do not cause interference with equipment within the building. This many include medical equipment, PCs, elevators, escalators, etc. The easiest way to reduce the risk of interference is to install a variable speed drive incorporating an RFI filter, following the manufacturer’s installation guidelines. If the variable speed drive has a built-in RFI filter it can save time and costs during installation.
Attention to harmonic disturbances
Variable speed drives are also what are termed non-linear electrical devices. They are a source of harmonic currents in the electrical supply system. Harmonic currents result in increased RMS currents and therefore problems in the AC mains cables, supply transformers, and other components. Potential the currents can cause another form of electrical interference called harmonic voltage distortion.
The easiest way to avoid harmonics problems is to ensure the variable speed drive incorporates a harmonic filter (e.g. DC link chokes). Most HVAC dedicated variable speed drives incorporate these filters or offer them as options. These drives are designed for installation in commercial buildings which do not necessarily have a dedicated mains supply transformer.
Without such a filter there can be an excessive level of harmonic voltage distortion which could affect other electricity consumers connected to the same transformer. Just as with the RFI filter, if the chokes are built in the variable speed drive, this can save time and costs during installation. In addition, a variable speed drive with a harmonic filter will have a lower RMS input current than a drive without the filter. Therefore often smaller size fuses and cables can be selected resulting in further cost savings.
Requirements to the drives
Variable speed drives used in HVAC applications ideally have the following features:
Quiet motor operation – this is particularly important when the motor is driving an AHU (Air Handling Unit) fan. The noise will travel down the ductwork, a possible source of annoyance to the building occupants. In the case of a cooling tower fan the noise could annoy nearby neighbours to the building.
Reliable flying start – although a fan (e.g. cooling tower fan) may be off, it is very likely to be spinning due to natural air convection. If a variable speed drive does not incorporate a “flying start” function, which detects the fan spinning in either direction, when it is given a start command, it will likely cause a shock mechanical load on the fan and its mechanical coupling. This increases wear and tear and results in a major current pull by the motor, which will either cause the variable speed drive to trip or incur damage. HVAC dedicated variable speed drives incorporating a flying start function should be able to “catch” a spinning fan at any speed in either direction and smoothly control it when starting.
Automatic Energy Optimisation – some HVAC dedicated variable speed drives have this function. With this function they are able to typically provide an additional 5 to10% energy savings compared to variable speed drives without this function. They automatically optimise the magnetisation of the motor when the motor is operating at part load.
Built in hand-off-auto pushbuttons. This feature removes the need for the switches to be fitted to the switchboard/control panel.
Display of motor current, kWHr, pressure and/or temperature (as relevant) in engineering units.
Fan belt breakage detection – this should be built into the variable speed drive so it removes the need for a differential pressure switch installed across a fan to detect air flow in an AHU.
Dedicated, intelligent HVAC variable speed drives in decentralised building management system:
At first glance, the variable speed drive is often considered purely an expenditure item. But, in fact, they save money when properly installed. During the design phase of projects, variable speed drives are often one of the first components cut back when trying to reduce costs. However, the best cost efficiency can be realised by fully integrating variable speed drive technology.
As described in the section above, dedicated HVAC variable speed drives incorporate functions that diminish the need for other components in the system. If the variable speed drive is integrated into the building management system (BMS) using serial communications, it is possible to reduce further the initial costs. Through this and the resulting reduced operating and maintenance costs it is possible to realise a 2.5-fold return on the initial investment within 5 years.
Dedicated HVAC variable speed drives can be integrated into the BMS using a single serial communications cable. Using various HVAC protocols such as LON Works, BACnet, and Metasys N2, or others, they enable a vast array of monitoring and control capabilities via the single cable. This minimises the number hardwired I/O points and their associated engineering and commissioning costs, reduces installation and commissioning costs and increases the diagnostic information available to the BMS.
Often the variable speed drives analogue and digital I/O in a hardwired system used to control and monitor the variable speed drive, can be used as I/O solely for the BMS. Even if these are not used in the initial design they provide spare I/O capacity for future expansion at no extra cost.
The variable speed drive can receive its start/stop, reset, speed reference and set point commands via this single cable. It can provide the BMS its running and healthy/tripped status points. If the variable speed drive is being used to maintain a set pressure (e.g. static duct pressure in a VAV system) the variable speed drive can perform all the necessary closed-loop control to run the fan at whatever speed is required to maintain that pressure. And the BMS can monitor the pressure via this single cable and adjust the set point if necessary. The variable speed drive can provide diagnostic information such as motor current, which would otherwise require a separate current transformer and ammeter, and it can also provide information such as kWHrs consumed, running hours and information on many other operating variables.
Based on using real costs and load/operating profiles from real applications, the following sections describe how best to integrate a variable speed drive into a number of different applications and highlight what percentage of the cost to install a variable speed drive can be saved elsewhere in the system/installation. The savings are based on a comparison between:
An IP55 HVAC dedicated variable speed drive complete with Build In RFI and harmonic filters (Danfoss VLT® HVAC Drive FC 102) integrated into the BMS using serial communications
AND
A hardwired fixed speed motor/pump/fan with some other form of flow control, based on the motor’s starter housed in a wardrobe type panel consisting of a three-phase supply with motor starter and thermal overload, ammeter and run and trip lamps.
The cost considerations also allow for all I/O points to be wired and commissioned. Then the analysis highlights how quickly the “true cost” of the variable speed drive will be recouped in the lower operating costs compared to having no flow control or using other flow control methods.
Variable Air Volume systems
Variable Air Volume (VAV) systems are the most energy efficient method of maintaining a building’s environmental conditions. In a VAV system the temperature of the supply air is kept constant by modulating the cooling coil chilled water valves and the volume of air to each zone is varied by the VAV boxes modulating based on the temperature of the zone.
Some form of flow control (e.g. inlet guide vanes or variable speed drive) is used for the supply fan to maintain the required static pressure in the supply duct as the VAV boxes modulate open and closed.
Depending on the complexity of the design there may be temperature sensors in the supply air duct, mixed air and rooms to control or monitor the temperatures, and the digital controller maintains a constant supply air temperature by controlling ventilation and cooling loops (and heating if relevant), in sequence. Relative humidity control may also be provided by a duct humidifier.
Using a variable speed drive for variable air volume applications a pressure sensor, measuring the supply air static duct pressure, typically two-thirds of the way along the duct, is connected directly to the variable speed drive. Using its PID controller the variable speed drive operates in closed-loop control to maintain the static pressure at the required set point. As the VAV boxes close the increase in static pressure is detected by the sensor and the variable speed drive reacts to reduce the speed/flow of the supply fan to maintain the pressure at set point.
For laboratories and surgery rooms
A useful feature to have within the variable speed drive for the surgery room application is a two/three-zone PID controller. If the supply duct splits it is then possible to install static pressure sensors in both paths and the variable speed drive will ensure the static pressure is at least maintained at either point. Alternatively with a single path an additional static pressure sensor can be installed and used as a high static pressure sensor to prevent damage to the ductwork.
For this application it is possible to compare costs between using inlet guide vanes or a hardwired general purpose variable speed drive and a HVAC dedicated variable speed drive integrated into the BMS using serial communications.
The points below need only one cable when there is dedicated HVAC variable speed drive with supply duct and high supply duct static pressure sensors connected directly into the variable speed drive. Also, the variable speed drive should be connected into the BMS using serial communications.
Fan start/stop
Static duct pressure set point (both normal and high limit)
Actual static duct pressure (both normal and high limit)
Fan belt breakage detection
Hands-Off-Auto status
Running/Tripped status
Motor current and kWh consumption
Only the remaining 7 analogue points are required when integrating an HVAC dedicated variable speed drive. This is a reduction of nine I/O points. There are obviously costs involved with wiring and commissioning each point in a BMS. By reducing the number of points required, it is possible to achieve major cost savings.
More costs can be saved by the need for only a three-phase, fused supply instead of, for example, a star/delta starter with thermal overload, current transformer and ammeter.
Compared to using inlet guide vanes, installation costs are lowered by using and integrating the variable speed drive approximately 52% the cost of purchasing and installing the variable speed drive (i.e. the variable speed drive’s true cost was only 48% of the actual cost). This does not include the savings made by not installing the inlet guide vanes, which themselves can often be 40-60% the cost. When these are included, the variable speed drive would be the lowest cost option.
However, based on just the basic installation savings (ignoring the inlet guide vanes) and on the load profile from this application, which operates approximately 18 hours per day/340 days per year, the true cost of installing the variable speed drive was recovered within 6 months of operation, through energy savings.
In addition to this, and what has not been included in the analysis above, it should be remembered that with a variable speed drive there is no need for power factor correction capacitors. Only 3 cables are required instead of the 6 for the star/delta starter. The variable speed drive can display more than just the motor current (static pressure, kWh, running hours). It is easier to balance the system, the smooth acceleration and deceleration of the fan will minimise fan belt wear and running the fan at lower speeds will reduce mechanical wear resulting in lower operating costs. In addition the BMS can receive much more diagnostic information via the serial communications network.
Smart solutions
Life-saving fire mode, the VLT® HVAC Drive FC 102 can be set to over rule all its self and motor protecting features in order to secure operation for as long as physically possible; far longer than during normal operation.
The feature is standard when intended for use in functions where continued operation is vital in emergency situations, such as a fire.
This is often the case in HVAC applications, where air supply is vital for rescue personnel as well as for people inside a burning building.
When set the drive will ignore warnings and alarms and may run to destruction. The fire mode is clearly indicated in the VLT® HVAC Drive FC 102 display to prevent any confusion.
If a bypass is available, the VLT® HVAC Drive FC 102 will not only sacrifice itself in case of fire, but is able to bypass itself and connect the motor directly to mains, and this way keep operation going as long as power is provided and the motor is functioning.
Real time clock
Products with built-in real-time clock are provided for retrofit of existing heating and ventilation systems.
Real-time clock makes it possible to control the ventilation system and pre-program actions, such as, increases or decreases in temperatures in different time-windows each day and the load on the BMA System is significant reduced.