In Harmony with Nature…
The movement toward sustainable building designs is being driven largely by environmentally-sensitive building owners and/or their prospective tenants. There are also heightened concerns about assuring a proper indoor environment at all times and conditions for the building occupants. In addition to providing temperature control, a fully effective HVAC system must also address many other indoor environmental issues that affect occupant comfort, productivity and health such as ventilation air, air distribution, humidity control, noise levels, etc.
As owners and their consultants weigh their HVAC system alternatives, they often find that active chilled beams are the ideal “green” solution for many buildings. Indeed, active chilled beams systems can be key to reducing energy when striving to meet the energy efficiency requirements of the Federal Energy Policy Act (EPAct 2005), Executive Order 13423, Energy Independence and Security Act 2007 (EISA 2007), GSA FY 2010-2015 Strategic Sustainability Performance Plan, and Executive Order 13514.
There is a persuasive overall comfort and economic argument for the use of active chilled beam over other more conventional systems. Used widely in Europe and in other parts of the world for many years, active chilled beams are a proven technology and have become very popular in North America. Most recently, ‘Chilled Beams’ were included in the 2012 ASHRAE Handbook – HVAC System and Equipment.
With active chilled beam systems the building’s primary/ventilation air is continuously supplied to the active chilled beam terminal units by the central air handling system. This primary/ventilation air is cooled or heated to partially handle the temperature-driven sensible loads, while in the summer being cooled/dehumidified enough to handle all of the internal moisture-driven latent loads.
Primary/ventilation air is ducted to the beams primary air chamber (1) and introduced into the beams lower chamber (housing the beams heat exchanger) through a series of nozzles (2). This induces room air (3) up into the beam via a perforated face and in turn through the heat exchanger coil (4). As elevated temperature chilled water or low temperature hot water is pumped through the heat exchanger coil (from the central plant) the induced room air is cooled and/or heated to the extent needed to control the room temperature. The now cooled or heated induced room air is then mixed with the primary/ventilation air and the mixed air (5) is discharged back into the room via lateral linear slots
Fan Energy Savings
In general the design intent is for the central system to circulate only the amount of air needed for ventilation and latent load purposes, with the active chilled beams providing the additional air movement and sensible cooling and/or heating required through the induced room air and secondary water coil. In this manner the amount of primary air circulated by the central system is dramatically reduced (often 75-85% less than conventional “all air” systems).
Essentially active chilled beams transfer a large portion of the cooling and heating loads from the less efficient air distribution system (fans and ductwork) to the more efficient water distribution system (pumps and piping).
The net result of this shift in loads with active chilled beam systems is lower energy consumption and operating costs. Studies have shown that fans are the largest consumer of energy in most commercial buildings in North America. With active chilled beam systems the fan energy is dramatically reduced due to the relatively small amount and low pressure of the primary air being circulated by the central system.
Chiller Energy Savings
While the size of the chiller in an active chilled beam system would normally be the same as that needed in a conventional “all air” system, its effective hours of operation (or loading) could be significantly less if the system employs a water-side economizer to serve the Active Chilled Beams. This is due to the relatively warmer secondary water temperatures (typically 56 — 58 °F) used by the Active Chilled Beams which allows the cooling load to be satisfied for more hours using the water-side economizer.
Also, if separate chillers are serving the central air handlers and the active chilled beams, the COP of the chiller serving the Active Chilled Beams would also be much higher due to the relatively warmer water temperatures used by the Active Chilled Beams.
Heating Energy Savings
As the Active Chilled Beams normally provide sufficient heating capacities at relatively moderate hot water temperatures (110 – 130 °F) there is also an opportunity to maximize the efficiency of condensing boilers through the relatively low water temperatures being returned to the boilers. With the relatively low water temperatures required, using geothermal heat pumps to satisfy the heating loads is also practical.
Excellent Indoor Air Quality and Odor Control
The full ventilation air requirements are delivered to the zones at all times and at all load conditions.
Superior Humidity Control
Humidity control at all sensible load conditions is also assured as the constant volume primary air is delivered with the proper moisture content to satisfy the latent loads.
Excellent Air Movement and Uniform Air Temperatures
Improved comfort through excellent air movement and uniform air temperatures throughout the room, with little concern about potential drafts and dumping at part load conditions. As the airflow and resulting air movement is constant at all load conditions and the induced room air is typically 3-4 times the amount of primary air, the temperature of the mixed air being continuously discharged into the room is generally more moderate than with conventional systems.
With Active Chilled Beam systems, the ductwork system size is greatly minimized providing space savings in the ceiling plenums and vertical air shafts. In some cases, the building’s floor-floor dimension can be reduced lowering the building’s installed cost or yielding more rentable floors for the same building height. Also the size of the mechanical room can often be reduced due to the smaller central air handlers serving the terminal units.
With active chilled beam systems and their lower central system airflows, the size of the central air handlers and ductwork system are also similarly reduced. Often times these size reductions more than offset the increased first cost of the active chilled beams over other more conventional types of terminal units. Other factors positively affecting the overall building costs when using active chilled beam systems include:
There are no electrical line power connections to the active chilled beams which can significantly reduce electrical wiring installation costs. In some cases this can result in reducing the overall electrical infrastructure in the building due to greatly reduced fan power requirements.
Simple low cost zone valves are used for temperature control as opposed to the rather complicated and expensive controls used in other terminal unit types such as VAV units, unit ventilators, etc.
Easy Commissioning of the active chilled beams requiring only adjustments to the water balancing valves and primary air balancing dampers through simple pressure readings.
No regular maintenance of the active chilled beams as there are no moving parts (other than infrequent vacuuming of the unit’s coil as required).
Very Low Noise Levels
When the active chilled beams are sized at the typical inlet static pressures of 0.5″ w.c. or less, very low noise levels are achieved as the new technology nozzles are whisper quiet and there is no terminal unit fan or motor in or near the occupied spaces. DADANCO’s patented nozzles rapidly induce secondary air to reduce the momentum and height the primary air jet which significantly reduces the noise generated.
The low noise levels possible with active chilled beams can be most helpful in areas where the ANSI/ASA S12.60-2002 Classroom Acoustics Standard has been adopted. The standard requires a maximum background noise of 35 dB and reverberation time of 0.6 – 0.7 seconds for unoccupied classrooms.
The following are frequently asked questions on DADANCO ACB™ Active Chilled Beam systems. To assist you in finding an answer to your question, the FAQ’s have been put into the following groupings:
1. Introduction to DADANCO Active Chilled Beam systems
1.1 What is an Active Chilled Beam system?
An Active Chilled Beam is an air-water system that uses the energy conveyed by two fluid streams to achieve the required cooling or heating in a space.
The air supplied by the central air handler to the active chilled beams is called primary air. The primary air is supplied to the active chilled beams at a constant volume and at a relatively, low static pressure (typically under 0.5″ wc). Within the Active Chilled Beam terminal unit the primary air is discharged into a mixing chamber through a series of nozzles. A zone of relative low pressure is created within the mixing chamber, thereby inducing room air through the secondary water coil into a mixing chamber. The induced room air is called secondary air.
In the cooling mode the primary air is cool and dry, satisfying a portion of the room’s sensible load and all of its latent load. The secondary water coil within the active chilled beam terminal unit is supplied with chilled water to offset the remaining internal sensible load of the room. The chilled water temperature is always provided above the room design dew point temperature to preclude sweating/condensation on the water coil.
1.1a What is the Entrainment Ratio?
Entrainment ratio is a ratio of secondary (entrained) air volume to primary air volume. Typically this ratio defines a measure of effectiveness of induction performance of active chilled beams. This ratio is typically in the order of 2.5 to more than 4 for better performing active chilled beams. This ratio shall be taken into consideration when evaluating different active chilled beam units. However, it has to be noted that entrainment ratio is directly affected by the fin density of the secondary coil.
1.1b Isn’t an Active Chilled Beam an Induction Unit?
Yes – well sort of. Active chilled beams operate on the basis of well established Induction principle. The main difference between old induction units and active chilled beams is that the latter utilises very low primary air static pressure (typically below 0.5″ W.C.), and are installed in the ceiling (as opposed to floor/wall mounting). Also, DADANCO active chilled beams utilize novel, patented nozzles which greatly reduce the noise generated when compared with the older induction units with conventional round nozzles.
1.2 What are common applications of Active Chilled Beams?
Active chilled beams are ideal for zones with medium to high sensible cooling and heating requirements. The reduction in primary airflows as compared to conventional “all air” systems such as VAV in these situations is dramatic, often 75-85% less. Common applications include offices, meeting rooms, open plan spaces, laboratories, universities, hospitals, schools, existing building refurbishments and libraries.
In addition due to the very low noise levels of active chilled beams buildings that have special noise levels requirements are good candidates. Finally zones where there is high concern about indoor environment quality are ideal candidates as the rooms are provided with proper ventilation air and humidity control at all times and under all load conditions.
Due to the dramatic energy savings possible with active chilled beam systems, probably the most common application is in those buildings that are striving to achieve LEED certification by the US Green Building Council. There are a number of areas where active chilled beams can help achieve LEED credits including energy efficiency, indoor air quality and individual temperature control.
1.3 How can specifically DADANCO ACB™ active chilled beams help me reduce the installed cost of an Active Chilled Beam system?
There are many elements DADANCO units can reduce installed costs including:
Potentially less primary air resulting in smaller ductwork system
Shorter unit lengths fitting into conventional T-bar ceiling modules
Optimal number of units to cover specific area
Potentially reduced floor to floor dimension due to the smaller ductwork system resulting in reduced building height or alternatively more floors can be built within the same overall building height
Small risers and plant rooms, giving more lettable area.
2. Comparing DADANCO and other systems
2.1 Can you compare the “old” induction units with the DADANCO ACB ™ units?
The major differences between the active chilled beams and the older induction units are:
Ceiling-mounted as opposed to floor/wall mounted
Significantly lower fan static pressures
Lower noise levels
Generally requires less primary airflow
2.2 How do you compare an active chilled beam system with an all-air system like VAV?
Both systems will have the same required refrigeration and heating capacity and, as a result, common chiller and boiler plant. The main differences, and the basis for comparison, are in the air handling system. With the greatly reduced primary airflows and pressure the fan energy savings of active chilled beam systems over VAV systems are dramatic. In addition as the active chilled beams have no moving parts, maintenance costs are at a minimum.
With respect to installed cost, the space required to accommodate AHU plant and ductwork and risers is dramatically reduced. The smaller ductwork also provides installed cost savings. There are no main power connections to the active chilled beams resulting in reduced wiring expense.
The following are major points of comparison:
|Item||VAV||ACB||Net for ACB|
|Air Side System Cost||Low||High||+|
|Water Side System Cost||Low||High||-|
|Risk of Condensation||Low||High||-|
|Control System Complexity||High||Low||+|
2.3 Are the pressure losses comparable in all-air and ACB systems?
Yes. When comparing the two systems, it is necessary to take into account the AHU losses, filters, outside air and return air path losses, the supply air duct loss and then the VAV box and its downstream ductwork and outlets or the ACB terminal unit. As the duct system for the active chilled beam system is much smaller, it’s possible to design the duct system at lower velocities with no concern about space constraints, thereby reducing the system fan operating pressures. Duct system design aside, the pressure losses of the VAV box, downstream duct and supply outlets will be very close to or higher than the active chilled beams terminal unit typically selected at 0.5″w.c or less.
3.1 When comparing the energy savings of an ACB system with other systems, what items, in terms of energy usage, are not subject to the energy savings?
The chiller, cooling tower and the associated chilled and condenser water pumps. Additionally energy related to the usual equipment such as the various exhaust fans, sump pumps, etc. are unaffected.
3.2 When comparing the energy savings of an Active Chilled Beam system with other systems, what items, in terms of energy usage, are subject to the energy savings?
The power used by the fans is the main difference, with the ACB system primary air fans are handling much less air, and therefore requiring less energy. If the system had a return air fan, the savings would be greater, as this fan will also be smaller. The hours of operation are the same, as will be the cost of the energy.
3.3 The major energy savings with Active Chilled Beam systems is in fan power. What is the situation with pumping energy?
There is a modest increase in total pump energy as a result of the secondary water system. However, while the total pump energy for ACB systems, primary and secondary, is higher than an all-air system, it does not significantly reduce the energy savings achieved with the fan power.
As with any exercise in comparing energy usage between alternative systems, each installation must be looked at separately.
4. System Design
4.1 Can the one Air Handling unit serve all of the perimeter terminal active chilled beams for a floor?
This is not the ideal solution. The best method is to zone the AHU’s to serve each exposure and the interior zones. This will enable the primary air temperature to be reset to suit each zones requirement.
If separate air handlers by exposure are not possible, there is a potential for overcooling some rooms at low part loads while others are at their design cooling loads. One approach is to add reheat coils to the duct system serving each perimeter exposure. Another (often more preferable) approach is to provide a sensible heat recovery wheel downstream of the air handling unit in order to provide close the thermally neutral dry primary air to the active chilled beams. The room neutral dry air will be able to handle the room latent loads and the sensible load will be handled entirely by the active chilled beam. This method will minimize the times that you would potentially need to reheat the primary air. Yes you will be adding additional static to the AHU fan but this is far better than paying for significant re-heat to minimize over cooling of low load zones.
4.2 What is the effect of the fan motor heat on the primay air in a draw-through air handling unit?
The effect is to raise the temperature of the air leaving the air handler. The change can be represented on a psychometric chart as a sensible heat increase, with the air leaving condition shifted to the right by whatever is the rise in dry bulb temperature. It can be thought of as reheat. This rise needs to be factored into the system design, as the primary air condition being provided to the active chilled beam units must still be that used in the selection process.
4.3 How many units can be controlled from one control valve?
A single control valve can control several units in the one zone, with a single temperature sensor controlling that valve. The piping and valves after the control valve should be such that the water flow to each unit is at the required design flow to each.
You can achieve additional LEED credits if you provide each occupant with individual control of the air conditioning. This can be achieved by providing a control valve and temperature sensor to each active chilled beam.
5. Chilled Water Design
5.1 How is condensation avoided in high humidity environments?
Outdoor air is pre-conditioned and dehumidified in the primary air handling unit, along with any return air needed to make up the primary air total. The building is maintained at a slight positive pressure with respect to the outside to control infiltration of humid air. Once the dehumidified air is in the space, the dew point is monitored and the temperature of the primary air will be controlled maintain the room design humidity level in order to avoid condensation at the beam. If the system can not maintain the room design humidity level then the last resort would be to increase the temperature of the secondary chilled water.
5.2 What about the system shutting down at night? Won’t the humid air infiltrate and cause a condensation problem at start up?
If the HVAC system is intention to operate the HVAC system to maintain a reset set point temperature during the unoccupied periods, the system can and should be cycled at night with the outdoor air dampers closed to maintain setback temperatures to save energy. This could cause minor infiltration of humid air. In Singapore for example, our experience indicates that the space humidity can increase by as much as 10 – 15% over a weekend shut down.
To address this, at start-up after an unoccupied period the primary air system is operated while the secondary water system remains off. Gradually the primary air system dries out the building and lowers the humidity level. Once the humidity level has been reduced, the secondary water system is started. In this manner operation of the cooled and dehumidified primary air flushes the moisture out of the building before the secondary chilled water pumps are initiated.
5.3 How low can the secondary chilled water (SCHW) temperature be without causing condensation?
A basis for deciding on a secondary chilled water temperature is to relate it to the room dew point temperature. In theory, a surface at room dew point temperature has the potential to condense water vapor from the air. At certain air conditions the air film on this surface will act as a layer of insulation, and allow the temperature of the surface to drop below the room dew point before condensation commences. The effectiveness of the air film depends on the velocity of the air and the fluid velocity with in the tubes. High air velocity over the coil and the low water velocity inside the tubes of the coil minimize the potential for coil sweating. This has the effect of reducing the “apparent room dew point temperature” by about 2-3°F. Therefore for a room dew point temperature of 55°F, a minimum secondary water temperature would be 53°F. We recommend, however, that the SCHW temperature be at or slightly above dew point to provide some safety and to mitigate the risk when room conditions vary.
5.4 How do you maintain the secondary water temperature?
There are three methods:
By circulating primary chilled water through a plate heat exchanger with the secondary water passing through the other side. The variable speed secondary water pump circulates the full secondary water quantity through one side while a modulating valve controls the primary water flow to achieve the design secondary water temperature. A sensor in the outlet of the secondary water line controls the modulating valve.
Primary Chilled Water Circulation
The use of a mixing valve that allows the amount of primary water into the suction side of the secondary water circulating pump. There needs to be a connection back into the primary water loop to return a quantity of water equal to that introduced to maintain the secondary water temperature. A sensor on the leaving side of the secondary water pump controls the mixing valve.
Chilled Water Mixing
Separate dedicated primary and secondary chilled water plants. This option provides the ultimate energy efficiency of the thermal plant but typically has higher capital cost. The Primary chiller plant serves the AHU only and there is a dedicated secondary chiller plant that receives and serves secondary chilled water from the ACBs. Primary and Secondary Chilled Water Plants
5.5 What proportion of the load is handled by the primary air and what by the secondary coil?
If you are starting out on a design and need a feel for the division of the load between the primary air and the secondary water, allow for the primary air to handle the transmission load calculated as a steady state load. This is a quick and simple calculation using the design outdoor air less room air temperature, the areas of masonry and glazing and the respective U factors. The primary air will also handle all of the room latent load.
The secondary load is the sum of the people, lights, office equipment and solar load. The first three you will have based on normal design standards or the values in a brief, and the third by reference to solar load tables or your heat load calculation program. Do not include the outdoor air load, as the primary air handler will handle this.
5.6 How important is it to accurately commission waterside?
Accurate commissioning is more important than with regular air conditioning systems. Remember, more than 70% of total cooling capacity delivered by the active chilled beam is provided by the secondary coil. Also, water flows through coil are relatively low (around 1 – 2.5 gpm). This presents a challenge for commissioning engineers as well as for the designers as a minor drop in water flow to the unit will have significant impact on the delivered capacity of the unit. We recommend commissioning the waterside for each unit. This can be done by a balancing valve/circuit setter or automatic flow control valve which will ensure that each unit receives the design water flow.
6. Unit Performance
6.1 Can the terminal units be connected in series?
This is not recommended. The limiting factor is the volume and velocity of the primary air entering the first unit’s plenum. Too high a velocity can generate unwanted noise and excessive pressure drops. Generally the layout of ACBs does not lend itself to series connection. If design layout demands that units be connected in series, contact DADANCO to discuss a larger primary air inlets connection, and possibly a larger plenum.
6.2 Are ACB systems noisy? Why are they quieter than th eold induction based systems?
The older installations used circular nozzles in a variety of sizes and configurations. The patented multi-lobed DADANCO nozzle is much quieter, due to the nozzle configuration and partly as a result of the lower operating pressure it operates at.
7. Piping and Insulation
7.1 Do you need to insulate the return water line in the false ceiling?
Within ceiling plenums used for the return air, this depends on the temperature of the return secondary water. If the flow to the coils was at, say, 57°F, then the return water temperature could typically be 62-63°F. At these temperatures there is no danger of condensation and insulation on the return piping is not required. The ceiling plenum used by the return air would probably be at a temperature of about 77-78°F with a dew point of typically 55°F.
This is a practice observed in many older installations. However they invariably insulated the piping in the riser to the plant room as the air was no longer effectively still air and the temperature could be several degrees above the plenum ceiling temperature.
If the pipe is to be insulated, remember the insulation is to reduce thermal gains and not prevent condensation. The costly vapor barrier is not required. It may mean a few more lines in the specification to include a thermal insulation for piping, but the cost saving to the client is worth it.
8.1 Can active chilled beams be used successfully in overhead heating or should I still use finned-tube radiation?
Based on previous testing there are no concerns when the heat loss along the perimeter is 250Btu/lft or less. Between 250 and 350Btu/lft heating from overhead is acceptable if the warm air is directed toward the window horizontally and hits the window at a velocity of around 75 fpm. Between 350-400 Btu/lft it is sometimes acceptable to discharge the heated air from above and directly down the window. At these levels and above it is generally recommended that normal finned-tube radiation be used to address down draft concerns.
9. Testing, commissioning and maintenance
9.1 How is the primary airflow to the active chilled beam measured?
The way to accurately measure the primary air flow into the active chilled beam is by reading the static pressure from the commissioning tube sampling the primary air plenum. A primary air flow versus static pressure chart is provided for each unit. Do not utilize readings in the duct near the unit and presume it will be the same in the plenum for commissioning purposes. This measurement could be up to 0.3″ wc off.
9.2 Are inspections of the coil necessary, and if so, at what frequency?
For the dry coil operation, the coil should be inspected once a year for any foreign matter and vacuumed if needed.
9.3 Do I need to use lint screens with active chilled beam?
The use of lint screens is a legacy of older floor mounted induction systems where the coil was next to carpet lint and debris. With active chilled beams being installed in ceiling and with face velocities over the coil of less than 100fpm there is not sufficient force to induce lint, or debris up into the coil. As the coils are designed to be dry there is little chance of grime and grit clinging to the coil. While not needed, lint screens are available.
9.4 Will condensation occur if I have bad or poorly maintained controls?
It could. With quality controls installed, the sensors will detect changes in secondary water temperature and/or the high room dew point and will have initiated the pre-arranged controls program to raise an alarm with the maintainer. The first step would be to reset the temperature of the primary air downward to attempt to remove the moisture form the space. If after a set amount of time satisfactory conditions are not achieved then the last resort is to reset the secondary water temperature upwards. The final step would be a complete shut down of the secondary water pump until the fault is corrected.
The Product Range
DADANCO active chilled beam designs have been developed using the patented nozzle technology in order to provide very high cooling/heating capacities for a given unit length. DADANCO units can be provided to fit into a standard T-bar ceiling, dry wall/plaster ceilings and other ceiling types as required.
Our product range covers the following lengths:
- ACB™ 20, 40, 45, 50, 55 & HACB™ 20 – 2’ to 10’ long in two-feet increments as std.
- ACB™ 10, 30 & 35 – 2’ to 6’ in one-foot increments as std.
(Non-standard lengths available on request)
Note: The intake and discharge grilles or diffusers used with ACB™ 10, 30 & 35 are supplied as an optional extra and therefore the size and length of these varies dependent on the beam performance and style of grille or diffuser used and the aesthetic requirements of the HVAC design.
Our active chilled beams deliver superior Indoor Environment Quality (IEQ) and Air Change Effectiveness (ACE) in both perimeter and interior zones.
We have developed nine types of active chilled beams:
- ACB™ 55 – 1-way discharge Ceiling-mounted Cassette (1-foot wide)
- ACB™ 50 – 1-way discharge Ceiling-mounted Cassette (2-feet wide)
- ACB™ 45 – 2-way discharge Ceiling-mounted Cassette (1-foot wide)
- ACB™ 40 – 2-way discharge Ceiling-mounted Cassette (2-feet wide)
- ACB™ 35 – 1-way discharge Ceiling-mounted Concealed (with integral drain pans)
- ACB™ 30 – 1-way discharge Ceiling-mounted Concealed (with integral drain pans)
- ACB™ 20 – 2-way discharge Ceiling-mounted Cassette (2-feet wide with integral drain pans)
- HACB™ 20 – 2-way discharge Ceiling-mounted Cassette (2-feet wide with integral drain pans & MERV 8 filter)
- ACB™ 10 – Horizontal discharge Bulkhead-mounted Concealed (with auxiliary drain pans)
Coil piping arrangements can be either 2-pipe for cooling or heating only or a change-over systems or 4-pipe for a cooling and heating design.