Frequently Asked Questions

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:

Introduction to DADANCO Active Chilled Beam systems
Comparing DADANCO and other systems
Energy
System design
Chilled Water Design
Unit performance
1. Can the terminal units be connected in series?
2. Are ACB systems noisy? Why are they quieter than th eold induction based systems?
Piping and insulation
1. Do you need to insulate the return water line in the false ceiling?
Heating
1. Can active chilled beams be used successfully in overhead heating or should I still use finned-tube radiation?
Testing, commissioning and maintenance

To gain the most benefit from the FAQ’s, please take the time to read them all. This will give you a far better understanding of the Active Chilled Beams systems in general, and specifically DADANCO unit characteristics and relative performance. If a question that is not covered here, please feel free to contact us at (413) 564-5657.

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
AHU	Large	Small	+
Ductwork	Large	Small	+
Risers	Large	Small	+
Ceiling Space	Large	Small	+
Pipework	Small	Large	-
Fan Energy	High	Low	+
Pump Energy	Low	High	-
Tenant Satisfaction	Low	High	+
Air Side System Cost	Low	High	+
Water Side System Cost	Low	High	-
Individual Control	Low	High	+
Thermal Comfort	Low	High	+
Generated Noise	High	Low	+
Maintenance	High	Low	+
Risk of Condensation	Low	High	-
System Comlpexity	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. Energy

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.
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.
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.

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.

Automatic Flow Control Valve/Circuit

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?

No.
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. Heating

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.