To Beam or not To Beam?
During the last couple of years, there has been an addition to the conditioning system options available to mechanical design engineers in the U.S. I say this because active beams have joined VAV, fancoils, etc., as another way of conditioning a space. However, while they are a very effective solution to certain design applications, active beams are not the “golden bullet” they are often perceived to be.
This article focuses on the active beam applications that I would describe as most feasible. There are, of course, many other applications that can be conditioned by active beams, but these are the circumstances where I most often consider them.
EXPERIENCE WITH PASSIVE AND ACTIVE BEAMS
I first started designing passive beam systems while working in the Netherlands in the early ’80s. Active beams were developed in the mid 1980s and started to take off late in that decade. I wrote an ASHRAE paper in 1993 that described the performance of active and passive beams for a number of applications for office buildings. This paper also discussed occupant comfort conditions provided by beams. It was also during this period that I reviewed many laboratory tests of beams with most of the prominent manufacturers of that time.
With the emergence of beams in the U.S., IBE Consulting Engineers have witness-tested beam performance at several different manufacturers, provided specialist consultancy for clients, and designed many successful mechanical systems employing beams.
There are special tricks that need to be performed when designing beam systems. A load is a load and should be calculated by the engineer in the traditional manner. With that said, designers should pay special attention to latent loads in the space and the conditioning of these latent loads.
Always calculate the required cooling capacity using dynamic simulation software, taking into account the simultaneous heat loads as well as the transient heat transfer effects of the thermal mass of the construction. Unnecessary oversizing of the system increases the investment costs. The intentional oversizing for future flexibility is realized by paying attention to the sizing of water flow rates and ensuring the stable room temperature control.
For a good comfort sensation, the human body has to be in thermal balance with its surroundings. The heat exchange of the human body with its environment takes place through convection (air movement), radiation, and evaporation (sweating and breathing). Thermal balance can, depending on activity and clothing, only be achieved within certain environmental conditions. At a room temperature of 26°C, a person doing work sitting down in an office produces approximately 140 W of heat, of which 70 W (approximately 50%) is released through evaporation. The remaining heat is emitted through radiation and convection in roughly equal quantities.
The determining factors for a room’s thermal comfort are the temperature, the radiant temperature, the relative air humidity, and the air velocity. However, two quantities involved in comfort sensation depend upon the individual: the metabolism or the activity level of the body, and the clothing resistance of Iclo (the heat resistance of the clothing). In order to assess thermal comfort, one can make use of a number of additional and derived quantities: the black globe temperature, the operative temperature, the radiant temperature asymmetry, the turbulence intensity, the predicted mean vote rating (PMV), the predicted percentage of dissatisfied (PPD) rating, and the draught rating (DR) value. These concepts are explained in ASHRAE Standard 55. When climatic ceilings that cool or heat through radiation are being used, it is particularly the operative temperature and radiant asymmetry that are important.
To set requirements for thermal comfort, the following standards and publications are important: ASHRAE Standard 55, 2010, EN-ISO 7730:1995, DIN 1946:1994 (Part 2), and CEN TR 1752:1998. Based on this information, the following design starting points have been formulated:
- Summer conditions (cooling period)
- Winter conditions (heating period)
Over the last few years, a definite tightening of the comfort sensation requirements has occurred. High air volumes and high ventilation or circulation flow rates are now more likely to create air velocities considered unacceptable. Alternative cooling systems such as climatic ceilings, which cool through radiation as well as convection, are one way of solving this problem. An analysis has to be made about whether or not the installation design satisfies the requirements of thermal comfort. Depending on the required cooling load, the combination of cooling system and air supply system, and the availability of research data from similar conditions, simulation experiments might be needed, particularly since there is a definite interaction between an air supply system and a cooling system
Active beam systems are designed to optimize the delivery of cooling to the space, but the paramount consideration in sizing and located the beams within the room should focus on occupant thermal comfort. ANSI/ASHRAE Standard 55, 2010)defines limits on local air temperatures and velocities that will maintain acceptable levels of occupant thermal comfort. The standard defines the occupied zone as in which stationary occupants reside. The height of this zone is generally considered 67 in. (1.7 m) for standing occupants or 42 in. (1.1 m) for predominantly seated occupants. The standard prescribes that velocities within the occupied zone should not exceed 50 fpm (0.25 m/s), while occupied zone vertical temperature gradients should be maintained at 5.4ºF (3ºC) or less.
Active beams directly supply a mixture of primary and secondary air to the space and should therefore be treated like the other air distribution devices employed in fully mixed air distribution systems. Since the temperature of the chilled water supplying the coil within the beam must be at (or above) the space dewpoint temperature, it is typically 56º to 60ºF (13 to 16ºC); thus, the reconditioned air temperature leaving the coil is several degrees warmer than that of the primary air with which it is subsequently mixed. This results in beam design discharge air temperatures ranging from 58º to 60ºF, warmer than those normally used by conventional all air systems. As the required supply airflow rate is inversely proportional to the room to supply air differential, active beams must discharge 15% to 25% more air to the space to satisfy its sensible heat gains.
Summer conditions (cooling period)
- • The PMV shall satisfy -0.5 < PMV < 0.5.
- •The operative temperature shall be between 23°C and 26°C
- (e.g., 24.5°C, 1.5°C).
- •The vertical air temperature difference between 1.1 and 0.1 m above the floor (level of head and ankles) shall be less than 3°C.
- • The mean air velocity shall be such that the DR is less than 15%.
- •The radiant temperature asymmetry of windows or other warm vertical surfaces shall be less than 23°C (relative to a small vertical plane 0.6 m above the floor).
- • The relative humidity shall be between 30% and 70%.
Winter conditions (heating period)
- The PMV shall satisfy -0.5 < PMV < 0.5.
- The operative temperature shall be between 20°C and 24°C ( e.g., 22°C).
- The vertical air temperature difference between 1.1 and 0.1 m above the floor (level of head and ankles) shall be less than 3°C.
- The mean air velocity shall be such that the DR is less than 15%.
- The surface temperature of the floor shall normally be between 19°C and 26°C.
- The radiant temperature asymmetry of windows or other cold vertical surfaces shall be less than 10°C (relative to a small vertical plane 0.6 m above the floor).
- The relative humidity shall be between 30% and 70%.
COMFORTABLE INDOOR CLIMATE WITH BEAMS
Indoor climate target values should be taken into account when defining design values for beam systems. Limit specific cooling capacity and primary airflow rate to the range where proper operation conditions and comfortable thermal conditions can be ensured, e.g., by avoiding too high air velocities in the occupied zone. The higher the specific cooling output, the higher the induction rate (typically 1:3 to 1:5) and therefore the risk of draft (draught) is increased with high linear cooling capacities.
To ensure comfortable conditions in spaces, it is recommended that the building is designed so that heat loads can be maintained below 80 (max 120) W/floor-m2.
OPERATION AND CONTROLS
Like with most HVAC systems, beam controls may vary from simple to quite complex. In order to ensure that the system functions properly, controls systems include:
- Control of the primary air temperature, humidity ratio, and flow rate
- Control of the building outside airflow rate
- Control of the chilled and, where applicable, hot water supply temperature
- Control of individual zone ventilation rates, temperatures, and humidity levels
- Adequate prevention of chilled water system condensation
The primary air supplies for active beam systems are much closer to the space ventilation rates than is the case for all air systems, as much of the space’s sensible load is removed by the chilled water circuit. The primary control of space temperature is normally accomplished by varying the water flow while maintaining a constant ducted airflow rate and temperature. Varying the airflow can provide additional control of the room temperature. This minimizes the risk of overcooling spaces with highly variable loads.
TWO-PIPE WATER DISTRIBUTION SYSTEMS
Two-pipe water distribution systems are limited to supply either hot or cold water to the zones and are not capable of delivering both simultaneously. Zone temperature control is accomplished by a control valve that varies the beam sensible cooling rate according to space demand.
FOUR-PIPE WATER DISTRIBUTION SYSTEMS
Four-pipe distribution systems afford simultaneous delivery of both warm and chilled water to the individual zone control valves. There, the zone thermal demands determine which water medium (warm or chilled) will be supplied. Figure 3 illustrates the beam operation when four-pipe distribution systems are used. Upon cooling demand, the time averaged chilled water flow rate is increased as the space temperature rises above the room setpoint while the warm water supply remains off. During heating demand, the warm water flow rate rises in accordance with a drop in space temperature while the chilled water supply remains off.
ZONE TEMPERATURE CONTROL IN BEAM SYSTEMS
Most beam systems provide constant air volume delivery while affecting the time-averaged water flow rate through the beams in order to react to the changes in zone conditioning demand. Therefore, reconditioning of the recirculated room air is the primary method of space temperature control in most beam systems. In these cases, the sensible cooling contribution of the primary air remains constant.
Condensation control can be implemented with either a reactive, or proactive, or a combination of both strategies. In a proactive strategy, the control system acts to avoid or prevent the formation of condensate. In a reactive strategy, the control system acts in response to condensate that has formed.
In a proactive strategy, the dewpoint can be monitored in one of three ways:
- Contact dewpoint humidistats can be used by attaching the sensor to an uninsulated portion of the piping immediately prior to the supply connection to the beam’s coil.
- The dewpoint temperature of the space air is determined by continuous sensing of the space air temperature, and relative humidity. The calculated dewpoint must then be compared to the supply water temperature.
- To avoid condensation, the setpoint temperature of the supply water can be reset above the room dewpoint, or the supply water to the zone can be turned “OFF.”
In applications where primary air is supplied at conventional temperature (55°F to 57°F) to spaces with significant sensible load variations, it may also be necessary to reset the primary airflow rate or temperature during low-load conditions. One method of doing so is to vary the primary airflow rate in reaction to thermal demands and/or occupancy of the space.
Supply of primary air at or close to room temperature may also overcome the potential for space overcooling. This, however, will result in reduced beam cooling capacities and will necessitate the use of more or larger beams.
From the preliminary load calculation spreadsheet:
- Room area: 150 sq ft
- Room sensible gain: 4,618 Btuh
- Room latent gain: 220 Btuh
- Per design minimum air supply to office: 40 cfm.
- Supply air conditions: 55°F db, 51.9°F wb
- Summer room temperature setpoint: 75°F.
- Supply air cooling: 40 cfm x (75 to 55) x 1.08 = 864 Btuh
- Chilled beam cooling required: 4,618 to 864 = 3,754 Btuh
- Assuming fully mixed conditions in the office, resultant mixed room conditions = 74.5°F db, 62.4°F wb, dewpoint 55.2°F
Therefore, one can see that the resultant air dewpoint is below the design supply water temperature and the resultant average beam surface temperature of 60.5°F (based on 63°F chilled beam leaving water temperature).
SPACES WITH OPERABLE WINDOWS
For applications in spaces with operable windows or doors, occupants and staff should be educated on the effect opening them can have on their thermal environment. When windows or doors are open, the supply of chilled water should be halted to avoid risk of condensation, but more commonly, the supply of both air and water are stopped in order to prevent conditioning of the neighborhood.
Window contact switches can detect when a window is opened. Alternatively, moisture sensors may be used to discontinue the chilled water flow during periods of condensation risk. When these are applied, one sensor should be fitted to the chilled water supply pipe in each room (with operable windows or doors) the system serves. Remember that the chilled water flow will not be restored until the sensor determines that all moisture has been evaporated from the surface of the pipe.
CONDENSATION CONTROL STRATEGIES
Condensation control may involve pro-active (preventive) and/or reactive mitigation strategies.
In proactive strategies, the control system acts to avoid or prevent the formation of condensate. Proactive strategies usually involve monitoring the space dewpoint via one of the following means:
- Contact dewpoint humidistats whose sensors are attached to an uninsulated portion of the supply water piping, often immediately prior to the supply connection to the beam’s coil.
- Continuous sensing of the space temperature and relative humidity allow calculation of the space dewpoint. The calculated dewpoint is then compared to the supply water temperature, and corrective is action taken when necessary.
These strategies are often employed on a floor-by-floor basis, but they may be applied to every zone or beam dependent upon the expected degree of condensation risk.
Reactive strategies involve control systems that act in response to condensate that has formed. Condensation is generally detected by condensation sensors mounted on an exposed portion of the beam cool water supply pipe work at (or very close to) the final connection to a beam.
Aspects that influence the cooling rate include:
- The heat transfer from the water to the pipes, which depends on the flow velocity, the type of flow in the pipe (laminar/turbulent), and any contamination on the internal surface of the pipe.
- The dimensions and composition of the cooler battery.
- The volume of air displaced over the cooler battery by induction. This depends on the primary air volume;
- The injection velocity from the nozzles, the nozzle distribution, and effectiveness. The induced air has to actually be displaced over the active length of the cooler battery
- The air resistance on the underside of the unit resulting from any visual screening or air distribution plates.
- The mean water temperature, which depends upon the water supply temperature, the water volume, etc.
- The difference between the air entry temperature in the unit and the mean water temperature.
INCREASING COOLING RATE USING INDUCTION
Special attention, including tests if necessary, also needs to be paid to:
- The location of the units in relation to the walls and in relation to each other. This has to do with the velocity of the air supply flow and how this affects.
- The air velocities and temperatures in the occupied zone.
- The stability of the airflow.
- The extra amount of open ceiling surface area needed to extract the air.
- Sound aspects, with regard to airflow noise caused and the sound insulation between rooms.
Because they distribute the air more evenly in the room, double-sided induction systems provide the highest level of thermal comfort, especially when high loads are involved. The air volume that can be induced in a certain direction can be influenced to a certain extent by the opening and closing of certain nozzles.
The benefits of such a system are:
- The small amount of primary air and space needed for air shafts.•The greater cooling rate.
- The limited under-temperature of the air supply to the room.
- The compact construction of some makes.
Although the induction systems are less sensitive to disruption, virtually the same preconditions apply here as for the passive systems with regard to the building physical quality of the plenum.
Finally, a major aspect of increasing the efficiency of the active beams is the fan power required to supply the primary air to each active beam. Compared to a traditional VAV system, the volume of primary air is substantially less, and therefore the fan power reduction when compared to a VAV base case is favorable. ES
Simmonds is a senior associate and head of the Advanced Technology Group for IBE Consulting Engineers (Sherman Oaks, CA).