ASHRAE Journal: Designing Chilled Beams for Thermal ... - TROX

This article was published in ASHRAE Journal, Vol. 51, October 2009. Copyright 2009 American Society of Heating, Refrigerating and Air- 

Conditioning Engineers, Inc. Reprinted by permission at www.troxusa.com. This article may not be copied and/or distributed electronically or 

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DESIGNING 

Chilled Beams 

FOR 

Thermal Comfort 

By Ken Loudermilk, P.E., Member ASHRAE 

Active chilled beams have been used in Europe since the mid-1990s but 

only recently have become a popular alternative to all-air HVAC systems 

for nonresidential buildings in North America. Although active chilled beams 

are an in-room space conditioning device, they are also the room air diffusion 

device, so sizing and locating the beams is vital to providing acceptable levels 

of occupant thermal comfort as established by ASHRAE’s thermal comfort 

standard (55-2004). 1 

Designers also should devote adequate 

consideration to the specification 

and maintenance of space humidity 

levels where chilled beams are to be 

used. Designing for space humidity 

levels that are unnecessarily low can 

result in primary airflow rates that are 

higher than necessary, and neglecting 

space latent loads can lead to excessive 

space humidity levels and potential 

condensation issues. 

Principles of Operation 

Figure 1 illustrates an active chilled 

beam. Primary air that has been cooled 

and dehumidified (1) is ducted from 

the central air-handling unit to a distribution 

plenum within the beam from 

which it is injected (2) through a series 

of nozzles. The velocity of the primary 

air jets entrain room air (3) through 

the beam’s integral heat transfer coil 

where it is reconditioned (4), then subsequently 

mixed with the primary air 

before this mixture is discharged to the 

space (5). The volume of induced room 

air is typically two to five times that of 

the primary air, depending on the size 

and design of the induction nozzles 

used, thus the discharge air mixture is 

three to six times the primary airflow 

rate. The ratio of the induced airflow 

rate to the primary (ducted) airflow rate 

is referred to as the beam’s induction 

ratio (IR). 

About the Author 

Ken Loudermilk, P.E., is the vice president of 

technology development for TROX USA, Cumming, 

Ga. He is vice chair of ASHRAE Technical 

Committee 5.3, Room Air Distribution, and chairs 

the TC 5.3 subcommittee on chilled beams. 

5 8 A S H R A E J o u r n a l a s h r a e . o r g O c t o b e r 2 0 0 9

The sensible cooling coil within the beam is supplied with 

chilled water whose supply temperature is maintained at, or 

above, the space dew-point temperature to prevent condensation. 

The sensible heat removed by the coil typically constitutes 

50% to 75% of the required space sensible heat removal. As a 

result, the primary airflow rate required to accomplish the space 

sensible cooling can be reduced accordingly. 

Although primary (ducted) airflow rates associated with 

chilled beams are considerably lower than those in all-air systems, 

their discharge airflow rate to the room is always greater. 

Since the chilled water supplied to the beam is maintained 

above the space dew point, the beam’s off-coil temperature 

will be higher than the primary air temperatures used in all-air 

systems. The resultant temperature of the beam’s discharge 

mixture is typically 3°F to 6°F (2°C to 3.3°C) warmer than 

that of all-air systems. Therefore, a proportionally higher 

(20% to 30%) discharge airflow rate to the space must be 

provided. This higher discharge flow rate often contributes to 

greater draft risks, which may compromise occupant thermal 

comfort levels. 

V L ’ T L 

1 

Beam A 

V O ’ T O 

Q S 

Q A Q A 

2 

4 

5 

3 

A 

Beam B 

V C 

V O ’ T O 

Q S 

H1 

V H1 ’ T H1 

Designing for Occupant Thermal Comfort 

Standard 55-2004 2 defines the occupied zone as the portion of 

a space where occupants normally reside. It is further quantified 

as the volume of the room that is (1) no closer than 3.3 ft (1 m) 

from any outside walls or windows nor within 1 ft (0.3 m) of 

any internal wall and (2) is vertically bounded by the floor and 

the head level of the predominant space occupants. Although 

the head level is often accepted to be 67 in. (1.7 m) for standing 

occupants, the standard allows the designer to define that 

height according to the space occupancy. 

For example, if a space is predominantly occupied by seated 

persons, the occupied zone height could be considered as 42 in. 

(1.1 m). Chapter 20 of the 2007 ASHRAE Handbook— HVAC 

Applications 3 predicts the percentage of occupants who might 

express thermal dissatisfaction for various combinations of 

local air speed and temperatures. Figure 2 (from that chapter) 

can be used to predict the percentage of occupants that will 

object to various air speeds and temperatures at the neck and 

ankle regions. As active chilled beams are normally mounted 

overhead, the neck region is usually the most critical. Comfort 

cooling applications should strive to minimize dissatisfaction 

levels, and in all cases limit the percentage of occupants objecting 

to these local conditions to 20% or less. 

Room Air Distribution 

Active chilled beams distribute air within the room in a 

manner consistent with that of linear slot diffusers. As such, 

relationships between airstream terminal velocities and thermal 

decay of the supply airstream that apply to linear slot diffusers 

also apply to active chilled beams. Upon discharge to the open 

space, velocity and temperature differentials between the supply 

air mixture and the room begin to diminish due to room air 

entrainment. Linear slot diffusers exhibit relatively long throw 

characteristics and their velocity and temperature differentials 

1 m 

(0.3 ft) 

Occupied Zone 

(Height Determined By Designer) 

Figure 1: Application of active chilled beams. 

diminish at a rate that is proportional to the distance the air has 

traveled within the space. 

Manufacturers publish throw values that allow designers to 

estimate the travel distance of the airstream before it reaches a 

given terminal velocity. Most manufacturers present such data 

using isothermal air for terminal velocities of 150, 100 and 50 

fpm (0.75, 0.5 and 0.25 m/s). These data can be used to map 

the airstream and predict the local velocity at the point where it 

enters the occupied zone. As the room-to-supply-air-differential 

decays at a similar rate, its temperature also can be predicted 

at the entry point based on the initial temperature difference 

(ΔT O ) between the beam discharge temperature and that of the 

room into which it is introduced. Manufacturers supply selection 

software that can be used predict the value of local velocities 

and temperatures at critical locations such as that where the 

airstream enters the occupied zone. 

Figure 1 illustrates a space being served by two active beams 

with two-way discharge patterns delivering identical primary 

(Q P ) and discharge (Q S ) airflow rates. The discharge airflow 

rate is a function of the induction ratio of the nozzles chosen 

and is calculated by multiplying the primary airflow rate by the 

induction ratio. Assume a beam produces an induction ratio of 

2.5 and is sized to deliver 100 cfm (170 m 3 /h) of 55°F (13°C) 

primary air to a 75°F (24°C) room. Also, assume that chilled 

water enters the beam at 57°F (14°C) and leaves at 61°F (16°C). 

The discharge airflow rate to the space will be 3.5 times the 

O c t o b e r 2 0 0 9 A S H R A E J o u r n a l 5 9

primary airflow rate or 350 cfm (595 m 3 /h). The temperature 

(T OC ) of the air leaving the beam’s integral cooling coil can 

be conservatively estimated as 1°F (0.6°C) warmer than its 

mean chilled water temperature, which is the average of the 

entering and leaving chilled water temperatures. In fact, the 

leaving air temperature would typically be at least 2°F to 4°F 

(1°C to 2°C) higher than the coil mean water temperature. The 

beam manufacturer has this information as well as the beam’s 

IR. Upon identifying the primary air temperature (T PA ), the 

temperature (T Z ) leaving the beam as well as the initial room 

to supply air temperature difference (ΔT Z ) can be estimated 

using Equations 1 and 2. 

T Z = [T PA + (T OC × IR)] / (IR+1) (1) 

or, for this example, 

ΔT Z = T ROOM – T Z (2) 

T Z = [55°F + (60°F × 2.5)] / (2.5 + 1) = 58.6°F and 

ΔT Z = 75°F – 58.6°F = 16.4°F 

The initial velocity (V O ) of a supply airstream leaving the 

discharge slot can be calculated by dividing the supply airflow 

rate leaving each slot (for two-way beams this is 0.5 x Q S ) by the 

effective area of that slot. If the effective area is not available, 

V O can be conservatively estimated as 450 fpm (2.3 m/s) for the 

purposes of this calculation. The local temperature difference 

(ΔT X ) between the room and the supply airstream at any point 

along its travel can be estimated by Equation 3. 4 

ΔT X = 0.8 × ΔT Z × (V X / V O ) (3) 

For a beam with an initial discharge velocity (V O ) of 450 fpm 

(2.3 m/s) and an initial supply to room temperature differential 

(ΔT Z ) of 16.4°F (9.1°C), the local temperature differential (ΔT X ) 

at the point coincident with a 50 fpm (0.25 m/s) terminal velocity 

is about 1.4°F (0.9°C). Referring to Figure 2, this predicts 

that less than 20% of the occupants would be dissatisfied with 

these local velocity/temperature conditions. 

As the region near the outside walls is not defined as part 

of the occupied zone, local velocities and temperatures do not 

generally affect occupant thermal comfort. Care should still be 

taken that they are not so high that they can affect processes 

(e.g., fume hoods) along the outer wall and that they are sufficient 

to provide adequate heating where applicable. Chapter 56 

of the 2007 ASHRAE Handbook—HVAC Applications 5 recommends 

that outlets used for perimeter heating be selected and 

located such that their isothermal throw to 150 fpm (0.75 m/s) 

extends at least halfway down the outside surface or to a level 

5 ft (1.5 m) above the floor, whichever is greater. 

The area of greatest draft risk usually occurs directly below 

the point where two opposing airstreams collide. Figure 1 illustrates 

such a point and defines the collision velocity as V C . 

If the velocity (V C ) of the colliding airstreams is of sufficient 

velocity (greater than 100 fpm or 0.5 m/s), some of the velocity 

and temperature differential of the airstreams is dissipated by the 

collision and the velocity (V H1 ) at the point where the airstream 

Figure 2: Percentage of occupants objecting to drafts in air. 3 

enters the occupied zone is reduced accordingly. Figure 3 can 

be used to estimate the velocity at the entry point for various 

collision velocities and vertical distances (H1) between the 

point of collision and the top of the occupied zone. This figure 

illustrates that the velocity (V H1 ) entering the occupied zone 

directly below the point of collision of two airstreams will be 

less than half the collision velocity (V C ) if distance H1 (the 

distance between the ceiling and the top of the designated occupied 

zone) is greater than or equal to 3.5 ft (1.1 m). 

In cases where the collision velocity (V C ) is 100 fpm (0.5 m/s) 

V H1 would be less than or equal to 50 fpm provided H1 is greater 

than 3.5 ft (1.1 m). In cases where H1 is greater than 3.5 ft (1.1 

m), active chilled beams should be sized and located so that 

their throw to a terminal velocity of 100 fpm (0.5 m/s) does not 

exceed half the distance to another beam with an opposing blow. 

In cases where the collision velocity (V C ) is 150 fpm (0.75 

m/s), V H1 is less than or equal to 50 fpm provided H1 is greater 

than 6 ft (1.8 m). Active beams for which H1 is greater than 


or equal to 6 ft (1.8 m) can be sized such that their throw to a 

terminal velocity of 150 fpm (0.75 m/s) does not exceed half 

the distance to a beam with an opposing discharge. 

Figure 3 can be used to determine the maximum collision 

velocity that limits velocities within the occupied zone to 50 

fpm (0.25 m/s) or less for any given distance H1 using the following 

equation: 

V C = 50 / (V H1 / V C ) (4) 

where V H1 / V C is the value from Figure 3 that corresponds to 

the distance H1. 

For example, if H1 is equal to 4 ft, 

V C = 50 / 0.43 = 116 fpm 

Room Humidity Design Considerations 

The sensible cooling contribution with chilled beams affords 

the designer an opportunity to significantly reduce the 

primary airflow rate compared to all air systems. As 50% to 

75% of the sensible heat gains are typically removed by the 

chilled water coil, proportional reductions in the primary 

airflow rates within the system may be achievable. However, 

this should be done with caution as the beam must also deliver 

sufficient ventilation air and maintain acceptable space 

humidity levels. The primary airflow rate to the room must 

be the greater of that required to (1) ventilate the space in 

conformance to ASHRAE Standard 62.1-2007 6 (or other 

applicable ventilation codes); (2) offset space latent gains 

to control the room humidity level within ASHRAE Standard 

55-2004 recommendations; and (3) provide sufficient 

sensible cooling to complement the sensible heat removed 

by the chilled water coil. 

In most common interior space applications, the primary 

airflow rate required to offset space latent gains will exceed 

both the ventilation airflow rate and the airflow rate required 

to complement the coil’s sensible cooling. The space airflow 

rate will be determined by the latent gains and the design 

room humidity ratio (W ROOM ). In perimeter spaces, the 

primary airflow rate to the beams will be driven by the sensible 

load (laboratories may be exceptions due to their high 

mandated ventilation rates). The use of beams whose water 

side cooling capacity contributes to more than about 65% 

of the total space load may be impractical due to architectural 

constraints that limit the installed beam quantities and 

lengths in these spaces. The goal of the design should be to 

reduce the primary airflow rate to as close to the required 

ventilation rate as possible. 

Designing chilled beam systems to maintain room air humidity 

levels lower than necessary can result in considerably 

higher primary air requirements. Figure 4 is presented in 

Standard 55-2004 7 and prescribes acceptable ranges of room 

temperatures and humidity ratios. Assuming a clothing level 

of 1.0 clo (0.15 m 2 · k/W), this diagram defines the thermal 

comfort window for a dry-bulb temperature of 75°F (24°C) 

to include room dew-point temperatures as high as 62°F 

Distance H1 (ft) 

7.0 

6.5 

6.0 

5.5 

5.0 

4.5 

4.0 

3.5 

3.0 

2.5 

0.3 0.4 0.5 0.6 0.7 

V H1 /V C 

Figure 3: Velocities entering the occupied zone. 

Figure 4: ASHRAE Summer and Winter Comfort Zones. Acceptable 

ranges of operative temperature and humidity with air speed ≤ 40 

fpm (≤0.20 m/s) for people wearing 1.0 and 0.5 clo (0.15 m 2 · k/W 

and 0.08 m 2 · k/W) clothing during primarily sedentary activity 

(≤1.1 met [≤63.9 W/m 2 ]). 8 

(16.7°C). Where chilled beams are applied, the room dewpoint 

temperature must not exceed the chilled water supply 

temperature, so design for dew points above about 57°F (14°C) 

is not recommended. 

Most conventional HVAC systems condition delivered air to 

about a 52°F (11°C) dew-point temperature, which coincides 

with a humidity ratio (W PRIMARY ) of 58 grains (3.8 g). The 

airflow rate (Q PRIMARY ) to offset the space latent heat gains 

(q LATENT ) can be determined by Equation 5. 

Q PRIMARY = q LATENT / [0.68 × (W ROOM – W PRIMARY )] (5) 

Using this equation and assuming the humidity ratio of the 

primary air is 58 grains (3.8 g), the primary airflow requirement 


for a space whose latent gains total 400 Btu/h (117 W) can be 

calculated for various design humidity ratios: 

If W R OOM = 65 grains (50% RH) → Q PRIMARY = 84 cfm 



In this case, designing for 75°F (24°C) and 50% relative 

humidity would result in a primary airflow rate that is 58% 

higher than that required to maintain 53% RH in the space. As 

the 53% RH is within Standard 55-2004 recommendations and 

results in a space dew-point temperature of 57°F (14°C), it is 

probably a reasonable design goal. 

Chilled beams are often used with central HVAC equipment 

that includes heat recovery and enthalpy wheels. Lower space 

dew-point temperatures can be achieved when the dew-point 

temperature is further suppressed by these processes. In cases 

where room dew-point temperatures below 55°F (13°C) are 

desired, using such equipment is recommended. 

Summary 

Active chilled beams can be selected to remove large amounts 

of sensible heat while substantially reducing primary airflow 

requirements. However, this must be done with consideration 

Advertisement formerly in this space. 

of the occupant thermal comfort and space dehumidification. 

Because producing high levels of thermal comfort is the primary 

objective of any comfort cooling application, beams should be 

selected, sized and located with that in mind. 

While primary airflow reduction opportunities are an inherent 

characteristic of chilled beams, the reduction of such should 

be limited to that required to provide adequate space humidity 

control. All-air systems almost always deliver a sufficient 

amount of dry air to satisfy the space sensible load, therefore, 

engineers often do not consider space latent loads in their selection. 

Individual space latent loads should be considered when 

designing chilled beam systems. 

In conclusion, the following design guidelines should be observed 

when selecting, sizing and locating active chilled beams: 

• Chilled beams should not be used in low ceiling 

height applications where the distance between the 

ceiling and the top of the occupied zone is less than 

3 ft (0.9 m). 

• When applied in lobbies, atriums or other areas with 

high and/or uncontrollable infiltration rates, provide 

adequate condensation prevention strategies. 

• To maintain high levels of thermal comfort (velocities 

within the occupied zone no greater than 50 fpm 

or [0.25 m/s]), active chilled beams were mounted 

at least 3.5 ft (1.1 m) above the designated occupied 

zone should be sized and located such that their throw 

to a terminal velocity of 100 fpm (0.5 m/s) does not 

exceed half the distance between them and another 

beam with an opposing blow. Active beams mounted 

6 ft (2 m)or more above the designated occupied zone 

may be located such that their throw to a terminal 

velocity of 150 fpm (0.75 m/s) is as much as half the 

distance between the beam and an adjacent beam with 

an opposing discharge. 

• Smaller nozzles result in higher induction ratios and 

higher sensible cooling capacities per cfm (m 3 /h) of 

primary air. However, the use of smaller nozzles generally 

results in higher noise levels and inlet pressure 

requirements for a given primary airflow rate that 

increases the number of beams required. 

• Designing for space humidity levels lower than that 

actually required may result in significantly higher 

primary airflow rates. 

References 

1. ANSI/ASHRAE Standard 55-2004, Thermal Environmental 

Conditions for Human Occupancy. 

2. Standard 55-2004, p. 3. 

3. 2009 ASHRAE Handbook—Fundamentals, p. 20.13. 

4. Koestal, A. 1954. “Computing temperature and velocities in 

vertical jets of hot or cold air.” ASHVE Transactions 60:385. 

5. 2007 ASHRAE Handbook—HVAC Applications, p. 56.4. 

6. ANSI/ASHRAE Standard 62.1-2004, Ventilation for Acceptable 

Indoor Air Quality, Table 6-1. 

7. Standard 55-2004, p. 5.2.1.1. 

8. 2009 ASHRAE Handbook—Fundamentals, p. 9.12. 

6 4 A S H R A E J o u r n a l O c t o b e r 2 0 0 9

ASHRAE Journal: Designing Chilled Beams for Thermal ... - TROX

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