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              Email:    Dr. Stanley A. Mumma,
   Ph.D., P.E

   

      

   Since February 15, 2001


Displacement ventilation vs. ceiling diffusers:


This site takes the point of view that radiant cooling/chilled ceilings can only be considered practical when used in conjunction with a dedicated OA system (DOAS) capable of decoupling the space sensible and latent loads. Such DOAS's require low supply air dew point temperatures to decouple the loads. Consequently DOAS's are capable of delivering air at temperatures in the neighborhood of 45F, if no reheat is added. The comments that follow are intended to emphasize the importance of taking advantage of these low supply air temperatures to minimize the first cost of DOAS/Radiant cooling systems. European literature makes a case for using displacement ventilation, introduced at floor level at about 65F, with chilled ceilings to minimize mixing of room generated contaminants with the ventilation air at the breathing zone to improve indoor air quality. However in most "displacement ventilation" situations the buoyancy induced natural convection is sufficient to cause more mixing than the literature leads one to expect. Supplying the ventilation air fully 20F warmer than available in DOAS requires the radiant panels to absorb more sensible load than necessary, introducing a marked first cost penalty. Further if the DOAS air were supplied to the space via high aspiration diffusers (the cold air reaches room temperature in less than a foot of throw), capable of high secondary room air flow and motion, the convective heat transfer to the radiant panels is substantially increased. The overall increase in heat transfer is about 15% greater than when the panels are operating in still air, and 10% greater than when the panels are operating with displacement ventilation. The enhanced convective heat transfer performance further reduces the ceiling area devoted to radiant panels, and hence first cost. Finally, reheating the cold air required in DOAS's never reduces the cooling coil load as much as the lost sensible cooling. Consequently the actual chiller load and annual energy consumption is increased.

In conclusion, for the reasons enumerated above, displacement ventilation is strongly discouraged. Rather, the 45F air should be supplied to the space via high aspiration diffusers located in the ceiling. The throw of the diffusers need to parallel the longitudinal pattern of the radiant panels.


Radiant panel duty in a typical office building located in the northeast:


In a 6 story, 31,000
ft2 per floor building whose envelope conforms to ASHRAE Std 90.1, the design load based on a ventilation rate of 25,000 scfm (required by ASHRAE Std. 62.1-2004) is 506 tons. By shifting all of the ventilation load, all of the space latent load, and a part of the space sensible load (that accommodated by the ventilation air at 45F) onto the DOAS, less than 200 tons of cooling remains to be accommodated by the radiant ceiling system. As a result, on average less than 12 Btu/hr-ft
2 per gross floor area is required of the panels. As a result, less than 50% of the building ceiling has radiant cooling panels. This huge shift of the load onto the DOAS is required for cost effective applications of radiant cooling.


Transient occupancy changes and condensation:


It can be shown that it takes about 45 minutes for the dew point temperature to increase enough for condensation to begin formation after the occupancy density has increased 300% above design capacity. That means that short surges in occupancy would generally not cause any condensation on the chilled ceiling.

Another way to consider the condensation potential is in a steady state upper boundary condition. If each occupant has about 140
ft2 of floor space, and the radiant panel density is 50% of the floor area, then the condensation surface area per person is about 70 sq ft. A typical office workers latent load is 205 Btu/hr, or 0.2 lb of water vapor per hour. If it were assumed that the enclosure absorbs none of the moisture, and the ventilation air removes none of the latent load, then all of the 0.2 lb of water vapor condenses on the chilled ceiling. When converted to a volume and spread out over the 70 sq ft, the moisture thickness after one hour is 5/10,000 of an inch. Far to thin to ever see with the naked eye.

Consequently, occupant induced condensation occurs at a very slow rate, and offers considerable time to detect and take action.


Envelope Integrity:


Design of systems employing the DOAS/radiant cooling approach requires a good knowledge of the building envelope integrity. The rate of infiltration of both air and moisture impacts the required DOAS supply air volumes and thermodynamic state points, as well as the resulting space dew point conditions. The resulting space dew point temperature thereby dictates how the radiant panels will operate, i.e. the sensible heat extraction and the propensity for condensation formation. Therefore, for good design, it is strongly recommended that the building envelope conform to ASHRAE Std. 90.1-2004, Section 5.4.3 or better. For convenience, the pertinent portion is also posted below.

5.4.3 Air Leakage (Ref: ASHRAE Standard 90.1-2004 Energy Standard for Buildings
        Except Low-Rise Residential Buildings
)


5.4.3.1 Building Envelope Sealing. The following areas of the building envelope SHALL be
           sealed, caulked, gasketed, or weather-stripped to minimize air leakage:
  1. Joints around fenestration and door frames
  2. Junctions between walls and foundations, between walls at building corners, between walls and structural floors or roofs, and between walls and roof or wall panels
  3. Openings at penetrations of utility services through roofs, walls, and floors
  4. Site-built fenestration and doors
  5. Building assemblies used as ducts or plenums
  6. Joints, seams, and penetrations of vapor retarders
  7. All other openings in the building envelope.
    Outside air intakes, exhaust outlets, relief outlets, stair shaft, elevator shaft smoke relief openings, and other similar elements shall also be very low leakage.
5.4.3.2 Fenestration and Doors. Air leakage for fenestration and doors shall be determined in accordance with NFRC 400. Air leakage shall be determined by a laboratory accredited by a nationally recognized accreditation organization, such as the National Fenestration Rating Council, and shall be labeled and certified by the manufacturer. Air leakage shall not exceed 1.0 cfm/ft2 for glazed swinging entrance doors and for revolving doors and 0.4 cfm/ft2 for all other products.



Please visit the Papers section of this site for more information on
      Dedicated Outdoor Air Systems and Radiant Ceiling Panels.



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Copyright 2001 Bob Hedman.