OPTIONS IN RADIANT by jizhen1947

VIEWS: 9 PAGES: 4

									                           cooling
                           BY Geoff McDonell




OPTIONS IN RADIANT
There are several types of commercial radiant cooling systems     day, and keep the total area of the radiant panels to a min-
                                                                  imum. The greatest energy efficiency gains are found by
in use in North America. The most common are: suspended           using high performance glazing to reduce and eliminate
                                                                  the perimeter zones (high transient thermal loads).
radiant ceiling panels, applied capillary tubes and concrete
                                                                  APPliED cAPillARY TUBES
core conditioning systems (radiant slabs). Other types of         Applied capillary tube systems consist of very small di-
radiant cooling devices and systems, such as chilled beams,       ameter (three to four mm) plastic tubing imbedded into
and combined chilled ceiling panels with integrated air           walls and ceilings, usually in a layer of plaster finish. Com-
supply terminals, are not widely available. Residential           mercially available capillary tube mats are currently only
wood frame buildings may also use commonly available              manufactured in Europe using proprietary processes. They
radiant floor PEX tubing systems, applied as wall or ceil-        are generally made from durable, recyclable polypropylene
ing systems instead of floor systems.                             plastic. These can be imported to North America, but the
                                                                  costs are sometimes an issue on smaller-sized installations.
SUSPEnDED RADiAnT cEiling PAnElS                                    Polypropylene is not an oxygen barrier material, so any
Suspended radiant ceiling panels have been available for          capillary tube system has to be separated from the metallic
the last few decades, but early attempts at using them for        piping system by a stainless steel heat exchanger, and all
radiant cooling applications were less than successful. This      the pipe accessories in the plastic piping system must be
was primarily due to small panel areas being operated at          stainless steel or all bronze. The plastic pipe system does
too cool a temperature in high cooling load applications,         not require any chemical treatment, thus saving some
which results in condensation issues.                             capital and operating costs.
   The panels have a relatively                                                                       These systems are very
light thermal mass, therefore                                                                      popular and common in
they respond quite quick-            RADiAnT cEiling PAnEl SYSTEMS                                 Europe, and are well under-
ly when a control valve is           Advantages:                                                   stood by the building de-
opened or closed to modulate         • Commonly available in North America.                        signers there. The concept
the warm or cool water flow.         • Fit well into conventional dropped ceilings.                is to maximize the radiant
Typically they have a response       • Relatively energy efficient.                                surface area of the room to
time of under 20 minutes and         • Fast-acting and can be used with conventional               use the whole ceiling, wall,
can be controlled by standard            HVAC controls.                                            or both, to provide a large
HVAC controls for relatively         • Design guidelines for both heating and cooling are          radiant surface operating
fast acting temperature con-             available.                                                at very small temperature
trol functions and short term        • Easy to divide up into discrete room-by-room control        differences to the occupied
transient loads in the space.            zones as required.                                        space. This results in a very
Due to the rapid heat gain and       • Low maintenance requirements.                               energy efficient system, as
response time of these systems,      • Silent operation, with temperature exchanges at the         the main mechanical plant
a water chiller is required to           speed of light.                                           equipment does not require
provide sufficient cooling wa-                                                                     operation at extreme set-
ter at around 16C during the         Disadvantages:                                                points to provide heating
daytime occupied periods.            • Relatively high cost for large surface areas.               and cooling to the space.
   Radiant ceiling panels may        • Very light thermal mass, no ability to store and offset        These systems can be
be economically feasible if they         building loads to nighttime.                              more energy efficient com-
are used as “second stage” cool-     • Can be costly to relocate for tenant room revisions.        pared to the suspended
ing and heating temperature          • Additional seismic restraints required for heavier          ceiling panels due to the
control due to their ability to          suspended ceiling elements.                               increased thermal mass cre-
be controlled in discrete zones.     • Subject to damage in a fire or seismic event, but           ated by the plaster embed-
This will provide a bit of en-           system is easily accessible for repairs.                  ment and, in many cases,
ergy efficiency at the end of the                                                                  contact with the building
50   HPAc | APRIL 2007                                                                                      MODERN HYDRONICS
                                                     structure. This allows a lot more building mass thermal
                                                     storage, which provides a more stable indoor climate.
                                                     This also results in a relatively slower response time when
                                                     a control valve modulates. Typically response times of
                                                     15 minutes to an hour or more are common, depending
                                                     on the thickness of the plaster and its proximity to the
                                                     building’s structural mass. It is common to use slow acting
                                                     pulse width modulation controls, or regular modulating
                                                     control valves for faster response installations, to operate
                                                     these systems. Due to the relatively rapid heat gain and
                                                     response time of these systems (compared to a slab radiant
                                                     system), a water chiller is required to provide sufficient
                                                     cooling water during the daytime occupied periods.

                                                     CONCRETE CORE CONDITIONING                                        A radiant ceiling in a wood frame building, using strapping and
                                                     Concrete core conditioning or radiant slab ceilings, con-         thermal conductor plates to run the PEX tubing into. The next step
                                                     sist of plastic tubing (usually oxygen barrier PEX) cast di-      is adding insulation on top of the radiant, in between the joists
                                                     rectly into the structural slab of a concrete building system.    above them, and then fastening the ceiling material up.
                                                     A variation on this is a hollow core pre-cast concrete slab
                                                     system which uses cool nighttime air moved through the             underside of the structural slab is intended to be the fin-
                                                     hollow cores to pre-cool the concrete for the next day of          ished radiant ceiling surface in the occupied space.
                                                     cooling, or conversely, warm air to pre-heat the slabs.              Concrete core conditioning systems are becoming more
                                                        Basically this system is like a radiant floor slab, but is      popular in Europe after being pioneered in Switzerland
                                                     used as an overhead ceiling radiant slab, with the tubing          over two decades ago. Because the whole ceiling area is
                                                     kept as close to the underside of the slab as possible (usu-       the radiant surface, the operating temperature differences
                                                     ally on top of the bottom layer of reinforcing bars). The          relative to the occupied space can be kept very small for
                                                     PEX tubing is generally 5/8"- or 3/4"-diameter depend-             heating or cooling.
                                                     ing on the slab loop area/length and water flow rates re-            The key design issue is that the huge thermal mass takes a
                                                     quired for the heating and cooling loads. Tube spacing can         very long time to change temperature, so it cannot be con-
                                                     be anywhere from six- to 12" depending on the desired              trolled by conventional modulating control valves, or easily
                                                     average surface temperature of the ceiling. The exposed            divided into smaller discrete different thermal control zones.
                                                                                                                             This also means that the building glazing system and
                                                                                                                             solar control systems must be carefully designed and se-
                                                       APPlIED CAPIllARy TUbE sysTEms                                        lected to minimize the perimeter peak transient loads to
                                                       Advantages:                                                           be within the radiant slabs’ capabilities.
                                                       • Can cover very large surface areas.                                    What has to be understood by the designer is the
                                                       • “Built-in” to the building finishes and completely invisible        time lag/time response/thermal storage factors associ-
                                                           in the space. Can be integrated into a dropped ceiling or         ated with the radiant slab system and how it responds
                                                           panels depending on interior finishes.                            to short term “high peak” transient thermal loads ver-
                                                       • Good energy efficiency.                                             sus long term “low level” transient thermal loads.
                                                       • Relatively fast acting response and can be used with con-              Radiant slab systems, whether for heating or cool-
                                                           ventional HVAC controls.                                          ing or both, are normally operated by using a constant
                                                       • Easy to divide up and control in small discrete thermal con-        water flow with a variable temperature reset schedule.
                                                           trol zones as required.                                           Typically the maximum winter peak temperature of
                                                       • Conventional controls and low maintenance requirements.             a radiant ceiling slab surface is around 24.5C while
                                                       •   Silent operation.                                                 the minimum summer peak cooling temperature is
Photo Rob Brown, Northeast Radiant Technology, LLC




                                                                                                                             around 17.5C, which can provide comfortable space
                                                       Disadvantages:                                                        “resultant” temperatures.
                                                       •   Limited availability in North America, high capital costs.           In climates where cooler nighttime temperatures are
                                                       • Not easy to renovate or change for tenant revisions.                available in the summertime, the slab can be cooled
                                                       • Requires very clean water to fill the system to avoid plug-         off during the night with an evaporative closed circuit
                                                           ging the small capillary tubes.                                   fluid cooler type cooling tower. No large cooling plant
                                                       • Very little expertise or design information available in North      equipment is needed, and in most temperate climates,
                                                           America.                                                          there is no need for any refrigerant-based water cooling
                                                       • Very light thermal mass, no ability to store and offset build-      equipment. The slab essentially acts like a giant static
                                                           ing loads to nighttime.                                           heat pump since any “spot thermal loads” are absorbed
                                                       • Severe fire or seismic event will render the system inoper-         and spread out over the whole slab by the constantly
                                                            able and require extensive repairs.
                                                                                                                                               CONTINUED ON PAGE 52


                                                     MODERN HYDRONICS                                                                                         APRIL 2007 | HPAC      51
                             COOLINg
                             c o n t i n u e d f r o m PAG e 51
                                                                        RAdIANt SLAb SyStEMS
                                                                        Advantages:
                                                                        • Extremely good energy efficiency.
circulating water system. Imbedded slab temperature sen-                • Integrated into building structure.
sors coupled with space temperature sensors will allow the              • Very low maintenance requirements -- passive hydronic
system to be monitored and fine-tuned as necessary.                        system with very few controls, minimal moving parts.
   The total building costs for radiant slab systems with               • No system work required for tenant renovations,
high performance glazing systems have been proven to be                    other than air supply points.
equal to, or less than a conventional building approach                 • Can be installed to provide very large radiant sur-
with conventional all-air HVAC systems, in spite of the                    face areas at low capital cost.
premium for high performance glazing systems. Further                   • Silent operation and speed of light self-compensating
savings can be found by reduced floor to floor heights                     operation.
possible with the exposed slab ceiling and a raised floor               • Very long operational life.
(access floor) system, saving a great deal of building enve-            • Provides stable space comfort conditions.
lope costs for multi-storey buildings.
   One of the key design issues is to “design-in” future flex-          Disadvantages:
ibility to the slabs by providing cast in place inserts and             • Thermal mass storage and radiant temperature con-
empty “cans” for future services installations without hav-                 trol cannot be modeled accurately by conventional
ing to drill into the slab.                                                 HVAC system computer modeling software.
                                                                        • Requires an integrated design team.
PERFORMANCE APPRAISAL                                                   • Severe fire or seismic event will render the system in-
Radiant slab ceiling cooling systems in Europe have a track                 operable, with expensive repairs or alternate retrofit
record of over 20 years, and the energy consumption has                     HVAC systems required.
typically been 60 to 70 per cent below even the best con-               • Aesthetics of an exposed slab as a finished ceiling.
ventional “all-air” HVAC systems.1 North American ap-                       Exposed services have to be designed cleanly.
plications of large non-residential radiant heating/cooling             • Hard surfaces require acoustic design and review to
slab systems are few, but indications are that a properly                   ensure acceptable noise levels.
designed thermo-active slab building can be built at the
same or less cost than a conventional building approach.
   Radiant panels are easily available for creating an energy       in North America, there is generally a cost premium to create
efficient HVAC system but the cost of the radiant panels is         effective radiant surface panel areas to perform adequately.
such that there is a desire to minimize their area. This re-           Similarly, capillary tubes offer an excellent way of providing
sults in operation right at the extreme limits of the surface       large radiant surface areas, but the economics are a concern.
temperatures for cooling. Although there are some successful           The main design issue to consider when deciding on a
examples of operating radiant heating/cooling panels systems        radiant cooling system is that radiant cooling from the ceil-
                                                                    ing, at a radiant surface temperature of approximately 65F
 RAdIANt FLOOR HEAtINg SyStEMS are reasonably                       (18C), will provide about 80 watts/sq. meter (25 Btuh/sq.
 well understood, and many designers attempt to use radiant floor   ft.) of effective cooling capacity. But a radiant floor cooling
 slabs or concrete topping systems for both heating and cooling     system (see sidebar) at the same surface temperature will
 applications. While some radiant cooling capability may be         only provide an effective cooling capacity of 35-45 watts/
 found via the cooled floor, it is nowhere near as effective as a   sq. meter (10-14 Btuh/sq. ft.) due to the location relative to
 ceiling radiant emitter in terms of human comfort. A warm floor    the human occupants, and masking from furniture.
 certainly provides comfort, but a cool floor does not.
    Why floors are not cool:                                        n  Geoff McDonell, P.Eng. is a senior mechanical engineer
 • Floor coverings, furniture and finishes can affect and           with Omicron Architecture Engineering Construction in
 mask the output and floor surface temperature, limiting future     Vancouver, B.C. He is also a LEED Accredited Professional,
 room finish revisions.                                             as given by the United States Green Buildings Council.
 • Radiant cooling heat exchange from the floor relative
 to human comfort is poor compared to an overhead radiant           1 Lawrence Berkeley National Laboratories, Paper: “Hydronic
 surface emitter.                                                   Thermal Conditioning of Buildings by Means of Constant Supply
 • A raised floor air distribution system cannot be used.           Water Temperature”, by Helmut Feustel, 1998.
 • Future anchoring and drilling into the floors becomes
 problematic if the radiant tubing is close to the floor surface
 of the slab.
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 • Installing the radiant tubing in the floor slab requires                        number on the Reader Postcard. Thank you.
 labour consumptive methods to keep the tubing near the top             VERy USEFUL .................................................................130
 of the slab surface.                                                   USEFUL..........................................................................131
                                                                        NOt USEFUL ..................................................................132


52   HPAC | APRIL 2007                                                                                                           MODERN HYDRONICS
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