|
— By Mike Thoresen & Jim Regan
Comfort and control, flexibility, equipment loads, energy efficiency and reliability: Today’s
business environment is placing increased demands on building mechanical systems in a wide
variety of ways. At the same time, improved HVAC technologies and design strategies offer
many opportunities to meet these demands cost-effectively.
Facility executives have always been concerned with maximizing occupants’ comfort and
minimizing complaints. Today this is more important than ever. In fact, space comfort —
including temperature and air quality — is the number one or two complaint in most surveys of
office workers. Business owners are recognizing the effect of the office environment on their
ability to attract and retain highly qualified employees.
Part of the issue of comfort is control: the increased demand by occupants to control the
temperature and air flow in their immediate area. Where mechanical engineers used to group
four or five perimeter offices together on one thermostat, now there are often only three
offices, on average, per thermostat.
Occupants also are demanding improved air quality, whether that is achieved with increased
air flow, improved filtration or a combination of the two. With today’s attention on “green
building” construction in all its aspects — including energy efficiency, construction
materials, outside air exchange and purity — air quality issues must be addressed in just
about every project.
Flexibility Is Key
The increased churn rates in today’s commercial buildings require increased system
flexibility to accommodate reconfiguration of interior space. The issues of comfort, control
and flexibility are exacerbated by the need to operate businesses a greater number of hours a
day than in the past — in many cases, 24 hours a day, 7 days a week.
Although office equipment is becoming more efficient, requiring less power and putting out
less heat, there is more equipment per person — individual PCs, copiers, printers, etc. —
which means increased overall office equipment loads. In fact there has been a 50-percent
increase in design loads over the last few years from an average of 4 to 6 watts per square
foot.
Data center load density also is rising. Where 20 to 40 watts per square foot load density
was typical, now it is not uncommon to see design loads of 80 to 100 watts and higher. At the
same time, computer facilities are becoming more critical to companies’ business missions and
productivity, and there is increasing reliance on networked systems. This demands greater
reliability from the cooling system, therefore higher levels of system redundancy.
Increased Lead Time
Among the current challenges in any new construction or renovation project is increased
demand for the equipment itself. In today’s construction market, a project involving
supplemental air conditioning for a building space is typically on an 8- to 12-week
construction cycle, with an average lead time of 20 weeks to obtain an air conditioning unit.
One company specializing in cooling equipment for data centers requires over 40 weeks lead
time. This may require selection of an alternate off-the-shelf product, although it can also
affect the design strategy overall.
Renovations of older buildings present special challenges. Many older mechanical systems
simply cannot support increased loads, and supplemental systems must be installed. But each
space must have the spatial and structural capabilities to accept additional generation and
distribution equipment. Existing ceiling heights are often a problem. Rooftop space may be
limited or structurally inadequate. In some cases, there is no feasible alternative other
than taking floor space to house new mechanical rooms.
At the same time that demands are increasing on mechanical systems, improved off-the-shelf
technologies such as under-floor systems, fan-powered terminal units, variable air volume
(VAV) diffusers, direct digital control (DDC) systems and increased chiller efficiency are
providing opportunities to meet facility executives’ needs.
Below Deck
One of the most interesting developments is the underfloor air distribution system.
Traditional systems rely on above-ceiling installation of ductwork and large distribution
units. In this system, a raised floor is divided into separate plenums to control the
distribution, and these act as the “ductwork.” VAV boxes are attached directly to floor
tiles.
If the space needs to be reconfigured, those floor tiles are simply picked up and moved
elsewhere. Furthermore, the system replaces the traditional large overhead diffusers with a
greater number of small diffusers, allowing for more individual control of air flow. This
type of system carries a higher capital cost than the traditional above-ceiling distribution
system. But it typically saves space, design work and related costs in the initial
installation, and it may quickly pay for itself in a building with high tenant turnover.
That said, the system is great for new construction but is not appropriate for a retrofit.
Even in a building with good floor-to-floor height, elevator and stair landings and other
basic building construction elements present obstacles, and the cost of accommodation becomes
prohibitive.
Fan-powered Units, VAV Diffusers
A standard VAV box reduces the amount of air as the load decreases. In a variation on the
traditional above-ceiling distribution system, fan-powered terminal units can replace
standard VAV boxes to increase occupants’ comfort. A fan-powered VAV box reduces the amount
of primary or cold air as the cooling load decreases, but mixes in return air to keep a
constant circulation rate in the space, which tends to make people feel more comfortable.
VAV diffusers also can be used to offer control over air flow in individual offices, although
at a cost of 20 to 30 percent higher than a standard VAV system. Both fan-powered terminal
units and VAV diffusers can be introduced in a renovation without great difficulty, although
a fan-powered terminal unit is larger than a standard VAV box, presenting challenges where
there is limited space. In contrast, VAV diffusers require less space.
Other Systems
DDC systems provide more information about the operation of individual equipment in occupant
spaces (e.g., temperature and volume of air), centralized equipment control and improved
documentation of HVAC operation. A DDC system either can be overlaid on, or replace, an
existing control system — and it can control nearly all types of HVAC equipment. So it is a
good option in either renovation or new construction.
The new generation of chillers is more efficient. Fifteen years ago a fairly efficient
chiller was 0.7 kilowatts per ton; today they may be 0.5 kilowatts per ton and less. Amenable
to use either in new or existing facilities, new chillers can improve the building’s energy
efficiency, in some cases, to a significant degree.
In existing facilities, the distribution system’s energy efficiency can be improved by a
retrofit using variable frequency drives on fans and pumps. Although these have been around
for some time, they have gotten better, smaller and less expensive.
System Redundancy
When it is essential to keep computer networks and data centers up and running, building
owners take a serious look at the investment in system redundancy. N+1 has become pretty much
the minimum standard, providing back-up for every piece of equipment, if not throughout a
facility, then in key areas.
Take a brokerage house, for example, where a trading floor and data center may be supported
by two separate, interconnected, fully redundant systems — two separate piping risers, either
of which can support the system, two sets of pumps, and two sets of cooling equipment. If any
part of the system fails, the other takes over. The degree of redundancy implemented depends
on the degree of risk the owner is able and willing to take.
At the same time that new and improved HVAC technologies are being deployed, we are also
seeing improvements in other types of building systems and office equipment, which are having
a positive impact on the size of HVAC equipment and energy required to heat and cool office
buildings. For example, new glass and lighting technologies — even flat screen PC monitors —
reduce heat gain in the building and could reduce size requirements of the mechanical system.
Are there technologies over the horizon that will improve the operational efficiencies? Every
time HVAC system technology seemed to have reached its pinnacle, this assumption has been
proven wrong. The gains may come a little slower than in other fields, but they are probably
still out there. There is new sound-wave based refrigeration under development, for example,
which some day may be applicable to commercial HVAC equipment.
Given that the efficiency of HVAC technology generally improves in slow, steady increments,
rather than great leaps of efficiency, it pays to consider upgrades about every 15 to 20
years. At that point, the gains are usually sufficient to justify a capital investment. But
HVAC technology is only part of the story. An effective design strategy is also essential to
getting the most out of opportunities for increased comfort and control, flexibility,
capacity, energy efficiency and reliability.
Effective Design Strategies
The past few years have seen a greater recognition of the importance of advance planning in
HVAC systems design. Part of the shift has come about because of new computerized tools that
help maximize system design and operational efficiency.
Even more significant has been the realization of the benefits that can accrue from having
mechanical engineers involved as early as possible in the design of a new facility or
renovation project, rather than being brought in after many essential decisions about the
overall building have been made. The concept of “whole building design” requires architects
and mechanical, electrical and structural engineers to work as a team to develop a building
that not only meets an owner’s program needs, but performs efficiently and cost-effectively.
For example, consider the impact of heat gain, building siting and orientation, lighting
strategies, and wall and roof insulation systems on the mechanical system. Unfortunately,
whole building design is the exception rather than the rule. Yet it can have a big impact on
the HVAC system.
HVAC today is characterized by new demands for comfort and control, flexibility, capacity,
energy efficiency and reliability, and new opportunities to apply improved off-the-shelf
technologies to meet building owners’ and managers’ needs. To ensure the best outcome, bring
all members of the design and engineering team together from the outset. The measure of a
building’s operational success is more than the mere sum of its parts.
Mike Thoresen, P.E., and Jim Regan are vice presidents with Syska & Hennessy, a New York City-
based consulting engineering firm that provides technical solutions in such areas as
mechanical/electrical design, facility management, facility business management, technology
consulting and engineering, and turnkey design/build.
|
