# HVAC Systems

## ![](media/image1.png){width="4.5in" height="3.3743055555555554in"}Right Sizing

### 

### Summary

The need to provide 100% uptime with a near equivalent reliability in
critical facilities does not preclude the opportunity to reduce first
cost and improve efficiency by avoiding inefficient oversizing of
mechanical equipment. Providing redundant capacity and sizing for true
peak loads can be done in a manner that improves overall system
efficiency, but only if a system design approach is used that considers
efficiency among the design parameters.

Upsizing the duct and piping infrastructure used to supply ventilation
and cooling to the cleanroom offers significant operating cost and
future flexibility benefits. New VFD (variable frequency drive) chillers
significantly reduce the penalty from oversized chillers and are highly
recommended. Pursuing efficient design techniques such as medium
temperature cooling loops and waterside free cooling can also mitigate
the impact of oversizing by eliminating the need for electric chillers
during the lowest load periods of the year.

Cooling towers are a very small portion of plant energy consumption and
upsizing them can provide a significant improvement to chiller
performance and waterside free cooling system operation. There is little
downside to upsizing them: a slight increase in cost and footprint are
required. Boilers are a more difficult design problem. The gas
consumption associated with maintaining excess boiler capacity (at
operating temperature) at idle can be a significant cost. The use of
heat recovery, run around coils or recirculation air for reheat can
allow for the majority of a boiler plant to be shutdown during specific
seasons.

### Principles

-   Have designing engineers, operating engineers (process and facility)
    and owner explicitly set loads, oversizing and safety factors early
    in design. Use these collaboratively derived values, rather than the
    traditional ad-hoc approach that leads to multiple safety factors
    accumulating through various design stages.

-   Upsize ducts and piping to allow for future flexibility and to gain
    immediate operation and equipment downsizing benefits.

-   Size chillers and boilers with care to avoid efficiency penalties
    when operating at normal loads that are far below the required peak
    capacity. Unequal unit sizing and a modular approach to add more
    capacity as load grows is often called for.

-   Utilize variable frequency drives to acquire operational savings
    from oversized fans, pumps, cooling towers, and some types of
    chillers.

-   Deploy redundant air handler, scrubber and cooling tower units in a
    manner that they can be used in normal operation to reduce pressure
    drop, reducing power requirements and providing backup capacity.

-   Size coils and heat exchangers to provide the proper design
    temperature differential at all loads.

### Approach

Right sizing means taking an in-depth, analytical approach to designing
the *system*, not just components. Standard rules of thumb and design
assumptions can produce an operable system, but they often result in
grossly oversized and inefficient systems. While redundancy and future
flexibility requirements typically demand a certain amount of surplus
capacity, careful design can minimize (or reduce) the first cost and
actually improve performance.

To right size a facility's systems, an early coordination meeting
between the design engineers, the operating engineers (facilities and
process) and the owner is required. At the meeting, the required safety
factor, design redundancy and oversizing for future growth should be
explicitly stated and documented. A number of key design parameters,
such as W/sf (Watts per square foot) process loads, redundancy
requirement, and desired surplus capacity, have impacts throughout the
design. For example, the table below shows the evolution of the W/sf
design value in a typical, uncoordinated design effort where information
is handed off between disciplines with little in-depth coordination.

Table 1. Impact of Implicit Oversizing Creep

  ------------- -------------- -------------------------------------------
  W/sf          Cumulative     Design Participant
  Effective     Safety /       
  Design Value  Oversize       
                Factor         

  50 W/sf       1              Actual requirement. Typically, can only be
                               determined by direct measurement after
                               completion. Note that nameplate electrical
                               data is typically maximum startup value
                               only.

  100 W/sf      2              Process Engineers. A safety factor is added
                               to allow for unknown future loads or
                               equipment additions late in the design
                               process to set this requirement. Can also
                               include the influence of manufacturer's
                               data, which often has a large safety factor
                               added in by the manufacturer (particularly
                               in exhaust rates).

  100 W/sf      2              Architect / General Contractor. Typically
                               the client requirements are passed through
                               directly to engineers.

  150 W/sf      3              Mechanical Engineer. To provide a design
                               safety factor, an additional 50% - 100%
                               sizing factor is added. Influences design
                               from chiller plant to coils to fans.

  225 W/sf      4.5            Electrical Engineer. To provide for future
                               capacity, equipment sizes are rounded up.
                               Oversized mechanical equipment also needs
                               to be fully served, having a noticeable
                               impact on the total W/sf requirements of
                               the space.
  ------------- -------------- -------------------------------------------

The above table is an extreme example of what can happen in an ad hoc
design process where everyone adds in an additional oversizing factor.
Several design parameters can undergo such oversizing creep.
Communication across the disciplines is required from the initial design
stages to prevent the operating and first cost bloat that can result
from oversizing creep. Safety factors and future capacity oversizing
requests should be clearly stated and then conformed to by all
disciplines rather than allowing them to grow haphazardly. Wherever
possible, design criteria should be based on measurements of equivalent
facilities and oversizing factors should be clearly recognized as a
significant cost-adder to the project.

Different disciplines may have different redundancy requirements, such
as N+2 on electrical transformers, N+1 on makeup air handlers, and no
redundancy on emergency generators. Likewise, individual systems may
have different redundancy requirements, such as N+1 on makeup air units
but no redundancy on recirculation units where loss of a single unit may
result in a permissively small (and temporary) reduction in
recirculation rate. The oversizing requirements are complex issues that
require ongoing coordination efforts and should be treated accordingly.
Further notes on the effects of oversizing on individual systems follow.

#### Ducts and Pipes, Fans and Pumps

Traditional approaches to sizing ducts and piping have been developed
over the years with a typical office building bias. Office buildings
typically operate for only half the year, 4,300 hours or less, and the
rules of thumb for duct and pipe sizing reflect this. In a critical
facility the energy cost of operating the system is a proportionally
higher concern than the first cost. Sizing air handlers for 500 fpm
(feet per minute), ducts for 0.1 in. w.g. (water gauge) constant
friction or piping for 3 ft. w.g./100 ft. is often a poor tradeoff of
first costs versus high operating costs incurred in 24-hour use
facilities. Sizing decisions should consider the actual operating costs
of the system in the context of its cleanroom application.

Lower pressure drop systems offer an immediate savings in smaller fans,
pumps and electrical power distribution. When sizing duct and piping
circuits, consider the first-cost savings that can result from reducing
the size of fan or pump motors, motor drives, and associated circuits.
The most efficient system designs will use large ducts and pipes on the
critical circuits and small fans and pumps; the fan and pump downsizing
helps to offset the additional first cost of larger ducts and pipes and
yields ongoing energy savings.

The future exhaust, ventilation and cooling requirements of cleanrooms
can be very difficult to predict. Sizing ducts and piping to provide for
future growth of the facility is a prudent measure. Due to the square
relationship between pressure drop and flow (doubling flow requires four
times the power), ducts and pipes tend to be the limiting factor when
looking to increase flow. During expansions, it is usually a feasible,
if not simple, task to upsize a pump and motor or add an additional air
handler, but increasing the duct or piping capacity throughout an
operating cleanroom tends to be very difficult and expensive.

#### Air Handlers and Scrubbers

It is common practice to supply redundant air handlers and scrubbers to
ensure the highest system reliability. These air handling devices
represent a significant portion of the pressure drop in the air side
system. If the fans are equipped with VFDs and the units are installed
in a parallel configuration that allow all of the air handlers or
scrubbers to be run at a reduced speed during normal operation,
significant energy savings can be realized.

For example, running three air handlers at 67% capacity rather than just
two at 100% capacity reduces the total power required to overcome the
internal air handler pressure drop by over 50%. A similar savings occurs
with scrubbers, although it will be slightly less due to increased
circulation pumping power. Operating all units, including the redundant
unit, in parallel has the additional benefit of having the redundant
unit as always operational and prepared to provide seamless backup. By
running the redundant air handler(s), slightly more maintenance will be
required. However, maintenance costs are outweighed by ongoing energy
cost savings.

#### Chillers, Chilled Water Loop, Cooling Towers

Standard constant speed centrifugal chillers operate most efficiently at
or near their design capacity. Depending on the number of paralleled
chillers used to meet a cleanroom's peak load, chillers rarely run at
peak capacity. Variable frequency drive (VFD, also referred to as
variable speed drive) equipped centrifugal chillers are a mature
technology to handle part load conditions without a significant
efficiency penalty. When equivalent chiller selections (similar
condenser and evaporator tubing and configuration) are compared, the VFD
chiller usually offers superior part load efficiency.

Figure 1. High Efficiency Centrifugal Chiller Comparison

![](media/image2.wmf){width="5.872916666666667in"
height="3.9611111111111112in"}

> 800 ton chillers operating at 44°F chilled water supply and 65°F
> condenser water inlet temperatures.

Chillers should be staged to minimize the number of chillers operating
at any given time. Barring an unusually advanced and complex pumping
system, staging on an additional chiller causes an immediate drop in
total plant efficiency as condenser water pumping power and tower fan
power jump up. At sub-design conditions, where modern chillers taking
advantage of condenser water reset should operate for many hours of the
year, a chiller can supply in excess of its nameplate capacity. The
controls system should stage chillers such that this extra capacity is
used to delay the staging on of an additional chiller with its
associated pumping costs.

The chilled water loop components should be designed for a large
temperature differential (delta T) to minimize pumping requirements. If
a single low temperature loop is used to supply medium temperature
loads, such as process equipment cooling water or recirculation sensible
cooling, a heat exchanger should be selected that provides a large delta
T. Due to the inherent surplus capacity of critical facility cooling
plants, low delta T syndrome (below design temperature chilled water
return at design flow) is a significant problem. By carefully selecting
all components on the loop to meet or exceed the design delta T, a
common problem of having to stage chillers on due to flow requirements
rather than load requirements can be avoided.

There are few downsides to having surplus cooling tower capacity
installed. In fact, an efficient design will allow all towers, including
the redundant unit(s) to be run simultaneously. Running all the cooling
towers in parallel maximizes the natural convective cooling and reduces
fan power requirements. This strategy requires that the cooling towers
utilize variable speed fans and have a suitable flow turndown range. By
running the redundant cooling tower(s), slightly more maintenance will
be required. However, ongoing energy cost savings outweigh the
maintenance costs.

#### Boilers

The boiler plant should be sized to minimize the energy used by idle
boilers. Due to dehumidification reheat loads, it is not uncommon to
find boiler plants operating throughout the summer. The fuel costs of
maintaining a boiler at operating temperature can be considerable.
Ideally, reheat requirements will be eliminated through the use of heat
recovery, run around coils, or simply a distribution system that allows
for the use of recirculation air for reheat. Such a system approach can
allow for the majority (possibly all in very high internal load
facilities) of the boiler plant to be shutdown in the heating season.
Barring this level of system design, the use of different sized boilers
with an intelligent seasonal switchover control is recommended to
minimize the idle boiler capacity that is kept fired during low load
periods.

### Related Best Practices

Low Pressure Drop Air Systems Variable Speed Chillers

Recirculation System Types Cooling Tower and Condenser Water
Optimization

Air Change Rates Variable Speed Pumping

Exhaust Optimization Dual Temperature Cooling Loops

Fan Filter Efficiency Control of Chilled Water Systems

### References

1)  

### Resources

-