**Contents**

[HVAC Water Systems 2](#hvac-water-systems)

[Variable‑Speed‑Drive Chillers 2](#variablespeeddrive-chillers)

[Summary 2](#summary)

[Principles 3](#principles)

[Best practice approaches 4](#best-practice-approaches)

[Case studies 5](#case-studies)

[Related best practice 6](#related-best-practice)

[References 6](#references)

[Resources 6](#resources)

# HVAC Water Systems 

## Variable‑Speed‑Drive Chillers 

### Summary 

![](media/image1.wmf){width="4.490277777777778in"
height="3.448611111111111in"}Cleanroom energy benchmarking data shows
that there is a variety of chiller plant designs and operating
efficiency for cleanroom facilities. Chiller plants usually serve
cleanroom facility and adjacent spaces simultaneously and use
significant energy and water. The efficiency level of the overall
chiller plant is influenced by the efficiency of individual components
and subsystems in the plant. Major components include chillers, water
pumps, and cooling tower or condenser fans. Figure 1 and Figure 2 show
chiller energy end‑use in typical semiconductor cleanroom facilities. In
both cases, the portion of chiller energy usage was significant. It is,
therefore, important to design, select, operate, and control chillers to
achieve high efficiency and to lower life‑cycle costs for cleanrooms and
their adjacent spaces.

Figure 1 Benchmarked HVAC energy usages in a semiconductor cleanroom
facility

![](media/image2.wmf){width="5.995833333333334in" height="4.1in"}Figure
2 Benchmarked cleanroom energy usages in a semiconductor cleanroom
facility

### Principles

The type of chiller selected and its operation normally has the most
significant impact on overall chiller plant efficiency and energy use in
the facility. Normally, water‑cooled chillers and those with evaporative
condensers are more efficient than their air‑cooled counterparts. That
is, an air-cooled chiller plant is less efficient than a combined
water‑cooled chiller and cooling tower or evaporative condenser system.

It's important to note that chillers operating at partial load
conditions usually correspond to lower energy efficiency, as illustrated
in Figure 3. This was benchmarked in one of the cleanroom facilities.

At part-load conditions, the pumps and heat rejection device often
operated at constant speed, thus increasing their fractional
contribution to reducing the overall efficiency (kW/ton). Installing a
variable‑speed drive (VSD) may increase the operating chiller part-load
efficiency.

In addition to energy savings from installing VSDs, there are other
potential benefits such as capital savings, lower sound level, reduced
wear of compressors, better electrical protection, and improved power
factors and reduced harmonics.

![](media/image3.png){width="4.083333333333333in"
height="2.5284722222222222in"}

Figure 3 Chiller efficiency vs. cooling load

### Best practice approaches 

Chiller efficiency is affected by many factors, including size,
components such as chiller motor efficiency, temperature delta (or
'lift') of chilled water, sizing of the evaporator and condenser heat
exchangers, cooling loads, and condensing temperatures. While the energy
efficiency of individual components influences the overall chiller plant
efficiency, how the equipment is controlled and operated also largely
affects the actual chiller system efficiency and therefore its energy
usage.

Providing variable‑speed drives for chillers typically improves their
operating efficiency, because the chiller operation with a
variable-speed drive is generally more efficient at part loads and more
responsive to varying loads than constant‑speed drive chillers with
their throttling or staging load controls. Electric power input can be
reduced by incorporating a variable-speed drive in a chiller when the
chiller is operating at a partial load, e.g., half load or less. Best
practice requires that relevant efficiency measures be first taken to
reduce the load. Based upon such a new load, the sizing and life‑cycle
cost benefit of a variable-speed drive can then be best evaluated and
implemented when it's feasible. Chiller efficiencies over the range of
operating conditions, with and without VSDs, are available from the
chiller manufacturers to facilitate the analysis.

![](media/image4.wmf){width="2.702777777777778in"
height="2.1104166666666666in"}

Figure 4 Chiller with a VSD in the large rectangular enclosure

### Case studies

In general, measures such as chilled water temperature reset, providing
VSD and improving control will improve the operating efficiency of
chillers. Chillers operate more efficiently when the lift (the
difference in temperature between the hot and cold refrigerant)‑ is
minimized, and when equipped with variable-speed drives.

Often, a chilled water plant may serve various loads that call for
different chilled water temperatures. In one case study, the chilled
water temperature for cleanroom process required was 60°F, chilled water
temperature for the re‑circulation air handlers and makeup air handlers
was 55°F, and chilled water for dehumidification in the makeup air
handlers and for the air handlers in other buildings served by the plant
was 38°F. Chiller efficiency is directly affected not only by chilled
water supply temperature, load, and chiller type, but also system
controls such as the use of a VSD.

Figure 5 shows the variations in water‑cooled chiller efficiency for a
group of cleanroom facilities. The chilled water temperatures ranged
from 36 to 40°F, while the chiller efficiency ranged from 0.4 to 1.2 kW
per ton. The best practice water‑cooled chiller efficiency was 0.4
kW/ton based upon the benchmarked data, for the chillers producing 43°F
chilled water. Opportunities exist to improve operating chiller
efficiency as illustrated in Figure 5. For example, providing VSD option
for water chillers for the facilities (e.g., Facility C) shown in Figure
5 would further improve operation efficiency of the chillers because
most of the chillers benchmarked were commonly operating under partial
load conditions.

![](media/image5.wmf){width="5.992361111111111in"
height="3.957638888888889in"}Figure 5 Water‑cooled chiller efficiency

### Related best practice 

-   Right sizing

-   Cooling tower and condenser optimization

-   Control of chilled water systems

-   Variable-speed drive water pumping

-   Free cooling

-   Dual temperature cooling loops

### References 

1)  http://hightech.lbl.gov

### Resources 

-   Energy Design Resources, Design Brief: Chiller Plant Efficiency,
    http://www.energydesignresources.com/resource/24/

-   ASHRAE handbook -- HVAC systems and equipments.

-   PG&E's CoolTools, http://www.hvacexchange.com/cooltools/