# HVAC Water Systems

## ![](media/image1.png){width="3.875in" height="2.90625in"}Waterside Free Cooling

### Summary

Free cooling utilizes the evaporative cooling capacity of a cooling
tower to indirectly produce chilled water for use in medium temperature
loops, such as process cooling loops and sensible cooling loops. Free
cooling is best suited for climates that have wetbulb temperatures lower
than 55°F for 3,000 or more hours per year. It is most effectively
applied to serve process and/or sensible cooling loops that between
50°F--70°F chilled water.

Table 1. Free Cooling Application

  ----------------------------------- -----------------------------------
  **At least 3000 hours per year      **Applicability**
  where wet bulb temperature is       
  below:**                            

  55°F                                Process cooling water

  45°F                                Process cooling, sensible cooling

  35°F                                All chilled water use
  ----------------------------------- -----------------------------------

1.  Assumes process cooling water is between 60 to 70°F.

2.  Assumes sensible cooling water is between 50 to 55°F.

### Principles

-   Chilled water systems use chillers that typically operate at 0.5 to
    0.7 kW/ton in a partial load regime while free cooling systems
    typically operate at 0.05 to 0.15 kW/ton.

-   A flat plate heat exchanger is used to isolate the chilled water
    loop from the open tower condenser water. With few exceptions, tower
    water is not reliably clean enough for use directly in the cooling
    loop.

-   The approach temperature on a cooling tower is critical for best
    results. A low approach temperature on the tower is critical to
    achieve the highest energy savings.

-   A traditional chiller is used to provide cooling during hot periods
    and as an always-available emergency backup. For a portion of the
    year, free cooling offers a non-compressor based backup to the
    traditional chiller.

### ![](media/image2.wmf){width="6.375in" height="3.95625in"}Approach

Figure 1. Psychrometric Chart for San Jose, California

Free cooling operates on the principle that during cool weather
conditions, particularly at night for 24 hour operation facilities,
process cooling water can be produced by the cooling tower alone,
bypassing the energy intensive chiller entirely. Free cooling reduces or
eliminates the chiller's power consumption while efficiently maintaining
strict temperature and humidity requirements. Free cooling can also
offer an additional level of redundancy by providing a non-compressor
cooling solution for portions of the year.

For application beyond medium temperature loops, use of a chilled water
reset is an integral part of optimizing free cooling. A chilled water
reset increases the chilled water supply setpoint during mild conditions
when lower temperature chilled water is not required to meet cooling
needs and, conveniently, when the cooling towers are able to produce the
lowest temperature free cooling water. It is often possible to configure
cooling tower systems to isolate an idle tower during cool (far below
design conditions) weather for use as a free cooling tower. A redundant
tower can also be configured to serve free cooling since a free cooling
system is inherently considered to be an intermittent cooling source.

A cooling tower utilized for free cooling should be selected to provide
an approach temperature (*Leaving Tower Water Temperature minus Wet Bulb
Temperature*) between 5 and 8°F. A lower approach temperature generally
results in a physically larger tower since more surface area in the
tower is required for the heat rejection process. A variable speed drive
(VSD) for the fan motor should be used in a free cooling tower to
minimize on-off cycling and maximize controllability to maximize energy
savings.

![](media/image3.wmf)

> CHWS/R -- Chilled Water Supply /Return
>
> PCWS/R -- Process Cooling Water Supply/Return
>
> HX -- Heat Exchanger

Figure 2. Free Cooling Loop Schematic

Free cooling requires that the cooling tower produce low temperature
water, often lower than a chiller will accept for condenser water. There
are two common design approaches to address this concern. One option is
to hydraulically isolate a tower and dedicate it to free cooling only.
This is the best approach, but requires careful piping configuration and
control valve placement and operation. A redundant backup tower can be
provided with automatic isolation valves and used for free cooling use.
Since free cooling operates during low temperature weather conditions,
the chilled water plant load is often low enough that even non-backup
towers are available for free cooling use provided the proper automatic
valving is implemented.

The other common solution is to share a single condenser water loop and
towers between free cooling and the chillers by running the loop at a
low, free cooling temperature and providing a bypass at the chillers.
The chiller-side bypass is used to mix warm condenser water leaving the
chiller with the low temperature condenser water to produce a suitable
condenser water supply temperature. The bypass is controlled in a manner
similar to bypasses provided at the cooling tower to control the minimum
condenser water temperature. Locating the bypass at the chiller end of
the loop instead of at the cooling tower brings low temperature water
into the main plant area, in many cases greatly reducing the cost of
piping to implement free cooling. Providing such a bypass mixing valve
arrangement also ensures that starting up the chillers during cold
temperatures with a cold tower sump can be done reliably. This approach
is popular in retrofit situations or where the pipe run to the cooling
towers is too long to economically allow a second set of pipes for free
cooling. Some efficiency is lost by producing lower temperature water
for the chillers than is used, but typically this is far outweighed by
the reduced chiller compressor energy consumption.

Added costs for a waterside economizer result from controls, heat
exchangers, pump, and piping. In a typical critical facilities
installation, no additional cooling tower capacity is required.

Both existing and new cleanroom facilities can benefit from free
cooling. Cleanroom facilities are typically designed to be able to
handle a peak load. Peak loads only occur 2 to 5% of the time in a year.
Cooler weather along with lower wetbulb temperatures allow existing
facilities to retrofit chilled water plants to provide free cooling. The
common use of medium temperature process or sensible cooling loops
integrates very well with free cooling. Re-piping, particularly using
the chiller-side bypass approach, and/or adding valves and controls is
usually cost effective. New facilities, on the other hand should be
designed with the concept in mind. Added cost for free cooling is
minimal and paybacks are short when such a system is accommodated in the
initial design stage.

### Real World Experiences (Benchmarking Findings/Case Studies)

![](media/image4.wmf){width="5.624305555555556in"
height="3.751388888888889in"}

Figure 3. Cooling Tower Benchmarked Data

Figure 3 shows the LBNL Benchmarking project data collected on cooling
towers at participating cleanroom facilities. The efficiency of cooling
towers is evaluated by comparing the cooling tower fan energy to the
chilled water system output. The cooling towers were operating between
0.013 and 0.19 kW per ton with an average efficiency of 0.07 kW/ton.
Chillers typically operate at 0.5 to 0.7 kW/ton in a partial load
regime. By using a cooling tower operating at 0.07 kW/ton for free
cooling, approximately 90% of energy can be saved when the chillers are
bypassed entirely.

The implementation of free cooling at Facility C, an 80,000 sf facility
consisting of 17,000 sf of ISO Class 5 and 6 cleanrooms was evaluated in
the benchmarking study. The facility had 900 tons of installed cooling
capacity and was measured to be operating at an average of 561 tons.
With the excess cooling tower capacity, a heat exchanger to isolate the
condenser water from the process cooling loop, a minimal amount of
piping and controls would be required to implement free cooling. An
annual energy savings of 1,140 MWh and a payback of 1.2 years were
projected.

### Related Best Practices

Chilled Water Plant Controls Variable Speed Chillers

Dual Temperature Cooling Loops Cooling Tower/Condenser Optimization

### References

1)  

### Resources

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