Water Chillers: HVAC Water Chillers and Cooling Towers Electric-Drive Chillers

Water Chillers:

HVAC Water Chillers and Cooling Towers

Electric-Drive Chillers

Electric-Drive Chillers

Electric-Drive Chillers:

Electric-drive chillers may be configured as hermetic or open-drive machines. With
open-drive chillers, the compressor and motor are separated, with their shafts being
connected via a flexible coupling. The advantage of this concept is that in the event
of motor failure, it does not contaminate the refrigerant and the motor can be readily
replaced. The disadvantage is that the chiller motor is cooled by ambient air and
these large motors may impose a high heat gain in the mechanical equipment room.
The alternative, and by far more popular design, contains both the motor and
the compressor within a common, sealed enclosure. In this configuration, the
compressor is rigidly connected directly to the motor shaft, eliminating the need
for a flexible coupling. The motor is cooled by the refrigerant flow and thus
imposes no heat gains that must be separately addressed. The only disadvantage
is that in the event of motor failure, the refrigerant system is often contaminated,
requiring a difficult and expensive cleaning in addition to replacing the motor.
The energy consumption by a rotary compressor chiller decreases as the
imposed cooling load is reduced, as shown in Figure 1.7. These chillers operate
efficiently at between ~30% and 100% load and most efficiently between 40% and
80% load. Within this capacity range, the refrigerant gas flow rate is reduced, yet
the full heat exchange surface of the cooler and the condenser is still available,
resulting in higher heat transfer efficiency.
Below about 30% load, the refrigerant gas flow rate is reduced to the point
where heat pickup from the motor and mechanical inefficiencies have stabilized
input energy requirements.

Typical rotary compressor part load performance.

The vast majority of electric-drive rotary compressor water chillers utilize
a single compressor. However, if the imposed cooling load profile indicates that
there will be significant chiller usage at or below 30% of peak load, it may be
advantageous to use a dual-compressor chiller or multiple single-compressor
chillers.
The dual-compressor chiller typically uses two compressors, each sized for
50% of the peak load. At 50–100% of design load, both compressors operate. But,
if the imposed load drops below 50% of the design value, one compressor is
stopped and the remaining compressor is used to satisfy the imposed load. This
configuration has the advantage of reducing the inefficient operating point to 15%
of full load (50% of 30%), reducing significantly the operating energy penalties
that would result from a single-compressor operation.
Negative-pressure chillers are typically somewhat more efficient than positivepressure
chillers. A peak load rating of 0.5 kW/ton or less is available for
negative-
pressure chillers, while positive-pressure chiller ratings below 0.55 kW/
ton are difficult to obtain.
Positive-pressure chillers tend to be smaller and lighter than negative-pressure
chillers, which can result in smaller chiller rooms and lighter structures. Negativepressure
chillers generally have a higher first cost than positive-pressure
machines.
Driven by the evermore stringent requirements of each new edition of
ASHRAE Standard 90.1, manufacturers are constantly trying to improve the
efficiency of electric-drive rotary compressor water chillers. In the past few
years, a magnetic bearing compressor has been offered that reduces compressor/
motor friction losses, thus reducing power input requirements, by eliminating
bearings. In this design, a magnetic field holds the compressor/motor shaft
in alignment.

This technology was developed by Daiken Corporation of Japan and this
manufacturer
is currently the sole supplier of magnetic bearing motors that are
used with centrifugal compressors. Daiken (and Donfoss, under license) sells the
magnetic centrifugal compressor/motor for retrofit applications in the range of
70–270 tons.
McQuay/Daiken and JCI/York offer water chillers using magnetic bearing
compressors. McQuay offers a dual-compressor machine in the capacities of
140–
550 tons, while JCI/York offers their dual-compressor chiller in the range of
210–400 tons.
Packaged electric-drive chiller “modules” are available from several manufacturers
(Multistack, Tandem, etc.) in 20-, 30-, 50-, and 70-ton packages, each with
dual independent scroll compressors utilizing R-410A. Each chiller module is
designed to be mix–matched to form water-cooled chillers featured by a total
capacity of 20 to 600 tons.
Each chiller module is constructed with compressors, an evaporator, a condenser,
and a control cabinet mounted on a common frame with a very small
footprint, typically about 28″ wide and 48″ deep, allowing each module to pass
through a normal 30″ doorway for installation. Chilled water and condenser water
piping headers are included with quick-connect couplings for piping modules

together. Modules are wired to provide for single-point electrical connection to
serve each chiller configured from multiple modules.
The advantages of modular chillers over more typical factory-fabricated chillers
are flexibility, especially in retrofit installations where space and accessibility
are
limited, and ease of future expansion. Their disadvantages include higher maintenance
cost and lower energy efficiency (0.65 kW/ton or higher).

Engine-Drive Chillers:

>
In an open-drive configuration, natural gas- and propane-fueled spark ignition
engines have been applied to rotary compressor systems. The full-load cooling
COPs for engine-drive chillers are ~1.3–1.9 for helical screw compressors and 1.9
for centrifugal compressors. These low COPs can be improved if the engine water
jacket heat and exhaust heat can be recovered to heat service hot water or for other
uses.
Engine-drive chillers have been around for many years, but their application,
most typically utilizing natural gas for fuel, has been limited by a number of
factors:
1. Higher first cost
2. Air quality regulations
3. Much higher maintenance requirements
4. Short engine life
5. Noise
6. Larger physical size
7. Lack of integration between engine and refrigeration subsystems

Since the mid-1980s, manufacturers have worked very hard to reduce these
negatives with more compact designs, emissions control systems, noise abatement
measures, basic engine improvements, and development of overall systems controls
using microprocessors.
However, the maintenance requirements for engine-drive chillers remain
high, adding about $0.03–$0.04/ton-hour to the chiller operating cost.
Currently, the engines used for chillers are either spark ignition engines based
on automotive blocks, heads, and moving components (below about 400 tons
capacity) or spark ignition engines using diesel blocks and moving components
(for larger chillers). While the automotive-derivative engines are advertised
to have a 20,000 h useful life, the real life may be much shorter, requiring an
engine replacement every 2 years or so. The diesel-derivative engines require an
overhaul every 10–12,000 h (equivalent to a diesel truck traveling 500,000 miles
at 50 mph).
Newer engines use lean burn technology to improve combustion and reduce
CO and NOx emissions. By adding catalytic converters to the exhaust and additional
emissions controls, natural gas-fired engine-drive chillers can meet stringent
California air quality regulations.

Gas engine-drive chillers remain more expensive than electric-drive units, and
including maintenance costs, have higher overall operating costs (see Table 1.5).
However, engine-drive chillers may be used during peak cooling load periods to
reduce seasonal peak electrical demand charges (see the section “System Peak
Cooling Load and Load Profile” in Chapter 2).

Condensing Medium:

>
The heat collected by the water chiller, along with the excess compressor heat,
must be rejected to a heat sink. Directly or indirectly, ambient atmospheric air is
typically used as this heat sink.
For air-cooled chillers, the condenser consists of a refrigerant-to-air coil and
one or more fans to circulate outdoor air over the coil. The performance of the
condenser is dependent on the airflow rate and the air’s dry bulb temperature.
Air-cooled condenser airflow rates range from 600 to 1200 cfm/ton with a
10–30°F approach between the ambient dry bulb temperature and the refrigerant
condensing temperature. For typical HVAC applications, the condensing temperature
is about 105–120°F. Thus, the ambient air temperature must be no greater
than 95°F with a loss of efficiency. As the ambient air temperature increases, the
condensing temperature increases and net cooling capacity decreases by about
2% for each 5°F increase in condensing temperature.
Water-cooled chillers typically use a cooling tower (or some large water source
like a river, lake, or ocean) to reject condenser heat to the atmosphere and Chapters
9 through 17 of this book address this topic in detail. At the chiller, with 85°F (or
lower) condenser water temperature supplied from the cooling tower, condensing
temperatures are reduced to 94–98°F, reducing the “lift” required of the compressor
and significantly improving the chiller COP when compared with air-cooled
machines.

Chiller Efficiency and Estimated Energy Cost

Table 1.5 illustrates the relative efficiency and operating cost for the various
types of chillers with both air- and water-cooled condensing.

 
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