ABSORPTION CHILLERS Lithium Bromide Absorption Chillers

ABSORPTION CHILLERS
Lithium Bromide Absorption Chillers

Lithium Bromide Absorption Chillers

ABSORPTION CHILLERS:
Lithium Bromide Absorption Chillers:
The vast majority of water chiller systems utilized for HVAC applications in the
United States are vapor compression cycle systems. However, in some applications,
principally in large cities and at large universities and hospital complexes,
steam distribution systems are available. In the past years, the cost of steam was
often cheaper than the cost of electricity and was used to provide cooling, utilizing
the absorption refrigeration cycle. This is generally no longer the case and
little or no new absorption cooling is utilized except where a waste heat source is
available, such as with cogeneration or some industrial processes, or where the
use of absorption cooling during peak cooling load periods may allow a reduction
in seasonal electric demand charges (see the section “System Peak Cooling Load
and Load Profile” in Chapter 2).

Absorption chillers using lithium bromide are defined as indirect-fired or
direct-fired and may be single stage or two stage, as follows:

1. The indirect-fired single-stage machine uses low- to medium-pressure
steam (5–40 psig) to provide the heat for the absorption process. This
type of chiller requires ~18,500 Btu/h per ton of cooling effect, resulting
in a chiller COP of about 0.67.
2. The indirect-fired two-stage chiller utilizes high-pressure steam (at least
100 psig) or high-temperature hot water (400°F or higher) and requires
~12,000 Btu/h per ton of cooling effect, resulting in a chiller COP of 1.0.
3. The direct-fired chiller, as its name implies, does not use steam but utilizes
a natural gas and/or fuel oil burner system to provide heat. These chillers
are two-stage machines resulting overall COP of 1.0–1.1.
For the indirect-fired units, the overall COP must be reduced to account for the
losses in the steam production in the boilers. With a typical boiler firing efficiency
of 80–85%, this reduces the overall COP for the single-stage system to ~0.54 and
to ~0.80 for the two-stage system.
As absorption cooling has a COP of only 0.54–1.1, it competes poorly with
electric-drive rotary compressor chillers, as shown in Table 1.5. (A two-stage,
direct-fired absorption chiller will cost ~$0.107/ton-hour of cooling to operate,
almost twice the cost of an equivalent electric-drive chiller.)
Other factors that must be considered for absorption chillers include the
following:

1. Absorption chillers require ~50% more floor area than the equivalent
electric-drive (vapor compression cycle) chiller. Additionally, due to their height, mechanical equipment rooms must be 6–10 ft taller than the
rooms housing electric-drive chillers. Finally, because the liquid solution
is contained in long, shallow trays within an absorption chiller, the
floor must be closer to an absolute level.
2. Absorption chillers will weigh at least twice as much the equivalent
electric-drive chiller.
3. Due to their size, absorption chillers are sometimes shipped in several
sections, requiring field welding for final assembly.
4. While most electric chillers are shipped from the factory with their
refrigerant charge installed, the refrigerant and absorbent (including
additives) must be field installed in absorption chillers.
5. While noise and vibration are real concerns for electric-drive chillers
(see the section “Noise and Vibration” in Chapter 6), absorption chillers
(unless direct-fired) are quiet and essentially vibration free.
6. Due to the potential for crystallization of the lithium bromide in the
chiller if it becomes too cool, the condenser water temperature must be
kept above 75–80°F.
7. An emergency power source may be required if lengthy power outages
are common. Without power and heat input, the chiller begins to
cool and the lithium bromide solution may crystallize. However,
because an absorption chiller has a very small electrical load requirement
(usually less than 10 kW), a dedicated backup generator is not a
major element.

8. The heat rejection rate from the condenser is 20–50% greater than for
the equivalent electric-drive chiller, requiring higher condenser water
flow rates and larger cooling towers and condenser water pumps.
9. Finally, an indirect-fired absorption chiller will be at least 50% more
expensive to purchase than the equivalent electric-drive chiller. Directfired
absorption chillers will cost almost twice as much as an electric
machine, and have the added costs associated with providing combustion
air and venting (stack).

Direct-fired absorption cycle chillers should be carefully evaluated as an alterative
anytime an engine-drive vapor compression cycle chiller is being considered.
Even though the energy costs for the absorption chiller may be higher, the
increased maintenance costs associated with engine-drive systems may make the
absorption chiller more cost effective.

Ammonia Absorption Chillers:

Small-capacity (3–5 tons) ammonia absorption chillers and heat pumps are
marketed
for commercial and industrial applications. These units are direct-fired
(typically natural gas) air-cooled chillers and generally have a COP of
about 0.5.

CHILLED WATER FOR HVAC APPLICATIONS:

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Typical applications for chilled water systems include large buildings (offices,
laboratories, hospitals, universities, etc.) or any multibuilding campuses where it
is desirable to provide cooling from a central facility.
As shown in Figure 1.8, the typical water-cooled HVAC system has three heat
transfer loops:
Loop 1. Cold air is distributed by one or more air-handling units to the
spaces within the building. Sensible heat gains, including heat from
temperature-driven transmission through the building envelope; direct
solar radiation through windows; infiltration; and internal heat from
people, lights, and equipment, are “absorbed” by the cold air, raising its
temperature. Latent heat gains, moisture added to the space by air infiltration,
people, and equipment are also absorbed by the cold air, raising
its specific humidity. The resulting space temperature and humidity
condition is an exact balance between the sensible and latent heat
gains and capability of the entering cold air to absorb those heat gains.
The distributed air is returned to the air handling unit, mixed with the
required quantity of outdoor air for ventilation, and then directed over
the cooling coil where chilled water is used to extract heat from the air,
reducing both its temperature and moisture content so that it can be distributed
once again to the space.
As the chilled water passes through the cooling coil in counterflow to
the air, the heat extraction process results in increased water temperature.
The chilled water temperature leaving the cooling coil (chilled
water return) will be 8–16°F warmer than the entering water temperature
(chilled water supply) at design load. This temperature difference (range)
establishes the flow requirement via the following relationship:

where
Fchw = chilled water flow rate (gpm)
Q = total cooling system load (Btu/h)
Range = chilled water temperature rise (°F)
500 = conversion factor (Btu min/gal °F h) (1 Btu/lb °F × 8.34 lb/
gal × 60 min/h)

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Loop 2. The warmer-returned chilled water enters the water chiller where it
is cooled to the desired chilled water supply temperature by transferring
the heat extracted from the building spaces to a primary refrigerant. This
process, obviously, is not “free” since the compressor must do work on
the refrigerant for cooling to occur and, thus, must consume energy in
the process. Since most chillers are refrigerant-cooled, the compressor

Water-cooled HVAC system schematic.

energy, in the form of heat, is added to the building heat and both must
be rejected through the condenser.
Loop 3. The amount of heat that is added by the compressor depends on the
efficiency of the compressor. This heat of compression must then be
added to the heat load on the chilled water loop to establish the amount
of heat that must be rejected by the condenser to a heat sink, typically the
outdoor air


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