What is gas cooling ?
Gas cooling is generally defined as alternative energy,
non-electric cooling technologies and primarily includes absorption,
engine-driven chillers and packaged desiccant units, among others.
As the name implies, natural gas (or more appropriately the combustion
of natural gas) drives the equipment. Some gas cooling technologies
utilize the thermal energy from combustion as the driving force
as in the case of absorption and desiccants, while engine-driven
chillers utilize the thermal energy converted to mechanical energy
to drive a compressor. Absorption constitutes the majority of
the gas cooling market today.
What is the difference between single effect and double effect ?
Absorption chillers are often classified by the number
of effects or stages of solution reconcentration, e.g.,
single effect (one-stage) or double effect (two-stage). Each stage
increases unit efficiency at the expense of higher cycle operating
temperatures and additional hardware in the form of supplementary
heat exchangers. The Carrier 16JB single effect absorption chiller
reconcentrates the lithium bromide in the generator in one step
utilizing low pressure steam or hot water as the heat source.
Double effect chillers, such as the Carrier 16JT and 16DF, incorporate
both a high- and low-stage generator with the low-stage acting
as the condenser for the high-stage. The heat source for the 16JT
is high-pressure steam while the 16DF utilizes the heat from the
combustion of natural gas or No. 2 oil. Reconcentration of the
lithium bromide solution in two stages increases cycle operating
efficiency.
What shipping options are available for 16-Series absorption chillers to simplify rigging and/or installation in today's retrofit and replacement market ?
Standard shipping configurations (1-, 2- or 3-piece)
for Carrier 16-Series absorption chillers vary by type of unit
and size. All models offer the option of multiple-piece shipment
(when not standard) to accommodate rigging where access into the
equipment room may be limited. When shipped fully assembled, the
shellside is completely evacuated to allow charging of the lithium
bromide and refrigerant (water) at the jobsite. Units shipped
in multiple-pieces include a 5 psig holding charge of nitrogen
on the shellside to protect against air infiltration during transport
and rigging. Jobsite re-assembly is simplified by pre-erecting
the machine in the factory and by incorporating weld-type flanges
on all interconnecting piping. Once re-assembled, the unit should
be leak tested and evacuated prior to charging.
What tube specifications are standard in Carrier 16-Series absorption chillers ?
All Carrier 16-series absorption chillers incorporate
straight copper tubes in the evaporator, absorber and condenser
sections as standard. The low-stage generator on the 16JT and
16DF double effect designs also features straight copper tubes.
The use of copper tubes is made possible due to the highly effective
corrosion inhibitor utilized in all Carrier absorption chillers.
Other types of inhibitors may necessitate the use of exotic tube
materials in certain heat exchangers due to their less effective
inhibiting characteristics. 90-10 CuNi U-tubes are standard in
the 16JB generator and the 16JT (high-stage) generator to allow
the tube bundle to expand and adjust freely when subjected to
rapid temperature changes. Allowing for this thermal growth and
expansion reduces tube wear and helps to prevent premature tube
failure. Horizontally-positioned, carbon steel fire tubes with
flue gas on the inside are used in the 16DF direct-fired high-stage
generator to allow easy soot removal and tube cleaning.
Why is the 16DF double effect direct-fired chiller typically rated a lower COP than the 16JT double effect steam-fired chiller ?
Chiller efficiency is usually expressed in terms
of the Coefficient of Performance (COP) which is a dimensionless
number calculated by dividing the energy output of the chiller
by the energy input. The COP for the 16JT double effect steam-fired
chiller is around 1.20 while the 16DF double effect direct-fired
chiller/heater is rated at a COP of .97-1.0 depending on the fuel.
The difference is due to the combustion efficiency (approximately
80%) being taken into account on the 16DF direct-fired unit since
the combustion of natural gas occurs inside the machine. On the
16JT steam-fired absorption chiller, combustion takes place external
to the machine and is not considered in the calculation of unit
COP. Both units have the same cycle efficiency of 1.20
but are rated differently when it comes to unit COP.
What is the significance of the higher heating value (HHV) of a fuel when establishing the performance rating of a direct-fired absorption chiller ?
Direct-fired absorption chillers, like the 16DF,
most often utilize natural gas as the fuel source which varies
in heat content depending on a variety of factors. Heat content
or heating value is defined as the amount of heat produced by
the complete combustion of a unit quantity of fuel. Higher heating
value (HHV) accounts for condensation of the water vapor formed
during the combustion process. In comparison, the net or lower
heating value (LHV) is approximately 90% of the HHV since it does
not account for the latent heat of vaporization of water formed
during combustion. Heating values generally range between 900
- 1,200 Btu/ft3 with 1,000 Btu/ft3 considered
as the nominal HHV. When evaluating the performance of one chiller
to another, it is necessary to use the same basis (LHV or HHV);
otherwise the comparison will not be accurate. Fuel consumption
will vary depending on which heating value is used since the COP
based on the fuel LHV will be 11% higher than that based on HHV.
The use of higher heating values is a customary practice in North
America.
What is the minimum chilled water temperature that an absorption chiller is capable of producing ?
Most commercial absorption chillers utilize the combination
of lithium bromide and water as the fluid pair. Lithium bromide
(LiBr), a non-toxic salt, is the absorbent while water is utilized
as the refrigerant. As water is the refrigerant, the minimum temperature
of the chilled water leaving the evaporator tubeside is limited
to approximately 40F as anything lower could potentially result
in freezing of the refrigerant on the shellside of the chiller.
What are some typical applications for absorption chillers ?
Absorption chillers are used in many of the same
commercial or light industrial applications as electric vapor
compression chillers. Although limited to approximately a 40F
minimum leaving chilled water temperature, they are used extensively
in comfort cooling applications such as hospitals, office buildings,
institutional facilities, schools/universities and the like. One
of the largest market segments is hospitals and healthcare facilities
due to the available steam supply and the low noise level requirement.
Cogeneration plants are also a natural fit since waste heat in
the form of steam or hot water can be used to operate either a
16JB single effect or a 16JT double effect chiller.
What is the current ARI Standard for absorption ?
The current Air-Conditioning & Refrigeration
Institute (ARI) Standard for absorption chillers is ARI 560-92
entitled Absorption Water Chilling And Water Heating Packages.
Issued in 1992, it replaced the then-current 1982 standard. It
is presently going through a general revision in conjunction with
the Absorption Chiller Certification Program scheduled for initial
release and implementation in 1997. Copies of ARI 560-92 can
be obtained from ARI.
What is a "hybrid" chilled water plant ?
Hybrid chilled water plants refer to systems that
incorporate both electric and absorption chillers. This offers
the flexibility to base load one chiller while using the other
to handle peak load requirements. Hybrid systems have proven to
be an economical solution for many applications. In many geographical
areas, operating the electric chiller as the base loaded machine,
while using the absorption chiller during peak load conditions,
reduces or avoids electric demand charges. In today's current
environment of electric utility deregulation with future electric
costs uncertain, a hybrid plant with the combination of a Carrier
16-Series absorption chiller used in conjunction with an electric-driven
unit may be the most efficient and cost-effective combination
available.