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.