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Article Excerpt
INTRODUCTION
Current guidelines for the installation of gas-fired residential furnaces at altitudes above 2000 ft (610 m) require that gas input rate be reduced. Compared to standard sea-level operation, the furnaces should fire at a 4% lower rate for every 1000 ft (305 m) above sea level. The reason for this is the decreasing air density with altitude, which results in a reduced mass of oxygen for combustion in a given volume of air, as compared to sea level. Fuel flow rate was reduced to compensate for the reduced oxygen availability, which resulted in safe operation.
Installation codes, such as ANSI Z223.1-2002/NFPA 54-2002 National Fuel Gas Code (ANSI 2002) in the U.S. and CSA B149.1-00 National Standard of Canada, Natural Gas and Propa ne Installation Code (CSA 2000) in Canada, recommend deratings for all appliances, subject to certification. Furnace fuel flow derate is controlled by installing smaller fuel orifices and/or decreasing the pressure in the fuel gas manifold. The derating standard was implemented before it became common to build furnaces with fan-assisted combustion systems to either draw or force products of combustion through the combustion chamber and/or heat exchanger. Since the current furnace designs operate with fan-assist, it is necessary to re-evaluate the traditional derating practice and determine what altitude derating is appropriate for fan-assisted furnaces. A related issue, and the purpose of this paper, is what effect, if any, does increasing altitude have on steady state efficiency and nitric oxide (NO) emissions.
The 4% derating standard was based on work initially done in an altitude chamber by Eisman et al. (1933). A variety of appliances were studied. Included were the then common gravity-type furnaces, which used multiport burners and relied on buoyancy to vent the flue gases and distribute the warmed supply air to different locations within the building. Later studies in the 1940s and 1950s used both altitude chambers and field studies in order to augment this work. Because the appliances studied were essentially the same, the same conclusions were drawn for the magnitude of the derating required. It was not until 1988 (Sheridan et al. 1988), when the new fan-assisted furnaces using in-shot burners came into wide use, that a reex-amination of the derating guide was undertaken. This study, based on altitude chamber tests, concluded that the 4% derating guide was appropriate for furnaces with negative pressures in the vent, but not for those that were fan assisted and had positive pressures in the vents. On these latter appliances the fan-assisted combustion systems either drew or forced the products of combustion through the combustion chamber and/or heat exchanger. Steady state efficiencies and NO levels were not measured in the Sheridan et al. (1988) study.
The effects of altitude on in-shot burners and heat exchangers, as well as complete furnaces, has been previously studied. A study by Kam et al. (1995) examined these effects and also included a performance evaluation of vent and supply air fans and the then current control systems to determine the effects of altitude on performance. Both altitude chambers and field studies were used. While complete appliances were tested, the overfire gas input rate margin for 400 ppm carbon monoxide air-free (CO-AF) concentration in the flue gas was not evaluated. Thus, there was no information available on altitude effects on overfiring margins, the thermal efficiency of furnaces, or NO emissions. The findings did suggest that the then-current 4% derating guide was overcompensating and that a smaller derating of about 2% could safely be used. This lack of information for altitude effects on CO and NO emissions and steady state efficiencies led ASHRAE Technical Committee 6.10 to suggest the current study, which was sponsored by ASHRAE through RP-1182. Since all current designs utilize fan assist, it was necessary to re-evaluate the traditional derating practice, determine what altitude derating is appropriate for these appliances, and measure the CO and NO levels and the thermal efficiency at all test altitudes.
OBJECTIVES
The primary objective was to test gas-fired furnaces of Categories I and IV (see the section "Furnaces and Instrumentation" for category definitions) at three altitudes--sea level, 2250 ft (685 m), and 6700 ft (2040 m)--and to objectively determine if a new derating protocol, with less derating than is currently required by installation codes for operating natural gas-fired and propane gas-fired furnaces with fan-assisted combustion systems at high altitudes, might be acceptable. The test methods used came from ANSI Z21.47-2001/CSA 2.3-2001 (ANSI 2001).
In addition, the applicability and validity of testing furnaces near sea level, as outlined in CAN/CGA-2.17-M91 National Standard of Canada, Gas-Fired Appliances for Use at High Altitudes (CAN/CGA 1991), to demonstrate compliance with ANSI Z21.47-2001/CSA 2.3-2001 at altitudes up to 10,000 ft (3050 m) was to be investigated.
The testing focused on determining the effects of altitude on the following variables: carbon dioxide ([CO.sub.2]), carbon monoxide (CO), oxygen ([O.sub.2]), and nitric oxide (NO) concentrations in the furnace flue gas, burner and igniter operating characteristics, heat exchanger operating temperatures, steady state thermal efficiency, and blocked-vent shutoff combustion performance.
This paper presents results for only the steady state thermal efficiency and NO measurements. The other results are presented in a companion paper (Fleck et al. n.d.). Complete details of the study are available in the contract report from ASHRAE (Fleck et. al. 2007).
The performance of each furnace was determined using standard tests from ANSI Z21.47-2001/CSA 2.3-2001 (ANSI 2001). The specific tests are described in the following sections: 2.7, "Category Determination"; 2.8, "Combustion" (2.8.1 and 2.8.3); 2.9, "Burner Operating Characteristics"; 2.10, "Pilot Burners and Safety Shutoff Devices"; 2.11, "Direct Ignition Systems"; 2.16, "Allowable Heating Element Temperatures"; 2.22, "Draft Tests for Furnaces not Equipped with Draft Hoods"; 2.24, "Allowable Air Temperatures"; and 2.38, "Thermal Efficiency." Of these, only section 2.38, "Thermal Efficiency," applies to this part of the study. The NO results reported here are based on measurements made during the thermal efficiency testing.
The plan for the field evaluation of the furnaces was to install the furnaces in an industrial trailer, transport the trailer to the three different altitudes, and perform tests according to ANSI Z21.47-2001/CSA 2.3-2001 (ANSI 2001) at each location. The fuels used were natural gas and HD-5 propane gas, each taken from a single source and transported to the testing sites as needed.
THEORY
The calculation method recommended in ANSI Z21.47-2001/CSA 2.3-2001 (ANSI 2001), to determine the steady state thermal efficiency of a furnace, is to subtract from the total energy input to burner(s) of the furnace the sensible and latent losses from the furnace. The sensible losses are the energy contained in the noncondensable flue gases and the heat losses from the jacket. The latent loss is the energy contained in the condensable flue products (e.g., water vapor). Note that this calculation ignores the electricity used to run the supply air and draft inducer fans.
With some furnaces, an extra heat exchanger is added to condense some of the water vapor formed in the combustion process and improve the overall energy recovery of the system. Recovery of this latent energy can add significantly to the overall efficiency of the furnace, especially when burning fuels containing hydrogen, such as natural gas and propane gas.
ANSI Z21.47-2001*CSA 2.3-2001 (ANSI 2001) contains formulas, nomographs, and charts to assist with these calculations. The formulas are based on the volume of gaseous fuel used at standard conditions, while the nomographs are limited to noncondensing furnaces. Because these units of measurement are difficult to work with at a variety of altitudes, as occurred with this study, it was decided to work through the flue loss calculations from fundamental principles on a mass-of-fuel basis, accounting for the variation of properties of the gases with temperature and pressure. A detailed sample calculation carried out in SI units is included in Appendix to this paper.
EXPERIMENTAL METHODOLOGY
Five different furnaces were field tested within a specially instrumented mobile trailer...
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