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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 the gas input rate be reduced. Compared to 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 compared with sea level. Reducing the fuel flow rate is done to compensate for the reduced oxygen availability, resulting in safe operation.
Installation codes such as ANSI Z223.1/NFPA 54 National Fuel Gas Code (ANSI 2002) in the United States and the CSA B149.1 National Standard of Canada Natural Gas and Propane Installation Code (CSA 2000) in Canada recommend deratings for all appliances, subject to certification. The furnace fuel flow is reduced by installing smaller fuel orifices and/or decreasing the pressure in the fuel gas manifold. The present derating standard is based on work initially done by Eisman et al. (1933) in the 1930s in an altitude chamber. 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 arrived at on the magnitude of the derating required. It was not until 1988 (Sheridan 1988) when the new fan-assisted furnaces using in-shot burners came into wide use that a re-examination of the derating guide was undertaken. This study, based on altitude chamber tests, concluded that the 4% deration guide was appropriate for furnaces with negative pressures in the vent (Category I and II furnaces), but not so for those that were fan-assisted and had positive pressures in the vents (Category III and IV furnaces). On these latter appliances the fan-assisted combustion systems either drew or forced products of combustion through the combustion chamber and/ or heat exchanger. In 1995, the Gas Research Institute, now the Gas Technology Institute, (Kam et. al 1995) undertook a study of modern in-shot burners, current heat exchanger design, draft inducer and supply air fans and then current control systems to determine the effects of altitude on performance. Both altitude chambers and field studies were used. However, complete appliances were not tested, only components. The findings did suggest that the then current 4% derating guide was over compensating and that a smaller derate of about 2% could safely be used. The fact that complete furnaces were not tested in the 1995 study led ASHRAE Technical Committee 6.10 to suggest the current study which was sponsored by ASHRAE through RP-1182. The motivation for the study being that since all current furnace designs utilize fan assist, it really is necessary to re-evaluate the traditional derating practice and determine what altitude derating is appropriate for these appliances.
OBJECTIVES
The primary objective was to test gas-fired furnaces of Categories I and IV (see the section in this paper on 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 for operating natural gas-fired and propane gas-fired furnaces with fan-assisted combustion systems at high altitude might be acceptable. The test methods came from ANSI Z21.47/CSA 2.3 American National Standard/CSA Standard for Gas-Fired Central Furnaces (ANSI 2001).
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) levels, burner and igniter operating characteristics, heat exchanger operating temperatures, steady state efficiency, and blocked-vent shutoff combustion performance. The performance of each furnace was determined using standard tests from ANSI Z21.47/CSA 2.3 (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, the most stringent test was found in section 2.22, known as the blocked vent test, which is a test of the sensitivity associated with the pressure-activated shutoff switch. Test 2.22 of ANSI Z21.47/CSA 2.3 (ANSI 2001) requires that the flue outlet be gradually blocked to restrict the flue gas flow rate until either the furnace shuts off the gas or the flue is fully blocked. In either case the concentration of CO present in the air-free sample (CO-AF) of the flue gases shall remain less than or equal to 400 ppm. The standard has several related tests to ensure safe operation (e.g., see 2.8.1 and 2.8.3). However, during the course of this study it was determined that if a furnace's flue CO-AF concentration remained less than 400 ppm CO-AF, it would generally pass the other requirements.
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 the required tests. 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.
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 furnace compliance with ANSI Z21.47/CSA 2.3 (ANSI 2001) at altitudes up to 10,000 ft (3050 m) was to be investigated.
This paper will report on the flue gas CO-AF concentrations and minimal derating potential, as well as observations on the operation of the ignition system, heat exchanger temperatures and discolorations indicative of high thermal stresses, and the high temperature limit control switches. A companion paper (Part II) discusses the calculated steady state efficiencies and measured NO levels (Fleck et. al 2009). Complete details are available in the contract report available from ASHRAE (Fleck et. al 2007).
OPERATION OF VENTURI-STYLE BURNERS
All of the furnaces tested used horizontally oriented inshot venturi-type burners. Figure 1 shows a schematic of a typical burner and location relative to the heat exchanger in a furnace. Fuel exiting from the fuel gas orifice enters one end of the venturi as a jet. This induces the primary combustion air into the venturi where turbulent mixing occurs. About 60% of the air required for complete combustion (stoichiometric air) is drawn...
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