1、Reducing Sidewall Vent Plumes and Increasing Equipment Installation Flexibility in Low Energy Design Larry Brand Paul Glanville, PE Yanjie Yang Member ASHRAE Associate Member ASHRAE ABSTRACT High-efficiency natural gas-fired space and water heating systems are an integral part of Low Energy Design,
2、offering equipment installation flexibility at minimum operating cost. These systems are largely installed with sidewall vents in new and retrofit single family and multifamily buildings when venting through the roof is not practical, is prohibited by local code, or is beyond the maximum length of t
3、he vent system permitted by the manufacturers installation instructions. The industry continues to investigate sidewall venting systems design and installation practices to improve building design flexibility, improve vent performance under extreme conditions, avoid nuisance outages, and avoid other
4、 occurrences such as ice formation on nearby structures. This paper investigates sidewall vent plume formation and the potential for ice formation on nearby structures in very cold climates. A review of the literature on turbulent buoyant round jet formation and transition from momentum-driven to bu
5、oyancy-driven flow is discussed as well as results of computational fluid dynamics (CFD) modeling of several geometries. CFD is a finite difference model that calculates the fluid parameters based on the physics of the fluid flow and surrounding conditions. Recommendations are made for improving sid
6、ewall vent performance and supporting flexible Low Energy Design practices. INTRODUCTION Condensing furnaces and appliances are commonly installed in a direct vent configuration with vent termination on the side wall of the building. A direct vent configuration provides for combustion air from outdo
7、ors to minimize conditioned space energy loss through infiltration, with the venting of the products of combustion to the outdoors through horizontal or vertical gas tight and corrosion resistant vent pipes. These systems have been installed and operating since the 1980s with good long-term performa
8、nce records when installed according to the manufacturers installation instructions. Ice formation on adjacent structures is a phenomenon that has been identified in very cold climates where sidewall venting is used for two story residential buildings spaced closely together. Frost forms in low-wind
9、 conditions on siding or soffits with the potential to enter attic spaces depending on the placement of attic vents. In cold climates, it is possible for the outdoor temperature to remain below freezing for 30 days in a typical winter season, providing the opportunity for frost formation to become a
10、n ice build-up over time. This paper analyzes ice formation from Category IV condensing furnace sidewall vent geometries using horizontal turbulent buoyant round jet solutions from the literature, laboratory testing, and Computational Fluid Dynamics (CFD) models to investigate: 1. Mechanisms leading
11、 to ice formation on adjacent structures in very cold climates, LV-11-C063 2011 ASHRAE 5092011. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions, Volume 117, Part 1. For personal use only. Additional reproduction, dist
12、ribution, or transmission in either print or digital form is not permitted without ASHRAES prior written permission.2. Techniques to minimize ice formation through changing vent sizes, lengths, and terminations, 3. Other research that is needed. FURNACE MANUFACTURERS INSTALLATION INSTRUCTIONS Catego
13、ry IV furnace manufacturers provide information in their installation instructions regarding the maximum length of the vent system as a function of pipe diameter and furnace capacity. Additional information is provided on the design of the vent terminals, clearance to windows and doors, and distance
14、 above the ground. Table 1 shows the pipe length for various plastic materials (PVC, CPVC, and ABS) as a function of pipe diameter in the installation instructions for a typical furnace. This information sets the upper limit for the vent system design to provide adequate venting. For each input capa
15、city, modified for altitude, the maximum pipe diameter and maximum length with elbows is specified. Figure 1 shows the recommendations for clearances to nearby windows, doors, and other construction features for combustion air and vent terminations. Manufacturers permit a “snorkel” arrangement where
16、 the combustion air and vent pipes penetrate the building envelope, turn vertical, and then terminate at a height above ground limited by the length of exposed pipe permitted for the climate. Table 1 Typical Vent Sizing Maximum Pipe Length from Manufacturers Installation Instructions (Abbreviated) A
17、ltitude, ft. (m) Capacity KBtu/hr (kW) Termination Type Pipe Diameter, inches (cm) Maximum Pipe Length for Given No. of 90 Elbows, ft. (m) 1 2 3 0 to 2000 (0 to 610) 40 (12) 2 pipe or concentric 1 (3.8) 50 (15.2) 45 (13.7) 40 (12.2) 2 (5.1) 70 (21.3) 70 (21.3) 70 (21.3) 0 to 2000 (0 to 610) 60 (18)
18、2 pipe or concentric 1 (3.8) 50 (15.2) 45 (13.7) 40 (12.2) 2 (5.1) 70 (21.3) 70 (21.3) 70 (21.3) 0 to 2000 (0 to 610) 80 (23) 2 pipe or concentric 1 (3.8) 30 (9.1) 25 (7.6) 20 (6.1) 2 (5.1) 70 (21.3) 70 (21.3) 70 (21.3) 0 to 2000 (0 to 610) 100 (29) 2 pipe or concentric 2 (5.1) 45 (13.7) 40 (12.2) 3
19、5 (10.7) 2 (6.4) 70 (21.3) 70 (21.3) 70 (21.3) Figure 1 Typical Combustion Air and Vent Termination Diagram from Manufacturers Installation Instructions (Abbreviated) Source: Carrier 58MCB Installation, Start-Up and Operating Instructions for Sizes 040-140, Series 100 510 ASHRAE TransactionsThe curr
20、ent requirement for sidewall vent terminal height is 1 ft (0.3 m) above the ground or highest anticipated snow level. In this analysis 1 ft (0.3 m) is used as a minimum height to evaluate the worst case condition and 7 ft (2.1 m) is used as the maximum height for insulated exterior vent pipe using 7
21、0 inches (1.8 m) exposed length penetrating the wall at 1 ft. (0.3m) above grade. HORIZONTAL TURBULENT BUOYANT ROUND JETS The physics of horizontal turbulent buoyant round jets has been investigated thoroughly in the power industry. Atmospheric plume dispersion modeling and jet-to-plume transitions
22、are the two primary areas of interest. The governing equations include conservation of mass and momentum and the boundary conditions. Fan (1969) investigated buoyant plumes for effluent dispersal in uniform fluids, and Xiao (2008) investigated primarily vapor-phase buoyant round jets for steam plume
23、 dispersion. Xiao provides a solution for jets with small and large density variations transforming to buoyancy-driven vertical plumes using a numerical method to solve the differential equations. Figure 2, below, shows Xiaos solution for small density variation such as flue gas into air as function
24、 of Froude number, a measure of the relationships between velocity effects and buoyancy effects. The figure shows the general relationship between the Froude number and the vapor plume trajectory with wider variations at lower Froude numbers. Figure 2 Normalized trajectories of horizontal buoyant je
25、ts CFD modeling of turbulent jets using GASFLOW (Xiao) and FLUENT (GTI) generally supported the results from the analytical results, above. The velocity and density characteristics of the condensing furnace vapor plume problem provide a Froude number in the range of 8 to 13 for 2 in. (5.1 cm) to 3 i
26、n. (7.6 cm) vent diameters. In this project we combine manufacturers installation instructions, the Xiao solution to the buoyant round jet governing equations, and CFD analysis to investigate specific geometries, provide a general regression model, and propose methods for reducing vapor plume imping
27、ement and ice formation. 2011 ASHRAE 511MODEL DEVELOPMENT AND REGRESSION ANALYSIS FOR GENERAL CASE The Xiao model was analyzed using condensing furnace flue gas parameters. Regression analysis was performed on the Xiao data to identify a function relating distance between the buildings and initial f
28、low velocity so ice formation as a function of furnace capacity and vent pipe size could be studied. The curve in Figure 3 was developed. Figure 3 Froude Number 10 Curve Fit Using this fit, the characteristic length and height numbers were calculated and the centerline trajectory for three pipe size
29、s was calculated as in Figure 4. 01020304050600246810HeightAboveGrade,ft.Distance, ft.2 in.2.5 in3 in.02468101214161800.511.522.53HeightAboveGrade,mDistance, m5.1 cm6.3 cm7.6 cmFigure 4 Centerline Trajectories as a Function of Pipe Diameter (IP and SI) The results show that a 2 in. (5.1cm) pipe diam
30、eter would produce ice on an adjacent structure about 10 ft. (3.1 m) above grade with larger diameters reducing the momentum effect and impinging on the opposite surface at 25 ft. (7.7m) and 50 ft (15.2 m) above grade. 512 ASHRAE TransactionsLABORATORY TESTING Laboratory testing was conducted to det
31、ermine the ice formation rates and provide trajectory data for calibration of the CFD model. An experiment was set up in the Gas Technology Institute laboratory environmental chamber (see Figure 5). Figure 5 From Left to Right: Environmental Chamber, Vapor Plume, Ice Formation Testing was conducted
32、with the following parameters: 1. 100,000 Btu/hr (29.3 kW) 90% AFUE furnace venting horizontally with a 2 in. (5.1cm) PVC vent pipe discharging 8.5 ft. (2.6 m) from the opposite wall 2. Chamber setpoint of 0oF (-17.8 C) 3. Target placed on the opposite wall at 40 in. (101.6 cm) above vent outlet max
33、imum ice formation by visual inspection. For the experiment, the chamber was cooled to its setpoint and turned off. The target was placed on the opposite wall. The furnace was then cycled for 300 seconds. The chamber was ventilated to the outdoors during furnace operation. While the furnace operated
34、, and with the chamber refrigeration system turned off, the chamber temperature reached 25oF (-3.9 C). The furnace was turned off and the chamber restarted. After 6 total cycles, the ice weight was measured. The results supported an ice formation rate of 0.7 in. (1.8 cm) maximum over a 30 day period
35、 at 12oF (-11.1 C) average outdoor temperature, no excursions above 25oF (-3.9 C), furnace on 100%, and no wind effect. This result was used to calibrate the ice formation rate in the CFD model. The visual placement of the target was lower than the maximum ice deposition area from the trajectory pro
36、vided in the analytical results and in the CFD model results discussed later. Possible causes include the placement of the refrigerant coils above the target zone and cooling from the nearby wall of the chamber. CFD MODELING RESULTS A computational fluid dynamics model uses a finite volume technique
37、 to calculate the fluid parameters based on the physics of the fluid flow, the geometry of the problem, and surrounding atmospheric conditions. A CFD model was used to simulate the vapor plume using the following assumptions: 1. Two adjacent buildings 20 ft. tall (6.1 m) to the eaves, 36 ft. (11.0 m
38、) deep, and spaced 10 ft. (3.0 m) apart representing two story multifamily buildings separated by a driveway or setbacks. 2. Outdoor temperature and wall temperature of 16 oF (-8.9 C). Cooling of the vapor plume in the ambient air was an important aspect of this study. This temperature was chosen to
39、 simulate a Minneapolis winter condition. From weather data, Minneapolis experiences 30 continuous days of winter with no excursions 2011 ASHRAE 513above this temperature. 3. No heat transfer through the wall to simulate a well-insulated building. 4. Flue gas flow rate of 23 ft/s, (7.0 m/s) outlet t
40、emperature of 115oF (46.1 C), and water vapor mass fraction of 8.2% consistent with a 100,000 Btu/hr (29 kW) furnace at 90% AFUE. Figure 6, below, shows the geometry and plume formation associated with these assumptions. The deposition rate profile is shown in kg/m2s (1.0 x 10-6kg/m2s = 0.2 x10-6lb/
41、ft2s). Figure 6 CFD Model Geometry with Velocity Path Lines (left) and Deposition Rate Profile (right) A user-defined function for condensation rate was developed from Carey (1992) and the experimental data from the environmental chamber testing. The water vapor partial pressure (Pact) for the actua
42、l cell in the CFD mesh and the saturated case (Psat) are calculated. If the actual cell has excess water vapor pressure above saturation, ice condensation will occur. The following equation shows the relationship: R = Kcons * (Pact - Psat)* MW Where, R is the condensation rate (kg/m2s), Kcons is the
43、 reaction rate in Newtons/s-mole, Pact is the actual water vapor pressure in Pa, Psat is the water vapor pressure at saturation in Pa, and MW is the molecular weight of water in kg/kmole. Kcons was empirically determined by laboratory testing. This first order mass flux phase change model has been u
44、sed with success in similar applications (Perujo, 2004). Several geometries were run in CFD varying outdoor temperature, terminal height above the ground, pipe diameter, and vent terminal design. Vent terminals used in this analysis were straight pipe or standard elbows at the vent outlet. The CFD r
45、esults reported here cover only 45 degree, 90 degree, and T fittings, although others were modeled. Figure 7 provides an illustration of commonly available pipe elbows. The ANSI wind load test in the furnace standard was not considered when selecting options for this analysis. 514 ASHRAE Transaction
46、sFigure 7 Vent Terminal Options Table 2 CFD Results Case Vent Termination Fitting and OrientationHeight Above Grade, ft. (m)Ice Condensation Rate (Max.), lb/ft2s (kg/m2s)Mass Percent of Water Condensation from Flue Gas, %Ice Thickness Model for 30 days (Max.) in. (cm)1Straight 1 (0.3) 1.22E06 (5.97E
47、06) 3.0% 0.66 (1.68)245 degree elbow down 1 (0.3) 1.55E06 (7.57E06) 4.4% 0.84 (2.13)345 degree elbow up 1 (0.3) 6.77E07 (3.31E06) 1.1% 0.37 (0.94)490 degree elbow up 1 (0.3) 0 (0) 0.0% 0 (0)5Te terminal 1 (0.3) 4.05E06 (1.98E05) 3.4% 2.20 (5.59)6Straight 1 (0.3) 1.13E06 (5.51E06) 1.9% 0.61 (1.55)745
48、 degree elbow down 7 (2.1) 1.75E06 (8.58E06) 3.7% 0.95 (2.41)845 degree elbow up 7 (2.1) 3.54E08 (1.73E07) 0.0% 0.02 (0.05)9Te terminal 7 (2.1) 4.05E06 (1.98E05) 0.2% 2.20 (5.59)10 Tee terminal with 1 ft (0.3m) extensions 7 (2.1) 1.83E06 (8.95E06) 0.5% 0.99 (2.51)11 Straight with 3 in. (7.6 cm) diam
49、eter pipe 7 (2.1) 5.70E08 (2.79E07) 0.0% 0.03 (0.08)The CFD model results in Table 2 show that an upward-pitched vent terminal (less than 90 degrees) modifies the trajectory of the vapor plume and minimizes deposition on adjacent surfaces. Increasing pipe size also significantly reduces the flow velocity of the plume and can eliminate ice formation when used with a high vent terminal location of 7 ft (2.1 m). Tee fittings or 90 degree fittings produced similar results, eliminating ice formation on adjacent buildings but allowing i
