Dear friends . Last Monday, it was a banal day until I got a call at office hours and heard a familiar voice. The call from Edwin Chan, who was my formerly classmate in the secondary school date, we were educated in Bethel High School together in 1981-1985. Truly, his call actually brought me back to the school date because at that moment we were happy very much. For about fifteen years ago, the school was relocated to Yuen Long district, its original area is now a divinity school in Kowloon Tong.
I remember the school date in Bethel High School we all were naughty students and we founded a naughty student group "Tai Fu Bond 大褲幫", we were several members at the beginning but were 20 more in the end. Each of Tai Fu Bond members had to wear a pantalettes 多摺蘿蔔長褲and must a pair of cuspidal shoes 尖頭鞋, such dressing was viewed as the "IN-THINGS" at that moment. . Personally, I don't think such dreassing as School Uniform and joining in Tai Fu Bond at that age were all that bad, even at this age, as long as we can handle it and separate it. Teachers in the school saying that the principle of "play while playing and work while working" must work very well as long as we can use it. Having naughty act in the school date is actually quite normal but having without it the date to each secondary school student will be a mundane world. As a naughty age and a call, Edwin's call brought back many pleasant memories. . Edwin Chow currently is working at a financial planning company as a planning consultant, who had a nickname in the secondary school as "Little Hippo 河馬仔" but in my mind, he is still a little hippo,. is a little hippo in Tai Fu Bond. He is,.....he definitely is. .
(In a summer time in 1984, this picture shows the first left is "Four-Eyes Fat四眼肥", after the first is "Pui Pui佩佩", and then is "Little Hippo河馬仔" and the last is me). At that time, we all were teenagers and had the time to be spent, and had happy with days. Acutally I really had a wonderful time in Bethel High School. By this blog, I wish little hippo could find remarkable success in his work. Best Regards |
Basic Function of the gas-fired cooker The following figure is a view of a conventional gas-fired cooker (or it is said to be “Portable domestic gas-fired cooker”). As illustrated, the gas fired cooker comprises (1) a case for gas canister, (2) Gas Burner, (3) Ignition and Heat Control Knob, (4) Gas cartridge Locking Lever, (5) Pan Support, (6) Spill Pan Protection, (7) Brand Name (Ken Ngan, name of the Designer) and (8) Model Number of the gas fired cooker (07536141G).
The main function of “Ken Ngan” portable cooker is mainly used to produce heating, it is ideal for camping & outdoor cooking preparation. The model “07536141G” provides controllable heat from 2Kw flash-fry temperatures down to a steady simmer for fondue or sauce and an automatic Piezo electric ignition. By burning the same volume of gaseous fuels, butane gas (C4H10) provides the highest heating value whereas that of producer gas is the lowest.  When the weight of fuel is a major consideration, hydrogen has the highest heating value per unit mass which is almost 3 folds of that of butane gas. Producer gas also has the lowest heating value in MJ/Kg. Considering the heat value, producer gas is the poorest gaseous fuel and it will not be chose. . As it can bee seen from the above tables, hydrogen has the highest heating value per unit mass and butane gas has the highest heating value by comparing the same volume with other gaseous fuels. Hydrogen gas is very rare in the local market and it is not safe to be cooking fuel, however, other gaseous fuels such as Propane, Butane, Methane and Natural gas are commonly used in general commercial or domestic areas. The first table shows the Butane gas (C4H10) having the highest heating value in 119.7 MJ/m3, but the heating value of Butane in MJ/Kg (it is 49) is slightly lower than that of Methane (54.9), and even that of natural gas (53.6) and propane (49.8). . Basically, low cost and availability in local market are the main factor in my design assignment; Butane’s price is not expensive compared to the above competitors’ prices. For some reasons, the fuel gas I chose for this portable gas-fired cooker is Butane Gas. . Calculation for the number and the diameter of the ports: Having a design that meets the preset constraints (e.g. circle diameter of the gas burner: 160mm; burner consumption: 2KW; 40% primary aeration; flame height not exceed 20mm; and loading of each port not exceed 10W per mm2), the following calculations can help to check that the overall design makes physical sense. Step 1: Apply port loading constraint. The total port area is Atot = (πND2)/4 And the constraint is (ṁFΔhc) / Atot = 2000W / Atot mm2 = 10W/mm2 Thus, ND2 = 4 (2000) / 10π= 254.6mm2 If ND2 = 254.6mm2 Choosing a value for N (or D) and calculate D (or N) as a first trial for the design. In this design, I chose N to be 36 yields, therefore D, diameter of each port will be 2.66mm.  Step 2: Determine flow rate Ǭ= 2000W = ṁFΔhc ṁF= 2000W / 45,742,000J/Kg (this value as per the fuel properties, Butane LHV) ṁF= 4.37 x 10-5kg/s The primary aeration determines the flowrate of air premixed with the Butane fuel C4H10: C4H10 + a(O2+3.76N2)à Products a = 4+2.5=6.5 C4H10 + 6.5(O2+3.76 N2)à Products C4H10 + 6.5O2+24.4 N2à Products C4H10= (12 x 4 + 1 x 10) =58Kg 6.5O2+24.4 N2 = (6.5x16x2 + 24.4 x 14 x 2) = 208 + 683.2 à 891.2Kg Air/fuel ratio (stoic) = 891.2Kg / 58Kg Air/fuel ratio (stoic) = 15.37 . ṁA, pri= 0.4 (A/F) stoic ṁF = 0.4 (15.37)4.37 x 10-5kg/s = 2.69 x 10-4kg/s . A/F = ṁA, pri / ṁF A/F = 6.156 Equivalent Ratio Φ=A/F stoic / A/F = 15.37 / 6.156 Φ= 2.497 Φ> 1, it implies fuel rich mixture. . The total volumetric flowrate is Qtot = (ṁA, pri + ṁF )/ƥ . To determine ƥ, the ideal gas law will be applied where the mean molecular weight is calculated from the composition of the air-fuel mixture: X A, pri= NA/ (NA+NF) = Z/ (Z+1) Where Z is the primary molar air-fuel ratio = (x+y/4)4.76(% aeration/100) Z= (4+2.5) x 4.76 x (40/100) Z= 12.376 . X A, pri= Z/ (Z+1) = 0.9252, X F, pri= 1- X A = 0.07476 MWmix = 0.9252 (28.85) + 0.07476 (58) = 31.03, ƥ = P/ (Ru/MWmix)T = 101325/ (8315/31.03)300 ƥ = 1.260kg/m3 and Qtot = (2.69 x 10-4 + 4.37 x 10-5)/ 1.26 Qtot = 2.48 x 10-4 m3/s . Step 3: Check the flowrate per port is QPort =Qtot / N = 2.48 x 10-4 m3/s/ 36 = 6.89 x 10-6 m3/s Step 4: Check practicability of design. If 36 ports equally spaced on a 160mm diameter circle are to be arranged, the spacing between ports is L = rθ= (160mm/2) x (2π(rad) /36) L= 14mm  This spacing seems reasonable, although it is not clear whether or not the flames will form independently or merge. Since all of the constraints are satisfied with the 36 port design, iteration is not required. . Adopting Flame Type for gas-fired cooker In real life, a flame produced from the burner is impinged upon a surface to transfer heat obtained from the combustion via convention, radiation and thermo-chemical heat release. Heat will be transferred to the other side of the surface by conduction with a small amount of heat loss to the surroundings. The flame characteristics of this portable gas-fired cooker are required to have low emissions of air pollutants, rapid ignition and combustion, stable and safe over the entire range of operation. . Considering the emission problems of the cooker In the domestic gas-fired combustion systems or portable gas-fired cookers, emissions of particular matter, carbon monoxide, sulfur dioxide and hydrocarbons are rather low because of the use of very fine and clean fuels, the gaseous fuels. This is even obvious when a better gaseous fuel, such as natural gas, is used. . As for butane gas which is sold bottled as a fuel for cooking and camping. When blended with propane and other hydrocarbons, it is referred to commercially as LPG. Butane is also used as a petrol component, as a feedback for the production of base petrochemicals in steam cracking, as fuel for cigarette lighters. Butane gas belong to the alkanes family CnH2n+2, using these gaseous fuel as the gas for portable cooker will be most likely to decrease the emission problems because butane contain negligible sulfur and organic nitrogen, but carbon dioxide and nitrogen are sometimes present. In addition, non-combustible substances are very low. Best regards  Ken Ngan (Nov) |
