Efficiency and heat losses of indirect water bath heater installed in natural gas pressure reduction station; evaluating a case study in Iran

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1 Efficiency and heat losses of indirect water bath heater installed in natural gas pressure reduction station; evaluating a case study in Iran 1.Ebrahim Khalili,. Seyyed Mostafa Hoseinalipour, 3.Esmaeil Heybatian 1. Researcher of National Iranian Gas Company (NIGC). Associate professor of Mechanical Eng., Iran University of Science and Technology 3. General director of Shahrekord Gas Company Corresponding Author khalili@nigc-chbgas.ir Abstract Indirect fired water heaters are typically used to raise the temperature of fluids such as natural gas. Without heaters, gas freezing (because of Joule-Thomson effect) can occur, while it is passing through the pressure reduction installation, damaging valves and instrumentation, or even causing gas supply interruption. At present, water-bath heater is widespread used for gas industry to heat the natural gas. Its large size, the need for water reposition, its low thermal efficiency and large heat losses from flue gas (exhaust) are the main operational problems of the traditional equipment. In this paper energy calculations in all parts of a typical heater have been considered. Then main energy losses, specially from chimney was calculated and then the heater efficiency was obtained. As a case study, one city gate station (CGS) located in Shahrekord city with nominal capacity of 10,000 SCMH has been considered. Based on statistical data collected from station during one year, calculations showed that more than 38% of combustion energy will be lost through the stack and into the ambient. It also has been observed that about % of losses is from the heater surfaces. The heater efficiency of this test case is less than 47%. The total energy losses from stack and surfaces in the year under study were 4667Gj and overall gas consumption for burners was m³ which approximately costs about $. Keywords: Energy losses, gas pressure reduction station, Stack losses, efficiency 1

2 Fig.1: Typical configuration of water bath heater Fundamental of combustion and heat losses The process of the combustion in the fire tube heater is supposed to be a steady state process at a constant pressure and all gases are ideal. Natural gas is a complex multi component mixture of a number of saturated hydrocarbons: methane, ethane, propane, and butane and its isomers. Nitrogen, hydrogen sulfide, helium, argon, water vapor, and other components are contained in small amounts in natural gases. The analysis of natural gas from the test case location is shown in table 1 [1,]. Table 1: Volumetric Analysis Of Natural Gas (Shahrekord Station-Iran) Constituent Percent by volume Methane ( CH 4 ) 89% Ethane ( CH 6 ) 4.1% Propane ( C3H 8 ) 1.% Nitrogen ( N ) 5% Carbon dioxide CO 0.7% Considering above compositions, gives us a density of kg/m³ and the heat value of 8400 kcal/m³ for natural gas in the station. By thermo-dynamical calculations we determined the exact value of the constant-pressure specific heat of natural gas, as a function of temperature[3]: c Pmix= X i cp i (1) Where parameter X i represents the mass fraction of constituent and c p i is its specific heat. Substitution the related values for fuel gives: c ( kj / kg. K) = T T T () P where T varies from 50 K<T<100 K.

3 To ensure complete combustion, even modern equipment with many features must operate with excess air. Also for simplicity we supposed the methane as combustion fuel so the general equation of combustion reaction of this hydrocarbon fuel and air is written as[3]: CaH b+a(a+b/4) (O +3.76N ωH O) b (3) aco +[ A (a+b/4)ω]h O+(A-1) (a+b/4)o +3.76A(a+b/4)N Where parameter A represents equivalence ratio shows the percentage of excess air with respect to the theoretical air. The parameter ω is the amount of humidity ratio of the ambient air (combustion air). Excess air wastes energy by carrying heat up the stack. Regarding the 50% excess air and the humidity ratio of (based on site condition), the equation of combustion will be: CH +3O +11.8N H O CO H O+O +11.8N (4) 4 Again for determining the specific heat for combustion product (flue gases) we have: c =X Cp +X Cp +X Cp +X Cp (5) P CO CO HO HO N N O O With substitution exact value of mass fractions and specific heats the above formula is rewritten as: c ( kj / kg. K) = T T T (6) P In a water bath fired tube heater, the main losses are: dry flue gas loss, E & radiation or surface convection loss, E & surf [4]. Considering the water bath heater as a control volume, the energy conservation law is: E& = E& + E& = E& + E& + E& (7) Fuel losses NG stack surf NG Where E & NG is the amount of heat absorbed by the natural gas to increase its temperature. Required energy for heating the natural gas To determine E & NG, using the first law of thermodynamic neglecting the potential and kinetic energies, we use the following relation: Tout E& = NG m& NG ( h out hin ) = m& NG c T PdT (8) in Where: m& NG = mass flow rate of natural gas through the station (kg/s) h = enthalpy of the natural gas at the inlet and outlet of the heater (kj/kg.k) Stack Losses for Natural Gas Calculating stack losses for heater using natural gas containing no sulfur and fired with negligible CO and hydrocarbons, was done. In the process of combustion the hydrogen content of the fuel is converted to HO which normally leaves the stack as water vapor carrying with it the heat required to convert it from liquid to vapor [5]. stack and 3

4 If a sulphur-free fuel gas is used with uninsulated stacks, a minimum exit flue gas temperature of 50 F should be maintained to avoid internal stack corrosion [6]. The stack losses is calculated as: m, o (,, ) T T Tm i Tm o E& = m& ( h h ) = m& c dt + c dt (9) Stack products out in products P P Where m& products refers to mass flow rate of combustion products and T m, i, T m, o stands for the mean temperature at the inlet and outlet of the stack. Using formulas related to the internal flow (inside the stack pipe) and external flow (outside the stack pipe), the dependence of other parameters is shown bellow [8]: T T U A T T mc m, o S = exp m, i & p Where: 1 1 Ln( r / r1 ) 1 = Rt = + + (11) U AS π r Lh π kwl π r1 Lhi and: T = ambient temperature T = mean temperature of flue gases at the stack entrance m, i T m, o = mean temperature of flue gases at the stack outlet m& = flue gases (combustion product) flow rate c = specific heat of flue gases (combustion product) p L = stack height Also for external flow over the stack wall (outside the stack pipe), the Nusselt Number based on Churchill and Bernstein formula is [8]: (10) 1/ 1/3 5/8 0.6Re Pr D ReD Nu D = / 3 1/ Pr Where Re and Pr stand for Reynolds and Prandtl Numbers respectively. 4/5 (1) 4

5 Fig. : Schematic chimney stack internal and external flow Surface losses from heater vessel To obtain the heat losses from heater walls ( E & ), the internal heater wall temperature was supposed to be equal to the water bath temperature [7] so the equation will be: Tw -Tamb E& surf = (13) xsteel xwool xal K A K A K A h A steel steel wool wool Al Al air heater Where: x =thickness (m) k = conductivity ( w / mk ) A = surface vertical to the heat transfer direction ( m ) h = heat transfer coefficient ( w / m K ) T =the temperature of water bath (inner wall of heater) w T amb =the ambient temperature of air ( C ). Since Shahrekord Station is considered as a case test so related parameters are [1]: t steel =1cm, k steel =43w/mK, t wool =3cm, k wool =0.039w/mK, t Al =1mm, k Al =50w/mK. Based on the climate condition, the average wind velocity at the station is about 6m/s and the convection heat transfer coefficient (external flow) was calculated 3 w/m²k. The efficiency of the water bath heater is based on bellow formula [7]: output E& E & fuel E & losses E & fuel ( E & surf + E & stack ) NG ηheater = = = = (14) input E& E& E& fuel fuel fuel Results and Discussion Considering the above mentioned relations and assumptions, a case study namely Shahrekord CGS was considered. It should be noticed that during a year, some statistical parameters has been measured from the heater by installing appropriate devices such as thermometer, pressure gage and gas flow meter. These parameters surf 5

6 are: station gas flow rate, heater inlet and outlet temperatures, burner flow rate, stack inlet temperature and ambient temperature. Fig.4 shows the monthly gas flow rate variation in one year in this station. Also Fig.5 shows the stack losses and also required energy to heat the gas before reducing its pressure from nominal pressure 1000 psi to 50 psi. During hot months (from May till August) the heater will be off. Other details and results of the calculations are shown in Table. The results shows the maximum temperature as 476ºC from heater output (stack inlet) which severely wastes the energy. The efficiency of the heater is averagely less than 47% which isn t remarkable. Nowadays there is more than MMSCM (Million Standard Cubic Meter) gas consumption in domestic division in Iran that contains one-third of total consumption. Also there are a thousands indirect water bath heaters that operate with low efficiency. It should be profitable to install high efficiency heaters to achieve the goal without wasting a lot of energies. The common problem of these heaters is that their efficiency is low between 30% to 60% [4,5]. Stack losses can be minimized by reducing excess air which reduces the quantity of flue gas that is heated to exhaust temperature and by reducing the exhaust temperature. The latter can be accomplished by adding heat exchange surface such as economizers and air heaters. The former can be accomplished by improving burner performance through burner design or precise combustion controls [7]. Fig.3: Indirect water bath heater located in Shahrekord Station, Iran 6

7 Fig. 4: Monthly gas flow in Shahrekord gas reduction station (MCM= Million Cubic Meter) Energy (Gj) Gas Preheating Stack Losses 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Fig. 5: Comparison between required energy for gas preheating and the stack losses 7

8 Month Station gas flow (m³) heater inlet temperature (ºC) Table : Case study results of gas heater located in Shahrekord City Gate Station heater outlet burners Stack inlet Stack heater inlet gas preheating temperature gas flow temperature pressure (psi) E (ºC) (m³/h) (ºC) ( ) losses & NG Gj E& ( Gj) stack Surface losses E& surf ( Gj) Heater Efficiency (%) η Jan 39,70, % 10.3 Feb 37,800, % 8.5 Mar 3,14, % 13.7 Apr 14,880, % 15. May 9,393, Jun 5,889, Jul 5,068, Aug 4,99, Sep 5,053, % 14. Oct 7,110, % 1.8 Nov,560, % 8.9 Dec 3,850, % 5.1 Average Ambient Temp. (ºC) 8

9 References [1] Data Sheets, taken from Shahrekord Gas Pressure Reduction Station [] Statistical Reports, National Iranian Gas Company, [3]R.E. Sonntag, C. Borgnakke, G.J.Van Wylen, Fundamental Of Thermodynamics, Six Edition, 00 [4] W. Angelo, M. H. Mantelli, F. H. Milanez; Design of a heater for natural gas stations, 14th International Heat Pipe Conference; Florianopolis, Brazil, 007 [5] Hang-Yen Fang; Development and performance measurement and modelling of packed-bed fire tube heater; A dissertation in chemical engineering submitted to the Graduate Faculty of Texas Tech University, for the degree of doctorate; 1984 [6] Specification for Indirect Type Oilfield Heaters; API Specification, 1K, eight edition, October 008 [7] Measurement and augmentation of burners at indirect water bath heater in gas city gate stations; National Iranian gas company s reports; 010 [8] Frank P. Incropera, David P. De Witt, Introduction to heat transfer, third edition, 001 9