Environmental control system holds vital importance as it is responsible for passenger’s ventilation and comfort. This paper presents an analytical design of environmental control systems and represents the esti- mated design in three-dimensional. Knowledge-based engineering application serves as the base for design- ing and methodology for the environmental control systems. Flexibility in the model enables the user to control the size and positioning of the system and also sub-systems associated with it. The number of passengers serves as the driving input and three- dimensional model gives the exact representation with respect to the volume occupied and dependencies on the number of passengers. It also provides a faster method to alter the system to user needs with respect to the number of air supply pipes, number of ducts and pipe length. Knowledge-based engineering gives the freedom to visualize various options in the conceptual design process. Nomenclature Abbreviation Meaning ACM Air Cycle Machine cmp Compressor CATIA Computer Aided Three Di- mensional Interactive Applica- tion ECS Environmental Control Sys- tem HPWS High Pressure Water Separa- tion System LPWS Low Pressure Water Separa- tion System MHX Main Heat Exchanger PHX Primary Heat Exchanger RAPID Robust Aircraft Parametric In- teractive Design trb TurbineSymbol Description Units A Area m2 P Perimeter m T Temperature degree C k Thermal Conductiv- itykg mK K Kelvin K D Hydraulic Diameter m m Mass ﬂow ratekg s p Pressure Pa m Dynamics viscositykg ms 1 Introduction The Environmental Control System (ECS) is responsi- ble for passengers comfort and one of the most impor- tant systems of an aircraft. Temperature and pressure of the air vary on the altitude and ECS works on reg- ulating the air, takes-in the bleed air mixes it with the recirculating air so that the pressure and temperature of the air are safe for breathing. Also, the system pro- vides de-misting, anti-icing functions for the aircraft [1]. Conventional ECS, operates on an open loop sys- tem, with air acting as the refrigerant. Bleed air is conditioned and fed into the aircraft for passenger survival. Further, this system is classiﬁed as a Low- Pressure Water Separation System (LPWS) and High- Pressure Water Separation System (HPWS). A com- monly used system in aircraft, where the air is used as a refrigerant. Air is made to pass through heat ex- changers, Turbines, Condenser and water extraction loop to give the conditioned air. The difference between LPWS and HPWS is the water extraction loop, consisting of Reheater and Condenser. Although LPWS has fewer components and lesser weight, the absence of water extraction loop unit becomes the limiting factor. The absence of this loop can lead to icing near the Turbine unit. This ic- ing is not a desirable effect and thus, HPWS becomes the better choice and is used commercially on a large 1 Devadurgam, Rajagopal, Munjulury Fig. 1 . Electrical ECS Schematic diagram [1] Fig. 2 . Low pressure water separation system schematic diagram [1] scale. On the other hand, electrical ECS mainly oper- ates on reducing power consumption, by using elec- tric power instead of pneumatic power. Air is taken as the ram-air substitute and is conditioned before feeding into the fuselage. Electrical ECS comprises of two air conditioning packs, which are electrically powered and motor driven. Electrical ECS is gain- ing importance due to the lesser fuel consumption and eco-friendliness when compared to the conventional ECS. Lack of bleed air inlet essentially means cabin pressurization is provided by electrical compressors. These compressors also act as the source of air for the ECS, illustrated in Figure 1. 2 Aims and Objectives This paper aims at designing and sizing conventional & electrical ECS and air supply piping along with dif- fusers. These could be used for different sizes and types of aircraft with a varying number of passen-gers. The user gets the freedom to choose the type of ECS system along with the air supply piping and diffusers(single-aisle). While fulﬁlling the goals, the objectives considered were, Designing the ECS and components based on the type of aircraft or number of passengers. Calculate inlet and outlet temperatures and pressures of individual components. Validating the outputs temperatures (K) and pressures (Pa) of bleed air. Designing air supply piping for single and dou- ble aisle conﬁgurations, which could be instan- tiated along the length of the fuselage. 3 Methodology The various components that make an ECS are sized/modeled and presented below. Also, the as- 2 Analytical Design and Estimation of Conventional and Electrical Aircraft Environmental Control Systems Fig. 3 . High pressure water separation system schematic diagram [1] sisting equations are illustrated with numerical val- ues along with the assumptions made to in the de- sign process. Initial assumptions are based on Moir & Seabridge [1]: Cruising altitude of 11277m Mach number of 0.78 Inlet bleed air temperature at PHX ( Tbleed) is 473.15 K Ram air temperature ( Tram) is 217.15 K 3.1 Analytical component sizing Heat exchangers form the backbone of ECS. Cross- ﬂow heat exchangers are considered and only dry bleed air is made to pass through the ECS[2]. Each component of ECS is illustrated with the sizing equa- tions along with the assumptions. These equations in- crease the ﬂexibility and adaptability of the system. The number of passengers ( NPax) acts as the driv- ing input for the ECS sizing. Other limitations like stresses, size, servicing, material, and cost were ne- glected during the sizing/modeling of the individual ECS components. 3.1.1 Heat Exchanger The Primary heat exchanger (PHX) & Main heat ex- changer(MHX) are not the only heat exchangers in ECS, even Reheater & Condenser are also cross-ﬂow type heat exchangers. Thus, PHX, MHX, Reheater, and Condenser have the same sizing equations. These are discussed in this section with emphasis on the method employed for the sizing.Fin tube hydraulic diameter on hot and cold side of the heat exchanger [2]. Dh=4At Pt(1) where, At= Inside cross section area of the tube, ( m2) Pt= Perimeter of the tube, (m) Density of air on hot and cold side of the heat exchanger [2]. r=MP RT(2) where, r= Density, (Kg m3) M = Molar mass of the air = 29 P = Pressure of air, (bar) R = Gas constant=0.0831 (J mol:K) T = Temperature of air, (K) For dynamic viscosity, Sutherland’s law is used [2]. m=mre f(T Tre f)1:5(Tre f+S T+S) (3) where, m= Dynamic Viscosity, (Kg m:s) 3 Devadurgam, Rajagopal, Munjulury mre f= Reference Dynamic Viscosity, (Kg m:s) T = Temperature of air, (K) Tre f= Temperature of air at sea level, (K) S = Sutherland’s Temperature, (K) Gasmre f (Kg m):sTre f (K)S (K) Air 1:76105288.15 110.4 Table 3 . Sutherland’s Constant Flow velocity of air on hot and cold side of the heat exchanger [3]. V=m rA(4) where, V = Flow Velocity of air, (m s) r= Density, (Kg m3) A = Cross sectional area of the tube = (PtDh)=4, (m2) m = Mass ﬂow rate of air, (Kg s) Reynolds number of air on hot side and cold side of the heat exchanger [2]. Re=rVDh m(5) where, V = Flow Velocity of air, (m s) r= Density, (Kg m3) Dh= Hydraulic Diameter of the tube, (m) m= Dynamic Viscosity, (Kg m:s) Thermal conductivity of air on hot side and cold side of the heat exchanger (W/m.K) [2]. kair= ((( 1:521011)(T3 bleed))((4:86108) (T_bleed^2))+((1.02*10^-4)*(T_bleed))- (3.93*10^-4)) (6)Prandtl number of air on cold and hot side of the heat exchanger [4]. Pr= (mCp)=Kair (7) Nusselt number of air [2]. f= (0:790(log(Re))(1:64))2(8) Nu= ((f=8)(Re1000 )Pr)=(1+12:7((f=8)0:5)(Pr2 3)1) (9) Heat transfer coefﬁcient (W m2K) of hot and cold side of the heat exchanger [3]. h= (NuKair)=(Dh) (10) Number of tube passes in the heat exchanger [3]. Nt= (LaDh)=SL (11) where, SL= Longitudinal ﬁn pitch distance, (m) Total heat transfer area at hot and cold side of the heat exchanger [2] . AHS=2(Nt)(Lb)(Lc)(((sfT hf)+(2f inh))=sf) (12) ACS=2(Nt)(La)(Lb)(((sfT hf)+(2f inh))=sf) (13) where, Nt= Number of tube passes f inh= Fin height in, (m) T hf= ﬁn thickness, (m) La;Lb;Lcare the length,breadth and height, (m) Total surface temperature effectiveness of ﬁn [4]. Eft=1((1hf in))(AHS=ACS)) (14) where, hf in= Isentropic efﬁciency of ﬁn Overall thermal resistance [4]. RT R= (1=(Efth)) + ( 1=((Ahs=volume hx) /(A_hs/volume_hx)*h))+(( T_hf/ K_ﬁn)) (15) 4 Analytical Design and Estimation of Conventional and Electrical Aircraft Environmental Control Systems where, Kf in= Thermal conductivity of ﬁn , (W mK) volume hx=(La)(Lb)(Lc), (m3) Overall heat transfer coefﬁcient [4]. U= (1=RT R) (16) Heat capacity rate for both bleed and ram air [3]. Ca(max=min)=m(bleed =Ram )Cp (17) where, m=Mass ﬂow rate of bleed air or ram air, (Kg sec) Cp= Speciﬁc heat of air at constant pressure, ((J kg:K)) Stream heat capacity rate ratio (dimensionless value between 0 and 1) [3]. CaR= (Camax(bleed =ram)=Camin(bleed =ram))(18) Number of transfer units, [3]. NTU = (UACS)=(Camin(bleed =ram))(19) Effectiveness of the heat exchanger [4]. E= (1exp((exp(CaR(NTU0:78))1)= (Ca_R*NTU^-0.22))) (20) Total heat transfer rate [4]. Q= (E)(Camin(bleed =ram))(TbleedTram) (21) Camin=min(Camax(bleed =ram);Camin(bleed =ram))(22) Tube length per pass [3]. Ltube= (NTUCamax(bleed =ram))=(UN2pDh) (23) Bleed air outlet temperature (K) from heat ex- changer [4]. Tphx= (Tbleed)(Q=Camin(bleed =ram))(24)Pressure drop at the heat exchanger [4]. Um= (4m)=((rhop(D2 h))) (25) f ric coe f f = (1:58(log(Re))(3:28))2(26) DP= (f ric coe f f((rho(U2 m))=(2Dh)))Ltube (27) Outlet pressure from PHX (Inlet pressure to compressor) [3]. Pout=PinDP (28) 3.1.2 Compressor It is essential to ﬁnd the work done by the compressor and the radial speed of the impeller as part of the air cycle machine that takes in cold bleed air from the engine, pressurizes and increases the temperature of the bleed air. The outlet temperature of the compressor, Tcmpout =Tcmpin(pc)((g1)=g)(29) It is also important to ﬁnd the is-entropic temperature of the compressor, Tcmpis = (TcmpoutTcmpin + (hcmpTcmpin ))=hcmp (30) Work done by the compressor, Wcmp=m(Cp)(TcmpisTcmpin ) (31) Radial speed of the impeller, U2= ((Wcmp1000 )=S0:5 f cmp (32) Outlet pressure of the compressor, Pcmpout =pcPcmpin (33) For electric ECS, pressure of ram air at the inlet is ob- tained by considering the aerodynamic effects while the aircraft is cruising. Pcmpin (eecs)=Patm(1+0:2Ma2)3:5(34) 3.1.3 Turbine Turbine is another essential component of the air cy- cle machine. The temperature of the air drops as the air expands through the Turbine. Work done by the Turbine and the radial speed of the impeller is to be calculated for the model. The outlet temperature and 5 Devadurgam, Rajagopal, Munjulury pressure of the Turbine are given by Equations 36 and 39 respectively. Outlet temperature of the Turbine, TtrbOut =TtrbIn(1(htrb(1(ptrb)((g1)=g)))) (35) Isentropic temperature, TtrbIs = (TtrbOutTtrbIn + (htrbTtrbIn))=htrb(36) Work-done by the Turbine, Wtrb= (m)(Cp)(TtrbInTtrbIs) (37) Radial speed of the impeller, U2trb= ((Wtrb1000 )=S ftrb)0:5(38) Outlet pressure of the Turbine, PtrbOut =ptrbPtrbIn (39) 3.1.4 Pressure Losses in Pipes In general, the pressure loss is considered for all types of fully developed ﬂows i.e., laminar or turbulent, cir- cular or non-circular, smooth or rough surfaces, hori- zontal or inclined pipes. There are different kinds of pressure losses and the equations that were used to compute them are presented in this section. 1. Pressure loss in the pipe DPL= (r)(g)(hL) (40) 2. Pressure loss at the pipe bends Head loss at the bend. hLbend= (KL)(V2 2g) (41) 4 Results Numerical data obtained for the different components of the conventional ECS are analyzed and validated in this section. Also, results obtained for the three different aircraft types that were considered are being evaluated. The performance of ECS varies with the user input called the number of passengers. The re- sults obtained along with the assumptions made for the different components of the system are provided below. mbleed =Npaxmpax (42)Aircraft Type (–)Number of passengers (Npax)Bleed air mass ﬂow rate (kg sec)Ram air mass ﬂow rate (kg sec) 1 150 0.50 0.8 2 250 0.83 1.1 3 350 1.17 1.4 Table 4 . Bleed and ram air mass ﬂow rate 4.1 Conventional Environmental Control System 4.1.1 Heat Exchangers - PHX and MHX Aircraft Type (–)Lap (m)Lbp (m)Lcp (m) 1 0.25 0.23 0.15 2 0.30 0.25 0.15 3 0.32 0.20 0.18 Table 5 . PHX geometry for three aircraft types Aircraft Type (–)Lam (m)Lbm (m)Lcm (m) 1 0.25 0.23 0.15 2 0.30 0.25 0.15 3 0.32 0.20 0.18 Table 6 . MHX geometry for three aircraft types (a) PHX (b) MHX Fig. 4 . Outlet Pressure and Temperature of the bleed air at the outlet of the PHX and MHX 6 Analytical Design and Estimation of Conventional and Electrical Aircraft Environmental Control Systems The Primary heat exchanger (PHX) & Main heat ex- changer(MHX) are typically cross-ﬂow type heat ex- changers. Depending on the inlet mass ﬂow rate of the air the size of the component varies. As the air- craft type changes i.e., as the number of passengers increase the mass ﬂow rate of hot bleed air increases and vice-versa. For both PHX and MHX, the ﬁn height and ﬁn thickness are taken as 0.006 m and 0.0002 m respec- tively for all the three aircraft types [2]. The inside cross-sectional area ( At) and perimeter of the tube ( Pt) is taken as 0.002 m2and 0.54 m respectively [2]. The thermal conductivity of the ﬁn ( Kf in) is considered to be 177 (W m2K)[5] for all the aircraft. 4.1.2 Air Cycle Machine - Compressor & Turbine The compressor, Turbine, and fan are all mounted on the same shaft and Turbine drive the compressor. Fig- ure 5a and Table 7, represent the Turbine outlet tem- perature, pressure and the diameter of the compressor impeller for the aircraft types. The bleed air from the reheater cold side (CS) ﬂows into the Turbine. Figure 5b and Table 8, represent the Turbine outlet tempera- ture, pressure and the diameter of the impeller for the aircraft types. Aircraft Type (–)Outlet Diameter D2 (m)Inlet Diameter D1 (m) 1 0.076 0.038 2 0.098 0.049 3 0.11 0.055 Table 7 . The inlet and outlet geometry of the air cycle machine compressor impeller Aircraft Type (–)Outlet Diameter D2 (m)Inlet Diameter D1 (m) 1 0.076 0.038 2 0.099 0.049 3 0.11 0.055 Table 8 . The inlet and outlet geometry of the air cycle machine Turbine impeller The values in Table 7, were obtained using sizing equation’s considered in Section 3.1. The pressure ra- tio of the compressor (pc)is considered to be 1.4 [2]. The speciﬁc heat of bleed air at constant pressure andvolume is considered to be cp=1.005 (k j=kg:K)and cv=0.716 (k j=kg:K)[3]. The compressor efﬁciency (hcmp) is assumed to be 0.7 [2]. Slip factor of the compressor (Sf cmp)is taken as 0.9 [6]. The impeller rotations per minute (rpm) ( Ncmp) is considered to be 45000 [7]. The diffuser inlet diameter ( D1cmp) was assumed to be half of the ( D2cmp). The assumptions considered for the Turbine are as follows, the expan- sion ratio of the Turbine for all the three aircraft types is assumed to be 2. The slip factor of the Turbine is as- sumed to be 0.9. The isentropic efﬁciency of the Tur- bine is assumed to be 0.8 for all three aircraft types. (a) Compressor (b) Turbine Fig. 5 . Outlet Pressure and Temperature of bleed air 4.1.3 High Pressure Water Separation Loop The high-pressure water separation loop consists of Reheater, Condenser, Water extractor, and Turbine. The bleed air temperature and pressure at the cold side of the Reheater are assumed to be 320 Kand 260000 Pa[2]. Fig. 6 . High pressure water separation loop Conventional ECS [1] 7 Devadurgam, Rajagopal, Munjulury 4.1.4 Reheater (Cold & Hot Side) & Condenser (Low pressure side (LPS) & High pressure Side (HPS)) Reheater (REH) and Condenser are cross-ﬂow type heat exchangers. The bleed air from the MHX ﬂows through the hot side of the Reheater. Bleed air from the high pressure side of the condenser ﬂows into cold side of the Reheater. From Reheater hot side (HS), the bleed air ﬂows into the high pressure side (HPS) of the condenser. The Reheater geometry for three Aircraft types Aircraft Type (–)LaR (m)LbR (m)LcR (m) 1 0.10 0.10 0.10 2 0.14 0.14 0.14 3 0.18 0.18 0.18 Table 9 . The above table illustrates the considered Reheater ge- ometry for three aircraft types (a) Hot side (b) Cold side Fig. 7 . Outlet temperature & pressure of bleed air at outlet of the Reheater Here, the Turbine outlet bleed air acts as the coolant air and passes to the low pressure side of the con- denser. At high pressure side of the condenser, bleed air from Turbine outlet acts as the coolant and passes at the low pressure side of the condenser. The bleed air from the condenser high pressure side (HPS) ﬂows into the Reheater cold side (CS).The Condenser geometry for three Aircraft types Aircraft Type (–)LaC (m)LbC (m)LcC (m) 1 0.10 0.10 0.10 2 0.14 0.14 0.14 3 0.18 0.18 0.18 Table 10 . The above table illustrates the considered Condenser geometry for three aircraft types (a) High pressure side (b) Low pressure side Fig. 8 . Outlet temperature & pressure of the Condenser Bleed air from Turbine ﬂows into the low pressure side of the condenser. Figure 8b & Figure 8a, rep- resent the temperature and pressure of the bleed air at the exit of the condenser (low pressure side (LPS)) respectively. The heat transfer takes place between the bleed air ﬂowing at the condenser high pressure side (HPS) and bleed air ﬂowing at the low pressure side (LPS). Figure 7a & Figure 7b, represent the outlet temperature and pressure values of the bleed air at the exit of the hot and cold side of the Reheater respec- tively. 4.1.5 Validation of inlet and outlet temperature of ECS components The summarizes of the inlet and outlet temperatures of different components of the conventional ECS are 8 Analytical Design and Estimation of Conventional and Electrical Aircraft Environmental Control Systems shown in Figure 9. The values are validated with Lieb- herr ECS data [8]. Fig. 9 . Validation of inlet and outlet Temperature of components vs Liebherr data [8] 4.2 Electrical ECS The results for Electrical ECS, the sizing values with respect to individual components of the system are tabulated. Comparing with conventional ECS, com- ponents like Reheater, Condenser and Water extractor are absent. Components like compressors and fans are powered by motors running on AC power. 4.2.1 Ram Air Compressors In Electric ECS, bleed air from the engine is absent. Instead ram air is used from atmosphere as the source of air to electric ECS, fed by electric ram air compres- sors. Fig. 10 . Temperature and pressure ram air outlet of two electric compressors Aircraft Type (–)Outlet Diameter D2 (m)Inlet Diameter D1 (m) 1 0.14 0.07 2 0.18 0.09 3 0.21 0.105 Table 11 . Inlet and Outlet geometry of the impeller Compressed air passes through the ozone separator and ﬂows into the Primary heat exchanger (PHX). Theprimary heat exchanger is a cross-ﬂow type heat ex- changer same as the conventional HPWS-ECS. The compressed hot ram air from ram air compressors passes at one side of PHX and cold ram air passes from the other side of the PHX from plenum. The inlet pressure of the ram air is obtained using the inlet pressure Equation 34. The Mach number of the aircraft is considered to be 0.75 and is ﬂying at an al- titude of 11277.6 m. The atmospheric pressure( Patm) at 11277.6 m is 8100 Pa [1]. Table 11 illustrates the impeller geometry of the compressor. Figure 10 il- lustrates the temperature and pressure of the ram air at the outlet of the compressor. The pressure ratio of the two ram air compressors is considered to be 5 [9]. The slip factor of the ram air compressors is consid- ered to be 0.9 [6]. The isentropic efﬁciency of the ram air compressors is considered to be 0.7 [9]. 4.2.2 Primary Heat Exchanger & Secondary Heat Exchanger Aircraft Type (–)Lap (m)Lbp (m)Lcp (m) 1 0.13 0.11 0.11 2 0.22 0.20 0.20 3 0.27 0.25 0.25 Table 12 . Electric ECS - PHX geometry for three aircraft types (a) Outlet temperature and pressure of the air - PHX (b) Outlet temperature and pressure of the ram air- SHX Fig. 11 . Heat Exchangers 9 Devadurgam, Rajagopal, Munjulury Aircraft Type (–)LaS (m)LbS (m)LcS (m) 1 0.13 0.11 0.11 2 0.22 0.20 0.20 3 0.27 0.25 0.25 Table 13 . Electric ECS - SHX geometry for three aircraft types Compressed air passes through the ozone separator and ﬂows into the primary heat exchanger (PHX). The primary heat exchanger is a cross ﬂow type heat ex- changer. The compressed hot ram air from ram air compressors is passed into the one side of the PHX and cold ram air from plenum passes on the other side of the PHX. Table 12 & 13, represents the geometry of the PHX and SHX considered in this study for three aircraft types. The Figure 11a & 11b, represent the outlet tem- perature and pressure of air at the outlet of the PHX and SHX. 4.2.3 Air Cycle Machine - Compressor The dimensional and thermal results are illustrated in Table 14 & Figure 12. These results are obtained an- alytically by solving the equations considered in the Section 3.1, Turbine sizing. The air inlet pressure of the Turbine is equal to the outlet air pressure from the SHX. The is-entropic efﬁciency of the compres- sor (htrb) is considered to be 0.8. The slip factor of the compressor ( Sftrb) is 0.9. From PHX the air ﬂows into the ACM. In compres- sor, air gains the pressure and temperature. Table 14, Figure 12 are obtained using equation’s considered in the Section 3.1. The pressure ratio of the compressor (pc)is assumed to be 5. The speciﬁc heat of bleed air at constant pressure and volume is considered to becp=1.005 (k j=kg:K)andcv=0.716 (k j=kg:K)[3]. The compressor efﬁciency ( hcmp) is assumed to be 0.7. The slip factor of the compressor (Sf cmp)is as- sumed to be 0.9. The impeller rotations per minute (rpm) ( Ncmp) is considered to be 45000 rpm. The im- peller inlet diameter ( D1cmp) is assumed to be half of the (D2cmp). Fig. 12 . Outlet temperature and pressure of air at the exit of the compressor Aircraft Type (–)outlet impeller Diameter D2(m)inlet impeller Diameter D1(m) 1 0.16 0.08 2 0.20 0.10 3 0.24 0.12 Table 14 . The inlet and outlet geometry of the impeller in the compressor for the three aircraft types 4.2.4 Air cycle machine - Turbine The dimensional and thermal results are illustrated in Table 15. These obtained by analytically solving the equations considered in the Section 3.1, Turbine siz- ing. The air inlet pressure of the Turbine is equal to the outlet air pressure from the SHX. The is-entropic efﬁciency of the Turbine ( htrb) is considered to be 0.8. The slip factor of the Turbine ( Sftrb) is 0.9. Fig. 13 . The temperature and pressure of the air at the exit of the Turbine Aircraft Type (–)Outlet impeller Diameter D2(m)Inlet impeller Diameter D1(m) 1 0.12 0.06 2 0.16 0.08 3 0.19 0.095 Table 15 . The inlet and outlet geometry of the impeller in the Turbine for the three aircraft types 10 Analytical Design and Estimation of Conventional and Electrical Aircraft Environmental Control Systems 4.3 Pressure Losses in Pipes The hydraulic diameter ( Dh) of each pipe differs for each type of aircraft, ( Dh) for aircraft’s type 1, type 2 and type 3 are 30 mm, 35 mm and 40 mm. The pipe is assumed to be made up of aluminum (Al). The absolute roughness of the pipe is considered to be 0.001*103m [10]. Aluminum pipes are assumed for the calculations as they are cheap, the surface rough- ness ranges from 0.001*103m to 0.002*103m and are lesser in weight compared to the pipes made up of other metals. (a) Conventional ECS (b) Electric ECS Fig. 14 . Sum of pipe pressure losses in all components These characteristics of aluminum contribute eco- nomically and increase the efﬁciency of the system [10]. The pipe length between each component is con- sidered to be 0.3 m. It is constant for both conven- tional and electrical ECS and does not vary with the type of aircraft. The bend angle considered for each pipe connec- tion for both conventional and electrical ECS is 900 and the bend type is ﬂanged smooth bend. For the bend with an angle of 900the ﬂow is across bends, so the frictional factor value ( KL) is considered to be 0.3 [10]. Figure 14a & 14b, illustrate the sum of the pipe pressure losses for all the components of both conven- tional and electrical ECS. 4.4 Final pressure and temperature at the outlet of the conventional and electric ECS system In this section, the plots represent the ﬁnal tem-perature and pressure of the conditioned bleed air at the exit of the conventional and electrical ECS. Fig- ure 15a & Figure 15b, represent the ﬁnal pressure and temperature of the conventional ECS before the con- ditioned bleed air ﬂows into the mixing manifold. Fig- ure 16a & Figure 16b, represent the ﬁnal pressure and ﬁnal temperature of the electrical ECS at the exit of ECS and at the inlet of mixing manifold. (a) Final pressure of the conditioned air at outlet (b) Final temperature of the conditioned air at outlet Fig. 15 . Conventional ECS (a) Final pressure of the conditioned air at outlet (b) Final temperature of the conditioned air at outlet Fig. 16 . Electrical ECS 11 Devadurgam, Rajagopal, Munjulury 4.5 geometric models and RAPID Integration Knowledge-based Engineering approach is employed in CATIA [11], the knowledge is stored in the form of templates also known as User Deﬁned Features (UDF), these UDF’s are instantiated repeatedly with the help of a catalog and Knowlege pattern to obtain the desired result. The geometric models of the two pack conventional and electrical ECS are represented in Figure 17a and 17b, respectively. These are the geo- metric models of ECS, that would be instantiated into the aircraft in RAPID [12] [13] and the choice of the system is controlled manually using a parameter. Es- sentially, the user can choose which system would be required for the particular aircraft. (a) Conventional ECS System (b) Electric ECS System Fig. 17 . Final ECS geometric models The diffusers are connected to the main system through a mixing manifold. This air is then fed into the aircraft through the diffusers. The user has a con- trolling parameter to chose the type of ECS from and the sizing is done automatically based on the number of passengers that serves as the driving input. The geometric model of the air diffusers is shown in Figures 18a that is integrated into the RAPID [12] [13]. Single-aisle conﬁguration has the ducts that would be instantiated as shown. The number of dif- fusers to be instantiated into the aircraft can be con- trolled manually by a string parameter named "num-berOfInstances" by the user as per the requirements. This parameter is controlled by the knowledge pattern code. (a) Single aisle type diffuser ducts (b) Double aisle type air supply pipes Fig. 18 . Single and double aisle diffusers conﬁgurations (a) Single aisle (b) Double aisle Fig. 19 . Air supply piping in RAPID Double aisle conﬁguration is different compared to single-aisle, instead of diffusers, the system incor- porates a different design. The air supply pipes are designed to ensure the supply of air to the cabin di- 12 Analytical Design and Estimation of Conventional and Electrical Aircraft Environmental Control Systems rectly. Figure 18b shows the air diffuser pipes, that would also act as the ducts for the double aisle con- ﬁguration. This eliminates the need for the ducts, in turn reducing the piping cost and the gross weight of tubing. The different air supply piping are represented in Figure 19a & Figure 19b instantiated into RAPID [12] [13]. The user gets a parameter, which can be used to switch between the conﬁgurations for the aircraft. Figure 20, illustrate the ECS instantiated automati- cally into the RAPID. This automation is achieved by knowledge pattern scripting. The product knowledge template workbench is used for the implementation of knowledge pattern script into the CATIA model. Fig. 20 . RAPID aircraft with ECS & single aisle ducts 5 Discussion The automatic sizing of the ECS depends mainly on the number of passengers. This serves as the driving input and can be controlled using the equations in Sec- tion 3.1. This model is then integrated into the RAPID and can be tailored to meet the user requirements [12] [13]. From the results, it is observed that the conventional system results in a greater loss. This is due to the presence of bleed air in the system. Electrical ECS, on the other hand, utilizes electrical energy, thereby making it cleaner, energy efﬁcient and greener. Thus, electrical ECS is the go-to choice these days and is used in Boeing 787. The mass ﬂow rate of bleed air depends on the num- ber of passengers on the aircraft. Non-availability of data led to assumption of mass ﬂow rate of 0.8 ( (kg sec)) for 150 passengers. This value was increased by 0.3 ((kg sec)) for other aircraft types being considered. Ini- tial sizing of the heat exchanger was done ensuring that the pressure loss would be in the range of 38,000 to 40,000 Pa. The sizing was done in iterations until the desired pressure loss value was obtained. The number of tube passes ( Nt), deﬁned as the num-ber of passes in a heat exchanger. This is usually mea- sured across bleed and ram air ﬂow directions and is obtained by [Equation 11]. The number of tube passes varies with aircraft type. Here it increases as aircraft type changes from 1 to 3. Parameters such as length of the heat exchanger( La), hydraulic diameter of ﬁn tube ( Dh) and longitudinal pitch ( SL) are used in for- mulating the number of tube passes ( Nt). Values obtained are cross-checked and veriﬁed with Liebherr data[8] for accuracy. From Figure 9, the variations can be observed between the Liebherr ECS data and Conventional ECS system data obtained in this study. In this study, we consider the bleed air to be ideal gas, the connecting pipe bend angle be- tween the ECS components is assumed to be 900. The above mentioned assumptions in this study stand as a main reason for the variation between the Liebherr ECS data and Conventional ECS data obtained in this study. This serves as a validation for the assumptions made and ﬁnal values of the design. Variations in the range 0.2 to 2.5 % are observed, these are within the permissible range. These results give the much- needed validation and provide the assurance that the design could be used for advanced models. Aluminum is the preferred choice of pipe material for internal connection of ECS. The diameter of the pipe acts as a variable and varies with aircraft type. Pres- sure loss increases with an increase in the number of passengers in the aircraft. This might be due to the in- crease in mass ﬂow rate of bleed air or due to constant pipe bend angle. The geometric model of ECS and cabin air distribu- tion system for single and double aisle cabin conﬁg- uration are presented in Section 4.5. These models are parametric and knowledge pattern was used in au- tomation. The user has the complete freedom to al- ter the ﬁnal design by controlling or changing a few parameters. Knowledge pattern scripting controls the design in RAPID, based on user inputs. The scripting is faster and ﬂexible when compared with VBA [14]. The number of air ducts for the single-aisle conﬁgura- tion can be adjusted or controlled by a few parameters. The design being really ﬂexible can be customized easily for either single or double aisle conﬁguration. 6 Conclusion Analysis and working of conventional and electrical ECS are addressed along with temperature and pres- sure data for different aircraft conﬁgurations. These systems are designed and tested for different aircraft types with a different number of passengers. The tem- 13 Devadurgam, Rajagopal, Munjulury perature and pressure data obtained from the system for a speciﬁc cruise altitude are compared with the standard data for accuracy. Results show a minor vari- ation from the standard values, although this could be accounted for the constants that are assumed during the course of the calculation. This work can be taken as the ﬁrst step in ECS and can serve as the base for further improvements, including increasing the size for an extremely high or low number of passengers.