Structural Analysis and Design Optimization of Air Cooler Support Structure

Dilip K Mahanty, Vikas Manohar, Sailendra Meduri, Sumit Bhatnagar Tata Consultancy Services, India

K Ramamurthy Mutual Mecaplast Ltd., India

Abstract In the present day structural engineering arena, the call of the day is the optimum use of materials to its fullest capability in terms of strength and life. In this sense, the support structure of any engineering products need to satisfy the optimum use of material, minimizing the displacements and stresses developed. One of the methods to carry out this is through finite element analysis. In the present paper, the support structure of newly designed air cooler was analyzed through finite element method using ANSYS. The objective was to try different material, to obtain minimum weight and displacement values.

For this purpose, structural analysis of existing support structure of Air Cooler was carried out for three different types of plastic materials (SEB200, FPT30PPC and FPT20PPC) which was showing some vibration based failure. The different loads acting on the structure due to the pump load and the loads due to the blower fans, self-weight, eccentric loading etc. during operation was considered for the analysis. Of the two prominent load cases, the load case where both pump load and fan motor assembly load acting downward was the worst loading case.

The stresses developed in the structure when SEB 200 was used were found to be within the allowable limits for both the load cases. The displacement value obtained through finite element analysis and actual measurements were differing by about 6% only. This validated the existing design and the procedures considered for analysis. Based on these findings the redesign of the structure for reduced weight (thickness) and reduction in displacements was considered. The final design showed a reduction of 44.0 % in the total displacement at the desired locations for the material FPT20PPC and reduction of stress levels by 26 % for the material. Based on the finite element simulation the modified final design was found to be better than the existing design in terms of displacement and stress values, cost of the material used and reduction of the problem of vibration based failure of the support structure.

Introduction In the present day structural engineering arena, the call of the day is the optimum use of materials to its fullest capability in terms of strength and life. In this aspect, various design configurations are to be met with an end objective for maximum use and return on investment. To achieve these, one needs to do a thorough simulation of any engineering components with time and cost effectiveness in an optimized manner. In this sense, the support structure of any engineering products need to satisfy the optimum use of material and minimizing displacement and stresses developed. One of the methods to carry out this is through finite element analysis. Here the support structure of a newly designed air cooler was analyzed through finite element method, which was failing in the field due to vibration. The deflections for the pump base and the fan base were high. To understand the cause of failure and to optimize the design, finite element analysis (FEA) of existing support structure of Air Cooler was carried out for three different types of plastic materials (SEB200, FPT30PPC and FPT20PPC).

The support structure for the air cooler supports the blower fan assembly, fan motor and suction pump at the base of the structure. The pump is for pumping water to the cooling mat. For the assembly model of the support structure, refer to Figure 1.

Figure 1 - Solid Model of Support Structure with pump and motor locations The static structural analysis of the support structure of the air cooler was carried out with the objective to find out the maximum stresses developed in the structure based on the safety aspect due to two different types of load cases. The load cases were:

(i) Load due to centrifugal force, applied in horizontal direction.

(ii) Load due to centrifugal force, applied in vertical direction.

The different loads acting on the structure due to the pump load and the loads due to the blower fans, self-weight, eccentric loading etc. during operation was considered for the analysis. The other objectives were weight and cost optimization of the structure.

Procedure

The geometric model of the existing design made in Unigraphics (UG) was available (See Figure 2). The mid-planes were extracted from the geometric model in UG. The extracted mid plane was imported into ANSYS 5.5. The model was meshed with type SHELL 63 finite element of ANSYS 5.5 (See Figure 3). The loads and boundary conditions were applied on the finite element model as per the physical conditions (See Figure 4). For locational details of forces acting on the support structure, refer to Figure 5.

The basis for various loads to be applied for the above load cases are given below.

Figure 2 - Solid Model of Support Structure

Figure 3 - Finite Element Model of Support Structure

Figure 4 - FE Model of Support Structure with Loads and Boundary Condition Weight of the Fan Motor Assembly

Figure 5 - Forces acting on the structure The weight of the total assembly, W1 = 48.23 N acting at a distance of 13.07 mm from the center of gravity (CG) taking the coordinate system located at the position given by UG Model.

(See Figure 6 and also see Figure 5.

R1 = 18.15 N

R2 = 4.916 X 9.81 – (R1) = 30.08 N.

Figure 6 - Line diagram of reaction forces on structure supporting the fan motor assembly

Centrifugal Force Due to an unbalanced mass of 4 gm at 200 mm radius on fan blade, centrifugal force will act on the center of motor shaft. Also, a run-out of 1.5 mm has to be considered (See Figure 7 and also see Figure 5).

Figure 7 - Line diagram of reaction forces due to the centrifugal force

This centrifugal force acts in a radially outward direction. Two load cases are considered due to centrifugal forces:

1. Force acting vertically downward (in direction of gravity).

2. Force acting horizontally, in the plane of the motor shaft.

The magnitude of the force is computed using rmF 2ω=

where

m = 4 gm. Unbalanced mass acting at the periphery of the fan blade.

ω = 1400 r.p.m. Motor speed for the fan.

r = 201.5 mm. Radius of the fan blade with run-out.

( )

NW

NNW

rmW

32.17

324.17105.120060

14002104

2

32

32

22

=

=×+×

××××=

=

−− π

ω

Centrifugal force considered in the vertical direction Refer to Figure 7 and also see Figure 5.

Fan CG from global center = (57.465 + 64) mm

= 121.465 mm

NR78.107

6032.174

×=

R4 = 9.64 N and R3 = 26.70 N.

Centrifugal force considered in the Horizontal direction Refer to Figure 5.

The reaction forces from the structure in the horizontal direction are equal in magnitude to the forces in the vertical direction, therefore

R3 = 26.70 N applied on the front end of motor in the horizontal plane.

R4 = 9.64 N applied on the back end of motor in the horizontal plane.

Loading due to the Pump Refer to Figure 1.

Weight of the pump = 1.497 x 9.81 N

= 14.4 N

Weight of pump acting on each screw hole.

we = W/4 = 14.4/4 = 3.6 N

Force on Pump (due to flow rate of the pump) Flow rate for the pump is given to be Q =10 liters / min. Head developed by the motor is 1m.

The force acting on the pump, at the location of the pump nozzle due to the flow rate is:

aQ

vaF

2

2

ρ

ρ

=

=

where

ρ = 1000 kg / m3. Density of the fluid (water) ( ) 2

2

5854.74

01.0mme −=

×πa =

Cross sectional area: calculated from the diameter of the discharge nozzle of the pump measured to be 10mm.

v = Flow rate or velocity of discharge.

sec/8778.260

1010 323

me −=

×

−Q = 10 liters / min. =

NF

Ne

eF

35.05854.7

8778.21000

=−

−×=Volume flow rate for the pump,

This force is resolved in the x and z directions (See Figure 8). Fx = 0.35 sin 45o = 0.25 N

Fz = 0.35 cos 45o = 0.25 N

Distance between the pump mounting point and pump discharge point hp =163 mm. (From Figure 8). Moments due to force components in x and z directions Mx = Mz = 0.25 x hp = 40.75 N-mm.

Figure 8 - Forces acting on the pump mounting points

Forces on pump mounting points due to Mz: Distance between pump mounting points dp = 134 mm (From Figure 8).

(P1' + P4

') x dp = 40.75 N-mm.

(P2' + P3

') x dp = 40.75 N-mm.

P1′ = P4

′ = 0.30/2 = 0.15 N.in -ve y direction

P2′ = P3

′ = 0.30/2 = 0.15 N in +ve y direction

Forces on pump mounting points due to Mx: Distance between pump mounting points dp = 134 mm (From Figure 8).

(P1" + P2

" ) x dp = 40.75 N-mm.

P1″= P2

″= 0.30/2 = 0.15 N. in -ve y direction

(P3" + P2

" ) x dp = 40.75 N-mm.

P3″= P4

″= 0.30/2 = 0.15 N in +ve y direction

Final forces on structure due to reaction force at nozzle: From Figure 5.

N 0.30 P3 P1N 0.0 P4 P2

P4" P4' P4P3" P3' P3P2" P2' P2P1" P1' P1

====

+=+=+=+=

Overall Forces acting on the Pump Mounting Points Overall, loads on holes due to weight of pump and reaction force (See Figures 5 and 8):

Load on each of the pump mounting points we = 3.6 N.

P2 = P4 = we + 0 = 3.6 N

P1 = we + 0.3 = 3.9 N

P3 = we – 0.3 = 3.3 N

Load Summary The magnitudes of forces for load case 1 and load case 2 are presented in Table 1 and Table 2 respectively.

Table 1. Forces for load case 1: Centrifugal force in horizontal direction.

Position Magnitude (N)

R1 18.15 Vertical Forces R2 30.08

R3 26.70 Horizontal Forces R4 9.64

P1 3.9

P2 3.6

P3 3.3

At Pump Mounting points

P4 3.6

Table 2. Forces for load case 2: Centrifugal force in vertical direction.

Position Magnitude (N)

R1 8.51 Vertical Forces R2 56.78

R3 0 Horizontal Forces R4 0

At Pump Mounting P1 3.9

P2 3.6

P3 3.3

points

P4 3.6

Analysis The static analysis of the support structure was carried out for the load cases mentioned above. The analysis was carried out for three different types of materials SEB200, FPT30PPC and FPT20PPC. Of these materials, SEB200 is the costliest and FPT20PPC is the cheapest. Based on the analysis of the existing model of the support structure, nine different configurations for the support structure were created. The challenge in this was to stiffen the structure as well as to reduce the weight of it in such a manner that manufacturing of the component in large quantity was possible. All the modified designs were analyzed for the worst load case simulation (Load Case 2) (See Figure 9).

Figure 9 - Modified finite element model of the Support Structure of Air Cooler

Analysis Results & Discussion

Displacement Results for Existing Design The analysis of the existing design yielded results as specified in Table 3. Maximum displacement in y-direction (UY) and the maximum total displacement (USUM) before modifying the design for the three types of materials and for two load cases are given in Table 3.

Table 3. UY and USUM for the two load cases.

Material SEB200 FPT30PPC FPT20PPC

UY (mm) USUM (mm) UY (mm) USUM (mm) UY (mm) USUM (mm)

Load case 1 -2.13 2.14 -2.53 2.53 -2.99 3.00

Load case 2 -2.15 2.17 -2.53 2.56 -2.98 3.01

The displacement values as obtained for SEB200 in the field was 2.05 mm for load case 1. This, when compared with analysis results, there was a deviation of 6 % on the higher side.

Stress Results for Existing Design The maximum equivalent stresses (SEQV) for the load cases 1 and 2 before modifying the design are presented in Table 4.

Table 4. SEQV for the two load cases.

Material

SEB200 FPT30PPC FPT20PPC

Stress (SEQV), Mpa

Allowable Stress 85.0 32.5 30.0

Load case 1 6.11 5.96 5.91

Load case 2 8.02 7.86 8.12

From the above table, it can be concluded that the stresses in the structure are well within the allowable limits.

Displacement and Stress Results for Modified Design Maximum displacement in y-direction (UY), total displacement (USUM) and equivalent stresses (SEQV) after modifying the design for the load case 2 are presented in Table 5.

Table 5 Uy, USUM , SEQV for modified design (load case 2).

Remarks Case No. UY

(mm)

USUM

(mm)

SEQV

(MPa)

Material: FPT 20 PPC

Design changes Thickness (mm)

1 2.703 2.725 6.693 1. Thickness of the main sheet body.

2. Local thickening at beam- body Junction

3. Created 5 cross-ribs on the L Section.

4. Created 4 cross-ribs on the flared part of the pump mounting beam.

5. Created 1 rib running along the L Section on the Horizontal plate.

6. Local thickening at fins for bolts.

2.5

5.5

2.5

2.5

2.5

5.5

2 1.953 1.953

2.876

Iteration run without the Fan-Motor Assembly load. (Only the load due to the pump was considered on the pump mounting beams).

3 2.494 2.515

6.449

A local thickening of the main sheet–body (Region between the pump mounting beam and the main sheet body, up-to the 3rd vertical rib).

4.0

4 2.268 2.306

6.469

The thickness of the pump supporting beams was increased from 2.5 to 4.0 mm

5 2.576 2.592

6.649

The vertical rib running along the L section was moved to the edge of the L section. This created a C-section type of a profile.

2.5

6 2.183 2.213

6.422

The thickness of the whole beam increased to 4 mm while the thickness of the ribs kept at 2.5 mm.

Material : SEB200

7 1.647 1.670

6.251

The thickness of the whole beam increased to 4 mm while the thickness of the cross-ribs kept at 2.5 mm.

Material : FPT20PPC

8 1.664 1.680 6.044

Material : SEB200

9 1.266 1.278 5.873

The model was changed to meet the requirements of manufacturer (See Figure 9).

The thickness of the ribs & the pump-mounting beam was kept at 3mm.

Figure 10 - Total displacement plot for the existing design for the material SEB200

Figure 11 - Maximum equivalent stress plot for the existing design for the material

Figure 12 – Total Displacement Plot for the existing design for the material FTP20PPC

Figure 13 - Maximum equivalent stress plot for the existing design for the material

Conclusion The modified design showed a reduction of 41 % and 44 % in the maximum displacements at the desired locations for the materials SEB200 and FPT20PPC respectively. Also, the stresses were significantly reduced by 26.70 % and 25.60 % for materials SEB200 and FPT20PPC respectively. The design modification also led to weight saving of 1.8 % as compared to the original design. It can be seen that the modified model is better than the original model both in-terms of displacement and stress values. Based on the finite element simulation the modified final design was found to be better than the existing design in terms of displacement and stress values, cost of the material used and reduction of the problem of vibration based failure of the support structure.

References 1. ANSYS Theory Manual.

2. J. E. Shigley, C. R. Mischke, Mechanical Engineering Design, McGraw-Hill, 1989, Singapore.