THE MINI AUTOCLAVE SYSTEM FOR AERONEUTICAL

MATERIALS RESEARCH LABORATORY

Mehmet Çakıcı

Control & Automation Engineer

THE OUTLINE OF THE WHITE PAPER

1. Mechanical Design of the system

2. Heat calculation for warming up

3. Heat loss calculation of natural convection

4. Heater and Controller selection

5. Real Photos of the System

1) MECHANICAL DESIGN OF THE SYSTEM

The mechanical design and analyses of the mini autoclave system is made with

SolidWorks design program. The material of the main and the top components of the system are

selected as 316 AISI Stainless Steel, which have 8238 kg/m3 as density and 468 J/kg.Ko as

specific heat. The material selection is made with respect to phases of the production process.

The production process includes 0.7 mPa pressure and 180oC temperature. The mechanical

design must be durable to these pressure and temperature values. The dimensions of main

component as shown in Figure 1. The mechanical design of main and top components as shown

in Figure 2. The thickness of the top component is 20 mm and the other dimensions are same

with the main component.

Figure 1: The Main Component Dimensions in mm

Figure 2: Main and Top Components Design

The working principle of the system is depends on increasing temperature, stable pressure

and vacuum. The top and the main components connection is made with 16 tie rod. The air

leaking problem under pressure and vacuum is solved with the custom-produced leak proof

gasket, which is a type of O-Rings. This leak proof gasket is compatible with the channel, which

is located on the upside of the main component. When the top and the main components is

connected with tie rods, the leak proof gasket cannot leaks the inside pressure or vacuum. The

leak proof gasket location is shown in figure 3.

Figure 3: The Leak Proof Gasket (Grey Part)

2) HEAT CALCULATION OF WARMING UP PHASE

The major phase of the production process is warming up of the whole Stainless Steel

based parts from room temperature to 180oC with the 2oC/min steps. The temperature of the

system must be increase two degree in one minutes because of the procedure of the production

process. The whole mini autoclave system is shown in figure 4.

Figure 4: The Mini Autoclave System

The necessary energy of the system for warming up with 2oC/min is calculated in below.

The total weight of the system is 67.477 kg, because of the safety factor, the calculations are

made with 75 kg weight.

Cp = 468 J/kg.C M = 75 kg T = 2oC

In order to increase two degree; Q = M*Cp*T

Q = 75*468*2

Q = 70200 Joule Energy in minutes

The energy needs in second; Watt = Q/60

Watt = 1170 Watt Energy in second

The 1170 watt energy per second is the heat that the system need to take from heater for

warming up in ideal conditions.

3) HEAT LOSS CALCULATION OF NATURAL CONVECTION

The production process temperature ranges from 25oC to 180oC. The maximum

temperature is 180oC. It is known that when the system is warming up, the heat loss of the system

is increasing step by step because of the natural convection. The plate heater needs to compensate

this heat loss to the environment. The natural convection heat loss is shown in figure 5.

Figure 5: The Natural Convection of the Mini Autoclave

If the controller of heater can compensate this natural convection heat loss at 180oC, the

system can works at every temperature below 180oC. Because, it is know that the heat lost at

180oC is every time much more than below temperatures. The heater of the system have to give

1170 watt and natural convection heat loss energy in order to increase temperature with 2oC/min.

The natural convections heat loss at 25oC and 180oC are calculated as below. The result

shows that the natural convection at 180oC is much more than 25oC.

Natural Convection at 180oC:

Lc = As / p As = Surface Area = (38*9)*4 + (38*38) = 2812 cm2 = 0.2812 m2

P = Perimeter = 38*4 = 152 cm = 1.52 m

Lc = 0.185 m

Upper surface hot plate; RaL : 107 – 1011 Nu = 0.1*(RaL)(1/3)

Stainless Steel AISI 316: @300K ρ = 8238 kg/m3

k = 13.4 W/mK

Cp = 468 J/kgK

α = 3.48*10-6 m2/s

Tf = (Ts + T ͚ ) / 2 = 102.5 oC = 375.5 K Ts = 180 oC and T ͚ = 25 oC

@102.5 oC and 1atm Air ; k = 0.031125 W/mK

ʋ = 2.333*10-5 m2/s

Pr = 0.7106

β = 1 / Tf

RaL = [g* β*( Ts - T ͚ )*(Lc)3*Pr] / ʋ2 = 3.34734*107 (g = gravitational acceleration = 9.81 m2/s)

Nu = 32.2279

h = k*Nu / Lc = 5.422 W/m2K

Q = h*As*( Ts - T ͚ ) = 236.323 Watt Heat loss at 180oC

Natural Convection at 25oC:

Lc = As / p As = Surface Area = (38*9)*4 + (38*38) = 2812 cm2 = 0.2812 m2

P = Perimeter = 38*4 = 152 cm = 1.52 m

Lc = 0.185 m

Upper surface hot plate; RaL : 104 – 107 Nu = 0.59*(RaL)(1/4)

Stainless Steel AISI 316: @300K ρ = 8238 kg/m3

k = 13.4 W/mK

Cp = 468 J/kgK

α = 3.48*10-6 m2/s

Tf = (Ts + T ͚ ) / 2 = 25.5 oC = 298.5 K Ts = 26 oC and T ͚ = 25 oC

@102.5 oC and 1atm Air ; k = 0.025547 W/mK

ʋ = 1.5666*10-5 m2/s

Pr = 0.72946

β = 1 / Tf

RaL = [g* β*( Ts - T ͚ )*(Lc)3*Pr] / ʋ2 = 6.18478*105 (g = gravitational acceleration = 9.81 m2/s)

Nu = 16.5445

h = k*Nu / Lc = 2.2848 W/m2K

Q = h*As*( Ts - T ͚ ) = 0.6424 Watt Heat loss at 25oC

4) HEATER AND CONTROLLER SELECTION

The system needs to 1170 watt energy per second for 2oC warming up as it calculated.

The natural convection heat loss is calculated 237 watt energy per second. It can said that the

plate heater of the mini autoclave system have to give minimum 1170 + 237 = 1407 watt energy

per second to the stainless steel parts. The natural convection heat loss at every temperature from

25oC to 180oC is accepted 237 watt as safety factor. In addition, the other safety factor is

increasing 20 percent of the 1407 watt energy to the 1688 watt. The desired plate heater have to

give minimum 1688 watt heat energy per second to the mini autoclave system. If the plate heater

can give minimum 1688 watt heat energy to the system, the PID temperature controller can easily

control the system temperature as required.

The PID temperature controller is selected as ORDEL Temperature Control Products –

AC491 model. This controller takes feedback with thermocouple which is located inside of

autoclave system. The thermocouple is selected as ORDER Thermocouple which is directly

compatible with PID temperature controller. The diagram of the temperature control system is

shown in the figure 6.

Figure 6: Temperature Control Diagram

Temperature

Controller

Plate

Heater

The Mini

Autoclave

Thermocouple

Referance

Temperature

The thermocouple is located on lateral surface of the main component of the system as

shown in figure 7. The thermocouple is custom produced according to main component. The

shape of the thermocouple is screw type. The mechanical properties of screw of thermocouple is

directly suitable with the properties of main component. The thermocouple and the main

component are connected to the each other with using Loctite leak proof liquid. This liquid make

the mechanical connections leak proof.

Figure 7: Thermocouple of the system

5) REAL PHOTOS OF THE SYSTEM