Development of CO₂ Gassing System for Optimized use of Gas in Mould Hardening of Steel Castings

Development of CO₂ Gassing System for Optimized use of Gas in Mould Hardening of Steel Castings

Prasad Thakare 1,

Department Mechanical Engineering Department, Smt Kashibai Navale College of Engineering, Pune

Shubham Salunkhe3,

Department Mechanical Engineering Department, Smt Kashibai Navale College of Engineering, Pune3

Rohit Shelar2,

Department Mechanical Engineering Department, Smt Kashibai Navale College of Engineering, Pune

Nitin P. Sherje

Department Mechanical Engineering Department, Smt Kashibai Navale College of Engineering, Pune

Abstract – In CO2 mould hardening, once mould cavity is produced, CO2 gas is passed through it which reacts with sodium silicate to make the mould harder. CO2 moulding process due to its ability to produce harder moulds is widely used for casting variety of metals and especially high density alloys like steels. The study is focused on development of a system for optimization of CO2 consumption & process time. In previous practice, the CO2 from CO2 cylinder was directly passed into mould at lower temperature than desired & higher pressure due to uncontrolled throttling through air gun which resulted in loss of CO2, non-uniform mould strength & more process time. To overcome these problems, a gassing system has been developed which reduces CO2 consumption as well as process time. Also, an automation of the system is proposed to time the gas flow in more efficient manner.

Keywords: Gassing system, CO2 moulding, template, CO2 Consumption,CO2 properties, flow rate, gassing time.

1. INTRODUCTION

   In CO2 moulding process, CO2 reacts with the binder i.e. sodium silicate and mould becomes hard. However, the method of passing CO2 into the mould plays an important role in optimizing this process. The sponsoring company is a manufacturer of Steel Castings (viz. valve bodies). Silica sand moulds are mainly used in the company. Prevalent practice in the company was to pass the CO2 into the mould directly from CO2 cylinders through small hoses. The holes were produced arbitrarily in the mould and CO2 was passed through each hole using air gun. As a result there was no uniform distribution of CO2 into the mould and the gassing time totally depended on operators skill/judgement. Also CO2 losses due to leakage were present. The reaction between sodium silicate and CO2 is as follows:

   hazards associated with handling pressurized CO2 cylinders.

2. LAYOUT DESIGN

   Fig 2.1 Layout of System

   Fig 2.2 Distribution System

   In conventional practice, CO2 from CO2 cylinder was directly passed through hose pipe into the mould. The cylinder pressure being very high (50-60 kg/cm2) and if throttled into mould through air gun causing CO2 temperature to drop below room temperature due to Joule Thompson effect. But the Reaction best occurs at room temperature. So to control the parameters such as pressure, temperature etc as per requirement of reaction, above conditioning system is developed [1]. The high pressure CO2 from cylinders is passed through copper hoses to the

   Na O.xSiO nH O CO2 Na CO .xSiO

   nH O

   CO2 manifold. Then the gas is passed through throttle valve

   2 2 2

   Objectives:

   2 3 2. 2

   followed by flow control valve. The gas pressure is reduced to desired value (1-3 kg/cm2) after throttling. The

   Developing a controlled gassing system primarily to reduce

   CO2 consumption and subsequently achieve uniform mould strength through uniform distribution, to optimize both time and cost required for the process. Also eliminate the

   temperature of gas drops down to -400 C after throttling. But the reaction of CO2 and sodium silicate is favorable at room temperature. So a heat exchanger is employed to increase the temperature. After that a receiver is used to store the CO2. After conditioning of CO2,task is to

   distribute it uniformly into mould cavity, a distribution system is developed for which template is developed. Five anchor bolts of 8 mm at suitable positions are fixed and

   Now to determine the fin spacing, assuming length of tube = 2.5 m= 98.425 inch

   they are connected by CO2 carrying pipe. The time for

   Therefore, Fin spacing No.of

   fins

   which CO2 gas should be passed (Gassing time) in a standard ,frequently used job is also calculated to optimize CO2 consumption. An automatic control is proposed to

   inches

   make the process semi-automatic.

   Actual tubelength

   Calculated fin spacing

   assumedlength

3. DESIGN OF HEAT EXCHANGER (EVAPORATOR) The heat extracted (Q) in heat exchanger is given by

Q h.A.t

After throttling the pressure drops down from 60 kg/cm2 to 10 kg/cm2 and it is associated with temperature drop from 25oC (room temp.) to -40oC.

The Log mean temp difference (LMTD) is calculated as:

Actual fin spacing available

Design of CO2 Gas Receiver:

Selecting volume as per the requirement & design pressure (Pi) is already known.

Assuming length of cylinder reservoir= l =2*Di Where Di is internal diameter,

LMTD

dt1 dt2

ln(dt1/ dt2)

Now, = 32

For Natural Convection,

For the air velocity range of 0-20m/s, heat transfer coefficient (h) = 40 W/m2K

From psychometric chart, specific volume at 10 bar =

0.038 m3/kg

Where, V is the volume of cylinder.

Let material be IS: 2062 GRADE A Structural Steel for Fabrication,

Sut = 410 N/mm2, FOS = 5

Preferred flow rate of CO2

=1ft3/min

, = PiDi

2t

. Pr eferred flowrate Mass flow rate (m)

Design of Hose pipe at the outlet of CO2 cylinder:

Specific volume

Material selected Copper

Heat transfer area, A

Q

h t

Design pressure= 1.5×Pw

Type of tube selected – Transverse finned Tube Material selected – Copper

The maximum working pressure is 10 kg/cm2. Hence for safety, the system is designed to a pressure of 15 kg/cm2.

By the use of material and working pressure copper tube is selected from chart.

Let fin O.D = 24mm

Using above data, hose pipe is selected from chart. Design of Manifold collecting CO2 gas from cylinders: Material selected Brass

Design pressure= 1.5×Pw

O.D is selected from chart. Length required = 3 m (on 2 sides together)

Design of throttle valve & throttle valve line:

Selection based on the manufacturers catalogue complying with inlet pressure (range) & outlet pressure.

Fin surfacearea Af / 4( fin dia. copper tubedia. ) 2

2 2

No.of

fins required A

Af

DESIGN SPECIFICATIONS

Development & Testing of Gassing System:

Based on above specifications conditioning system is developed as below. The conditioning system contains throttle valve, flow control valve, pressure gauge, heat exchanger, receiver, outlet throttle valve etc. And for uniform distribution the template is designed as shown below.

Figure 2: Distribution Template

The amount of CO2 required to harden the mould of steel castings per month in prevalent system have been measured for the months of Nov 15, Dec 15 and Jan 16. Similar tests are carried for the modified system also in the months of Feb 16, March 16. Also during the test, overall time required for the operation was measured for both prevalent and modified system and compared.

Gassing Time for each template:

We know sand density and density of steel castings. Also, as per the conventional practice recommended by Indian Foundry, 1 kg of sodium silicate is required for 20 kg of sand and 1 ft3 CO2 gas is required per pound of sodium silicate. So CO2 required per kg of sand can be found out. Then by calculating mass of sand in the mould (in drag), CO2 required for considered mould can be find out.

For calculating mass of sand in mould,

Mould volume(drag) Length Breadth Depth Volume of sand in drag Mould volume Jobvolume Massof sand in drag Volume of sand sand density So, amount of CO2 required can be found.

For gassing time calculation,

AverageCO2 flowrate per template

Total flowrate for 20 20 mould 4 Avg. flowrate for 10 10template

Total gas sin g time for entire mould (T ) Amount of CO2 required for considered mould

Total flowrate for considered mould

CO2

gas sin g time per template T

4

RESULT & DISCUSSION

Figure 1: Developed Conditioning System

1. CO2 Consumption:

   CO2/ melting Vs Month

   CO2/ melting Vs Month

   0.08

   0.06

   0.08

   0.06

   0.055630.056390.05959

   0.055630.056390.05959

   0.047520.04643

   0.047520.04643

   0.04

   0.02

   0

   0.04

   0.02

   0

   Nov

   Dec Jan Feb

   March

   Nov

   Dec Jan Feb

   March

   month

   month

2. Gassing time required (per drag)

   1. Prevalent practice: 4 min 2 sec

   2. Improved method: 2 min 52 sec

CONCLUSION:

With the implementation of new system, CO2 consumption is reduced by about 18% and thereby reduced cost of CO2. It is observed that the average monthly savings of CO2 cost is Rs.11, 966/- which is significant and the payback period is found to be 6.5 months. In addition to that the use of template has shown significant reduction in gassing time and uniform distribution of CO2. The uniform strength of mould can be achieved by using the standardized template for the CO2 distribution as designed and implemented in this work. As CO2 gas always passes through manifolds under controlled conditions, hazards associated with handling CO2 as well as its leakage is prevented. Also, the improved system has been found to be highly cost effective as indicated by its simple payback period. As gassing time is calculated, arbitrary consumption of CO2 is avoided which in turn reduces the gassing time. Thus the gassing process is systematically optimized.

REFERENCES

1. Dr. M.Venkata Ramana, Modelling of CO2 moulding process, Global Journal of Advanced Engineering Technologies [2014]

2. J. Zych, Pulsating gas dosage in the moulding sands hardening process in the cold-box technology, Archives of Metallurgy and Materials [2013]

3. M.B.Parappagoudar, D.K.Pratihar, G.L.Datta., Modelling and analysis of sodium silicate bonded moulding sand system using design of experiments and surface response methodology, Journal of Manufacturing Science and Production

4. M Venkata Ramana, Modelling of the properties of sand mould made of reclaimed sand, International. Journal of Engineering Research and Applications, Vol. 4, Issue 12(Part 6), December 2014

5. M.VenkataRamana, Modelling of process parameters of silicate bonded CO2 mould made of reclaimed CO2 sand using artificial neural networks [2015], Vol. No. 10, Issue No. II, August

6. M Venkata Ramana, Optimization of process parameters of CO2 moulding process for better knockout property, International Journal of Advanced Technology in Engineering and Science [2014], Volume No.02, Issue No. 12