Overview:

For more specifications please download the FreeCad File:

LOX FreeCad Design (.FCStd)

Cooler Design:

The cooler is presented by a tunned refrigerator by adding seven cooling tubes inside it.

For more information check this report:

310821_ICPT_LOXPrototype(.docx)

• Calculation pressure of pipes

Equation: P = (2*S*T)/((O.D.-2*T)*S_F)

Where:

   P = Fluid Pressure (psi)

           T = Pipe wall thickness (in)

           O.D. = Pipe outside diameter (in)

           S_f = Safety factor (General Calculations 1.5 – 10, Use 1 For Bursting Pressure)

           S = Material strength (psi)

Ultimate Tensile strength or Yield strength can be used.

Ultimate should be used to determine the bursting pressure.

Yield can be used for estimating pressures at which permanent deformation begins.

1Mpa=144.55psi

In our case, We have 2 types of pipes: one made of aluminum (Al) and the other of copper (Cu). In the table below, information about these two types of pipes

Aluminum Pipe Equations Formulas Design Calculator [1]

[1] https://www.ajdesigner.com/phpaluminumpipe/pressure_rating_equation.php#ajscroll

Solving for pressure rating

Note:
 Under most cases, S = 7500 pound/inch2 for aluminum

Our technical calculators can calculate weight, working pressures, copper tube thickness and wall diameter requirements.[2]

[2] https://lawtontubes.co.uk/technical-calculators/pressure-calculators/

• Prototype design

Initially, it was suggested to replace the main cycle (open oxygen cycle for liquefaction of oxygen) with a closed cryogenic cycle running on nitrogen gas for liquefaction of oxygen. This is due to the increased cost of the oil-free oxygen compressor, but it turned out later that nitrogen gas also needs an oil-free compressor, and for this reason we decided to return to the basic suggestion attached below.

In our prototype, we decided to dispense with the heat exchanger in order to avoid expensive materials and manufacturing costs. However, through the theorical study, it was found that we will face a problem in reaching the required liquefaction temperature, in addition to the compressor failure due the low gas temperature at the compressor inlet.

Therefore, the following was decided:

• Cooling component

In our system, the “Kelvinator” refrigerator has been adopted as a condenser for the compressor outlet. The second refrigeration cycle in the Kelvinator works with refrigerant R-503. This refrigeration cycle needs to be filled with refrigerant R-503. Due to its unavailability in the market, it was replaced with refrigerant R-508b, due to its compatibility with compressor oil.

 13062022_ LOx prototype _ Mechanical design (docx)

A. Liquefaction of Oxygen _ Prototype 

1) Overview _ Flow chart

LOX prototype flow chart by EDraw

2) Pipe sizing

3) Lox cycle calculation

Calculation of oxygen liquefaction cycle

4) Yield factor

Yield: Y = mf• / m• = h₁-h₂ / h₁-h_f

Where Point 1: before compressor (inlet)

          Point 2: after compressor (outlet)

Y = (h1-h2)/(h1-hf)= (243.34-238.702)/(243.34-(-133.58)) = 0.012305 ≈ 1.231%

Y = m_f• / m• ⇒ m_f• = Y × m• = 0.012305 × 0.001243721_Kg/s = 1.53039869×10⁻⁵ Kg/s

m_f• = 1.53039869×10⁻⁵ _Kg/s × 3600 = 0.0550944 Kg/hr

mass flow/density = m_f•/ D  ; where density D of liquid = 1141.8 Kg/m³

= 0.0550944_Kg/hr/1141.8_Kg/m³ = 4.8252192×10⁻⁵ m³/hr = 0.048252192 L/hr = 48.252192 mL/hr

B. Heat exchanger

1. Type of heat exchanger

Types of heat exchanger used in cryogenic systems

We chose helical coil heat exchanger for many features

a. Shape of heat exchanger

Fig. 1 – Schema of a shell and helical tube heat exchanger [2]

[2] S. Bahrehmand, A. Abbassi, 2016. Heat transfer and performance analysis of nanofluid flow in helically coiled tube heat exchangers. Chemical engineering research and design 109 (2016), 628–637.

b. Characteristics of helical coil and shell

The copper pipe used has 9.62 mm outer diameter (O.D.) and 1.2 mm thickness. The coil pitch and the number of turns will be calculated in paragraph 2.c. The schema of the heat exchanger is shown in figure below. The shell inner diameter, outer diameter and height are 60 mm, 140 mm and 2.5 m, respectively.

c. Boundary condition

As can be seen in Fig. 1, hot fluid (Oxygen gas) at the specific temperature of -80 °C (193 K) with pressure 50 bar and mass flow rate inlet boundary condition enters the helical coil at the top and leaves at the bottom. Cold fluid (Oxygen gas) at a temperature of -183 °C (90 K) with 1 bar pressure and mass flow rate inlet boundary condition enters the shell at the bottom and leaves at the top.

Equal values of mass flow rate were specified for shell-side and coil-side fluids.

d. Performance analysis of the heat exchanger

Heat transfer enhancement was experimentally investigated by by Jamshidi et al.(2013) [3]. It was observed that the increase in coil diameter, coil pitch and mass flow rate in shell and tube can enhance the heat transfer rate.

It is also seen that the increase in tube diameter and coil diameter enhances the effectiveness because the heat transfer area increases.

Fig. 2: Variations of performance index vs. mass flow rate based on various parameters [3].

Furthermore, it can be observed from Fig. 2 that with the increase in tube diameter the performance index enhances remarkably. The reason can be attributed to the significant decrease in pressure drop, the increase in heat transfer area and enhanced secondary flow.

The heat transfer rate enhances with coil diameter due to increased heat transfer area and the pressure drop increases with coil diameter because of increased length of the tube.

The effect of coil diameter on pressure drop is more intensive than that of heat transfer rate; consequently, the performance index decreases with an increase in the coil diameter.

For all cases, the optimum value of mass flow rate corresponding to maximum performance index is found to be 0.1 kg/s.

[3] Jamshidi, N., Farhadi, M., Ganji, D.D., Sedighi, K., 2013. Experimental analysis of heat transfer enhancement in shell and helical tube heat exchangers. Appl. Thermal Eng. 51,644–652.

e. Advantage of Helical Coil Heat Exchanger

Helical coil heat exchanger has many benefits that make it a good choice:

• Highly efficient use of space, especially when it’s limited and not enough straight pipe can be laid.
• Under conditions of low flowrates, such that that the typical shell-and-tube exchangers have low heat-transfer coefficients and becoming uneconomical.
• When there is low pressure in one of the fluids.
• When one of the fluids has components in multiple phases (solids, liquids, and gases), which tends to create mechanical problems during operations, such as plugging of small-diameter tubes. Cleaning of helical coils for these multiple-phase fluids can prove to be more difficult than its shell and tube counterpart; however, the helical coil unit would require cleaning less often.

2. What need to consider when design Helical Coil Heat Exchanger

a. Materials

When designing the helical coil heat exchanger, the first thing you need to consider is what material you should use. Copper tube and Stainless-Steel tube are two most common choices. Copper tube have relatively higher heat exchange rate, because copper tube is softer. Stainless steel tube doesn't react with water, which make it last longer, especially when one of the heats transferring fluid is water.

In our case, we will use the copper tube, because stainless steel tubes are not easily available in Lebanon.

b. Design of heat exchanger

c. HX FreeCAD design

Helical coil heat exchanger freeCAD design (v0.17)