Designing A Vaccine Refrigerator For Worldwide Use: Principles And Assumptions

To protect population worldwide from harmful diseases vaccines are used. For making vaccines effective they should be stored in temperature limits 2^oC to 8^oC before use. This is very critical issue worldwide for developing and hot countries, where vaccines loose their effectiveness before use.

This task is to design a refrigerator as an engineer which can be used to store vaccines. Refrigerator capacity should be at least 10 liters and average temperature of refrigerator should be maintained at 5^oC. By using principles of thermodynamics and ideal refrigeration cycles establish own computed set of assumptions (power source, operating temperature range etc.) and test criteria/validation.    

In previous time, there are few designs made those are available on internet resources. One of design was made on concept of solar power. Capacity of refrigerator they decided was 4liters. But main thing I noticed in their design concept is cost they invested for manufacturing. Majority of designer suggested using solar power for small refrigerators. But these refrigerators are useful in nation like Africa. (Aslam, McPhail, McPhee, Rowaland, 2009)

Our main goal to make this design concept is make a refrigerator which should be useful worldwide. This concept is working on 12volts electric power compressor. Hence at any place in any condition this design concept is valid.

In this design we used vapor compression refrigeration system. This system is used in our household refrigerator. Components used to design this refrigerator are easily available in market. So in this design no need is required to manufacture any special kind of components.

Design is made by assuming ambient temperature equal to 22^oC. 200 doses of vaccines are assumed to be stored in refrigerator. From 10 liter capacity of evaporator heat has to be removed equals to 64.3KJ.

Power is supplied to the system through compressor. 40W compressor is used which is driven from 12V electric supply. Compressor is used to increase the pressure of working fluid from 2.9bar to 6.8bar so as to achieve the required refrigeration effect.

Evaporator dimensions of design are 270mm*193mm*193mm. Volume of evaporator is near about requirement of design which was 10liters. Temperature of evaporator is tried to design at 2^oC without considering any losses. Material of evaporator pipes used is of copper material whose thermal conductivity is equal to 385 W/m K. Five turn of copper tube of outer diameter 4mm with wall thickness 0.4mm is used at side of 270mm*193mm (width) of evaporator.

Design requirements

Material of condenser pipes used is of copper material whose thermal conductivity is equal to 385 W/m K. Six turn of copper tube of outer diameter 10mm with wall thickness 1mm is used at side of 270mm*193mm (height) of evaporator.

Expansion valve used is made up of copper material with thermal conductivity of 385 W/mk. Volume expansion used in expansion valve is 2.1359 times to inlet volume so as to reduce pressure of working fluid from 6.8 bar to 2.9 bar. Hence temperature of working fluid will be reduced from 299 K to 273 K. Diameter of inlet side of expansion valve is 4.822mm and outlet side of expansion valve is 3.3mm.

Working fluid used in system is R134a refrigerant. Refrigerant is selected because it is eco friendly and capable to produced desired refrigeration effect so as to temperature of evaporator should be maintain at 2^oC to 8^oC.

Theoretical C.O.P of system by using vapor compression cycle is around 8. This C.O.P is based on theoretical calculation of design. If this design is build successfully then round about results will be achieved.

Vaccine,

Assuming density of vaccine equals to water,

Number of doses to be store = 200 = 2liter

Mass of vaccines =

Air,

Density of air,

Mass of air=

Ambient temperature= 22^oC =295K

Evaporator temperature = 2^oC= 275K

Heat generation =

Dimensions of evaporator assumed to store 200 doses of vaccines= 270mm *193mm *193mm

Evaporator pipe size available in market

Material = Copper

Thermal conductivity, =

Outer radius,  =2mm, Inner radius, = 1.6mm, Thickness, = 0.2mm

Length of pipe in evaporator, = 270mm

Required internal temperature of pipe (  to maintain evaporator temperature equals to 275K by removing  of heat. Initial temperature of evaporator (  equals to ambient temperature= 295K

Internal temperature of copper pipe should be maintained at 273.03K.

Refrigerant used= R134a

From data available in the study books and above calculations temperature range decided for the design is in between 273K at evaporator side and 299K at condenser side. Condenser work is to change the phase of high pressure refrigerant by losing its latent heat to air. Assumed air temperature is 295K. Hence temperature at inlet of condenser is decided equals to 299K so as to it can be capable to release its heat with required pressure.

Theoretical working of compressor is assumed as adiabatically. Hence no change in entropy occurs in between compressor operation.

Literature survey

100% vapors are assumed to be released after evaporator. Hence at end of compressor superheated vapors with 303K temperature is released.

Data taken from steam table of R134a-: (Saturated refrigrent- 134a)

Evaporator side = 0^oC=273K

Condenser side= 26^oC=273K

Temperature (K)	Pressure (Mpa)	Enthalpy (kJ/kg)		Entropy (kJ/kg.K)
		Saturated liquid	Saturated vapor	Saturated liquid	Saturated vapor
273	0.29282	50.02	247.23	0.197	0.919
2999	0.6853	85.75	261.48	0.3208	0.9082

 Compressor outlet= 30^oC= 303K

Pressure (Mpa)	Temperature (K)	Enthalpy (kJ/kg)	Entropy (kJ/kg.K)
0.6853	303	265.37	0.919

Required refrigeration effect per unit mass flow rate

Initial assumption

Time taken to remove total heat = 1min.= 60sec

Heat transfer rate= 64.3 kJ/min

Mass flow rate of refrigerant required

Total work done by compressor = m* enthalpy difference b/w inlet and outlet point of compressor

But available power source is 40W pump working at 12volts input supply.( Sanheng refrigeration). Hence by reverse engineering

Mass flow rate of refrigerant required

Heat removal rate by refrigerant= mass flow rate * refrigeration effect

Time taken to remove total heat

Specific heat at constant pressure of refrigerant,

 Heat removal,Q=

Condenser pipe size available in market

Material = Copper

Thermal conductivity, =

Outer radius,  =5mm, Inner radius, = 4mm, Thickness, = 1mm

Major heat loss takes place due to phase change. Hence temperature inside condenser pipe, T_i= 299K

Ambient temperature, T_o= 293K

Length ,L required-:

After calculation, length required, L = 1662.78mm

Length of evaporator = 270mm

Number of turn required =

Input conditions of refrigerant

Pressure,  = 0.685MPa, Temperature, = 299K

Outlet condition

Pressure, = 0.29282, Temperature, = 273K

According to general fluid equation-:

At inlet of expansion valve-:

At outlet of expansion valve-:

Working fluid is same hence value of R is constant throughout. Mass flow rate is also same hence value of m is also constant. Variables are pressure, temperature and volume.

By dividing (1) and (2)-:

As length of both sides of expansion valve is same hence ratio of areas-:

From available pipes cross section and solidwork analysis of capillary decided are-:

Inlet side diameter= 3.3mm

Outlet side diameter= 4.822mm

Dryness fraction = 0.171468

Theoretical Co efficient of performance

Automatic initial mesh: On

Result resolution level: 3

Advanced narrow channel refinement: Off

Refinement in solid region: Off

Evaluation of minimum gap size: Automatic

Evaluation of minimum wall thickness: Automatic

Size

X min	-0.285 m
X max	0.287 m
Y min	-0.229 m
Y max	0.004 m
Z min	-0.004 m
Z max	0.004 m

2D plane flow	None
At X min	Default
At X max	Default
At Y min	Default
At Y max	Default
At Z min	Default
At Z max	Default

Heat conduction in solids: Off

Time dependent: Off

Gravitational effects: Off

Flow type: Laminar and turbulent

High Mach number flow: Off

Default roughness: 0 micrometer

Default wall conditions: Adiabatic wall

Thermodynamic parameters	Static Pressure: 101325 Pa Temperature: 293.2 K
Velocity parameters	Velocity vector Velocity in X direction: 0 m/s Velocity in Y direction: 0 m/s Velocity in Z direction: 0 m/s
Turbulence parameters	Turbulence intensity and length Intensity: 2.000 % Length: 5.417e-004 m

Water

Refrigerant R-134a

Fluid Subdomain 1

Thermodynamic Parameters	Static Pressure: 101325 Pa Temperature: 293.2 K
Velocity Parameters	Velocity in X direction: 0 m/s Velocity in Y direction: 0 m/s Velocity in Z direction: 0 m/s
Turbulence parameters type:	Turbulence intensity and length
Intensity	2.000 %
Length	5.417e-004 m
Flow type	Laminar and Turbulent
Default fluid type	Gas
Fluids
Faces	Face<2>
Coordinate system	Face Coordinate System
Reference axis	X

Environment Pressure 1

Type	Environment Pressure
Faces	Face <1>
Coordinate system	Face Coordinate System
Reference axis	X
Thermodynamic parameters	Environment pressure: 101325 Pa Temperature: 273.0 K
Turbulence parameters	Turbulence intensity and length Intensity: 2.000 % Length: 5.417e-004 m
Boundary layer parameters	Boundary layer type: Turbulent

 Outlet Mass Flow 1

Type	Outlet Mass Flow
Faces	Face <1>
Coordinate system	Face Coordinate System
Reference axis	X
Flow parameters	Flow vectors direction: Normal to face Mass flow rate normal to face: 1.250 kg/s

Type	Global Goal
Goal type	Static Pressure
Calculate	Minimum value
Coordinate system	Global coordinate system
Use in convergence	On

Type	Global Goal
Goal type	Total Pressure
Calculate	Minimum value
Coordinate system	Global coordinate system
Use in convergence	On

Type	Global Goal
Goal type	Temperature of Fluid
Calculate	Minimum value
Coordinate system	Global coordinate system
Use in convergence	On

Type	Global Goal
Goal type	Mass Flow Rate
Coordinate system	Global coordinate system
Use in convergence	On

Type	Global Goal
Goal type	Velocity
Calculate	Minimum value
Coordinate system	Global coordinate system
Use in convergence	On

Finish conditions	If one is satisfied
Maximum travels	4.000
Goals convergence	Analysis interval: 0.500

Save before refinement

On

Flow Freezing

Flow freezing strategy

Disabled

In the designed refrigerator vapor compression refrigeration cycle is used. Capacity of refrigerator is 10ltr. Designed refrigerator is easy to capable and portable because of its small size. Designed cycle can maintain temperature limits required to store vaccine those are in between 2^oC to 8^oC. Designed refrigerator can be used anywhere because compressor operates with electric supply of 12volts. Refrigerant is eco friendly that is R-134a because it doesn’t contain fluorocarbons. Limitation of designed refrigerator is quantity of doses stored. Doses can’t exceed more than 200. 

Conclusion

Eco friendly vapor compression refrigerator system is designed in these papers. R-134a is effective refrigerant while using in small refrigerators because of its lower critical temperature limits at more than atmospheric pressure. It is also eco friendly because of no fluorocarbon emission. Designed refrigerator system can be used anywhere because compressor of system runs on 12volts electric supply motor.
Required temperature limit can be maintained with this low energy input. In future to exceed the capacity of refrigerator we can achieve by altering dimensions of expansion valve. This will result increase in time taken to achieve refrigeration effect by using same compressor motor.

References

Dupont, “Thermodynamic properties of HFC-13a”,The miracles of science, Available:

https://www.chemours.com/Refrigerants/en_US/assets/downloads/h47752_hfc134a_thermo_prop_si.pdf (Retrieved March 11, 2017)

Josip, 2006, “Specific heat of R-134a”, Refrigration Engineer. Available:

https://www.refrigeration-engineer.com/forums/showthread.php?39894-Specific-heat-of-R134a (Retrived March 11, 2017)

“Saturated refrigrant 134a” Tempreature table, Available:

https://www.egr.msu.edu/classes/me417/somerton/R134a%20Tables.pdf (Retrived March 11, 2017)

Aslam, N. McPhail, K. McPhee, R. Rowaland, B. 2009, “ Vaccine Refrigrator for developing nations”, Engineering Design project, Michigan state university, Avaibale:

https://apptechdesign.org/wp-content/uploads/2009/05/finalfridgereport1.pdf (Retrived March 11, 2017) 

Sanheng refrigeration,” 12 Volt R134a Refrigerator Compressor”, Alibaba.com, Available:

https://www.alibaba.com/product-detail/12-Volt-R134a-Refrigerator-Compressor_60257551364.html (Retrieved March 11, 2017)