Design and ANALYSIS
There is a need to develop a device that can provide a minimum of 1 pint of water per hour. The process requires to design and
manufacture distiller's components and assemble them to a complete distiller machine. To do this , the following analysis should be considered:
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Required heat transfer energy, Qtotal
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Required power demand per hour
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Heater unit: conversion efficiency
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Heat loss of unit, outside surface temperature (safe touch)
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Surface temperature of heater if there is no water
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Water storage unit, dimensions and load steam flow rate
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Cooler materials: heat transfer specifications
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Cooler materials : durability/collosion
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Controller: heat load control
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Controller: heat cut off
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Material selection
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Bench mark process steps and time
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Inspection for quality
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1. REQUIRED HEAT TRANSFER ENERGY, Q TOTAL
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THERMAL ENERGY CALCULATIONS
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Q total = E1 +E2 E1 = Energy required to reach boiling point
E1= M (cp) (T2-T1)
= 1kg (4.2kj/kg/k) (100 – 15) = 357kj/kg
E2 = M (hg – hf) E2 = energy for vapor phase transformation
= 1kg (2675kj/kg – 418kj/kg) = 2257kj/kg
total = 2257kj/kg + 357 kj/kg = 2614kj/kg
Measured energy (ammeter) = 2808kj/kg (1hr/3600sec) (1000kj/kg) = 780 joules/sec or 780W
To calculate efficiency, efficiency = thermal energy/ electrical energy
= 2614kj/kg/2808kj/kg (100%) = 93.09
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2. REQUIRED POWER DEMAND PER HOUR
With Q total of 2614 kj/kg, determine the maximum power required per hour for the unit.
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2614kj/kg (1hr/3600sec) (1000kj/kg) = 726 joules/sec or 726 Watts.
With 726 W, can determine the maximum current draw required for,
120V, then P = IV , I = P/ V = 726 W/120V = 6.05 A
Therefore, the unit has electrical requirements as follows:
For 120 V, the unit use 6.05 A but it is rated 6.6 A, and 726 W but rated 800 Watts on the higher side.
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3. MATERIAL SELECTION
The material selected for the construction of the unit is stainless steel because:
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Good thermal conductivity
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Good machinability
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Ductility
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Resist corrosion
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Strength and stiffness
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4. Determine heater unit: conversion efficiency
The heater unit is predicted to have a heat energy efficiency of 95%. This means it is able to utilize 95% of the incoming heat energy to heat water to steam. The rest of the heat is lost through the lateral surfaces of the unit.
To determine the amount of energy utilized to heat water to steam,
Q utilized = Energy input * efficiency
Energy input (Q total) = 2614kj/kg or 726 Watts
Efficiency = 95% = 0.95
Q utilized = (726 W) (0.95) = 690 Watts
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5. Cooler materials: heat transfer specification
Once the steam enters the condenser, it travel through a long vapor path where heat transfer takes place ie cool air from the cooling fans causes the steam to condense.
The distillate is then re-routed to a take off pipe and collected into a reservoir. At this stage, volatile chemicals like chlorine, calcium are stacked at the top of the packing and are allowed to escape.
The condensing coil is made up of copper/aluminum and has a specially formed spiral innertube providing a long path for good heat transfer.
Condensing coil: heat transfer specification
The thermal conductivity of copper (401) is better than aluminum (238) and specific heat of copper is less than that of aluminum. So, copper is much recommended material for condensing coil because it transfer more heat than aluminum. One advantage of copper is that if there is leak, it can easily be fixed unlike aluminum. Some of its outstanding properties are:
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Good electrical conductivity
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Good thermal conductivity
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Corrosion resistance
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Easy to alloy
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Ductile
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Tough
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Easily joined
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Recyclable
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catalytic
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6.Determine the heat loss of the unit outside surface temperature (safe touch)
The transfer of heat energy is always from the higher temperature medium to the lower temperature one, and its stops when the two medium reach the same the same temperature. With the efficiency of 95%, the heater unit is expected to lose approximately 5% of the heat through lateral surfaces of the unit.
The amount of heat loss to the surface is the as the amount heat gained by the surrounding surfaces.
To determine the amount of heat lost to the outside surface:
Qin =2614kj/kg or 726 Watts Q utilized = (Energy input)( 5%)
Q utilized = (2614kj/kg)(0.05) = 1307kj/kg
To minimize amount of heat loss , the unit requires to be well insulated so that only a negligible amount of heat can pass through the surface.
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7.Controller: Heat cut-off
The distiller unit works on the principle of evaporation and condensation. An electric heating element (370 Watts) boils water in a stainless steel boiling chamber to 100 degrees which eventually turns to steam. When most of the water is boiled out of the boiling chamber, a float switch is recommended to turn off the power from the heating element.
This controls the minimum and maximum heat load in the boiling chamber because it shut off the element when it exceed. The controller also act as safety shut off switch which turns the unit off should it overheat. However, safety standards should be observed. It gives off heat and this factor should be considered when finding a place to place. It should be put on its own electrical circuit to prevent electrical overload.
Float switch is designed to mount through the wall of the boiling chamber (internally).
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