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Interview Q/A

Q1 what is critical radius of sphere and how does it vary? 

▶️ the radius at which the heat transfer occurred is maximum is critical radius. For sphere, it is equal to 2k/h. Heat transfer is minimum when there is no insulation. As we keep on increasing the insulation, the rate heat transfer keeps on increasing. This is because convective heat transfer is dominant over conductive heat transfer at this stage. Finally the rate of heat transfer reaches it’s maximum value when the radius of insulation is equal to the critical radius. If we keep on adding insulation after this, the rate of heat transfer will start decreasing.


Q2 How does a refrigerator work?

▶️ in a refrigerator, an evaporating gas called a refrigerant draws heat away, leaving the surrounding area much colder. Refrigerators and air conditioners both work on the principle of cooling through evaporation. A refrigerator consists of two storage compartments – one for frozen items and the other for items requiring refrigeration but not freezing. These compartments are surrounded by a series of heat-exchanging pipes. Near the bottom of the refrigerator, the unit is a heavy metal device called a compressor. The compressor is powered by an electric motor. More heat-exchanging pipes are coiled behind the refrigerator. Running through the entire system is pure ammonia, which evaporates at -27 degrees Fahrenheit (-32 Celsius). This system is closed, which means nothing is lost or added while it is operating. Because liquid ammonia is a powerful chemical, a leaking refrigerator should be repaired or replaced immediately. The refrigeration process begins with the compressor. Ammonia gas is compressed until it becomes very hot from the increased pressure. This heated gas flows through the coils behind the refrigerator, which allow excess heat to be released into the surrounding air. This is why users sometimes feel warm air circulating around the fridge. Eventually the ammonia cools down to the point where it becomes a liquid. This liquid form of ammonia is then forced through a device called an expansion valve. Essentially, the expansion valve has such a small opening that the liquid ammonia is turned into a very cold, fast-moving mist, evaporating as it travels through the coils in the freezer. Since this evaporation occurs at -27 degrees F (-32 degrees Celsius), the ammonia draws heat from the surrounding area.

This is the Second Law of Thermodynamics in effect. Cold material, such as the evaporating ammonia gas, tend to take heat from warmer materials, such as the water in the ice cube tray.

As the evaporating ammonia gas absorbs more heat, its temperature rises. Coils surrounding the lower refrigerator compartment are not as compact. The cool ammonia still draws heat from the warmer objects in the fridge, but not as much as the freezer section. The ammonia gas is drawn back into the compressor, where the entire cycle of pressurization, cooling and evaporation begins anew.


Q3 what is cavitation and what are the ways to eliminate it? 

▶️ it happens in suction lift. Due to low pressure in suction pipe, the boiling point of water decreases. This causes the water to boil at the temperature at which the process is happening. Due to this, vapours are formed resulting in formation of bubbles in impeller. When these bubbles collapse on the surface of pump, there is a jerk and this causes a cavity on the surface. This is cavitation. 

To avoid cavitation, the pressure at the pump inlet must exceed the vapour pressure by a certain value called npsh. This can be increased by increasing the height of suction side.

Another way to eliminate cavitation is to reduce the temperature of the pump and liquid. This will decrease the vapour pressure of liquid.


Q4 what is lmtd?

▶️ the log mean temperature difference is an equivalent temperature difference or an indicator of average temperature difference between the hot and cold fluids in a heat exchanger.


Q5 how will you calculate lmtd when you are given entrance and exit temperature of hot fluid and entrance temperature of cold fluid?

▶️since the heat released by the hot fluid is equal to the heat gained by the cold fluid, the outlet temperature of the cold fluid can be calculated by using the formula Q=m.Cp.∆T 

After calculating the outlet temperature of cold fluid, we can determine the lmtd by using the formula 

Lmtd= (θ1-θ2) / ln(θ1/θ2)

Where θ1 and θ2 are the differences between temperatures at inlet and outlet respectively


Q6 what are the assumptions of Bernoulli equation?

▶️ 1. The flow should be steady

2. The flow should be ideal

3. The flow should be irrotational


Q7 what is meant by utility?

▶️utility refers to piped services such as water, nitrogen, compressed air, LP, HP,MP(low pressure, high pressure, medium pressure), steam etc that are needed to facilitate the the process.


Q8 what do know about UOP?

▶️ Universal Oil Products is a multi-national company developing and delivering technology to the petroleum refining, gas processing, petrochemical production, and major manufacturing industries. Moreover, UOP is the leading international supplier and technology licensor in these fields. UOP provides reliable technical service and support, with expertise in plant operations, asset management, customized training, strategic and tactical support, troubleshooting, and engineering services. UOP’s worldwide sales, support and manufacturing facilities includes locations in the Americas, Asia, Europe and the Middle East. Moreover, Honeywell is ranked 77th in the list of fortune 500 companies.


9) Types of Convection. Explain them. 

▶️~Natural :- a type of flow, of motion of a liquid such as water or a gas such as air, in which the fluid motion is not generated by any external source (like a pump, fan, suction device, etc.) 

~Forced:- a type of flow in which fluid motion is generated by an external source (like a pump, fan, suction device, etc.). 


10) Give a relation for Natural Convection. 

▶️~Grashof number, Gr:- It is the ratio b/w buoyancy forces and viscous forces. 

Gr= gβ∆TL^3/ν^2

Where β is the thermal expansion coefficient, ∆T is the temperature difference, L is the characteristic length and v is the kinematic viscosity.

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