Heat Transfer through Evaporators

In summary, when trying to calculate heat transfer through an evaporator in a refrigerator, it is important to consider the complexities of the system, such as the refrigerant being in a mixture of saturated liquid and saturated gas. While Newton's Law of cooling can be used for a simplified calculation, results may not be accurate and it is recommended to use the heat convection equation or an energy rate equation for a more precise calculation. The energy rate equation, specifically, can be simplified to only require the flow rate of the refrigerant and the change in enthalpy, making it a convenient option for approximating heat transfer in evaporators.
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Dear all,

I am a bit stuck trying to find out how heat is transferred through an evaporator in a refrigerator. I was using Newton's law of cooling before I realized there are more complexities in the equation as the refrigerant is in a mixture of saturated liquid and saturated gas in the evaporator.

Newton's Law of cooling (heat transfer from a solid surface to a fluid):

QL=UL*AL*LMTD
where
QL= heat transfer
UL= overal heat transfer coefficient
AL= external surface area of the evaporator
LMTD= log mean temp different over the evaporator tube. i.e. The average temperature difference between the solid surface and the fluid (air in this case) across the evaporator inlet and outlet.

My Question

Is Newton's Law of cooling applied to refrigerator evaporators and/or condensers with this equation, for a simplified calculation? Or would the results be so far from correct that calculations must always take into consideration the boiling and condensing of the refrigerant?
 
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The heat convection equation will provide a good calculation of the heat transfer but it requires a knowledge of LMTD meaning you need to know the hot fluid inlet temp, hot fluid outlet temp, cold fluid inlet temp and cold fluid outlet temp. You also need to be able to calculate U, which requires the use of empirical correlations (for condensers anyway, not sure about evaporators). It can be difficult to accurately calculate all these terms.

If you are simply looking for a good approximation of the heat transfer, an energy rate equation (enthalpy eqn) should suffice. In the case of an evaporator, the working fluid (refrigerant) acts as if it has an infinite specific heat capacity and can be assumed as isothermal. When adding energy to this fluid, the enthalpy of vaporization is the major contributor and sensible heat can probably be neglected, provided the liquid is relatively saturated at inlet and outlet (isnt too subcooled or superheated) If this is the case, the energy eqn simplifies to the heat/energy added to the fluid;

[itex]\dot{Q}[/itex]=[itex]\dot{m}[/itex][itex]\Delta[/itex]hfg
All you need to know is the flow rate of the refrigerant and the change in enthalpy from inlet to outlet (get this in thermo tables for whatever fluid you are looking at).
 

1. How does heat transfer occur in evaporators?

In evaporators, heat transfer occurs through the process of evaporation. This is when a liquid is converted into a gas, absorbing heat energy in the process. The heat energy causes the molecules in the liquid to become more energetic and break away from each other, forming a gas. This transfer of heat from the liquid to the gas is what causes the liquid to evaporate.

2. What factors affect the rate of heat transfer through evaporators?

The rate of heat transfer through evaporators can be affected by several factors, including the temperature difference between the liquid and the surrounding environment, the surface area of the evaporator, the rate of flow of the liquid, and the physical properties of the liquid, such as its viscosity and density. Additionally, the design and efficiency of the evaporator itself can also impact the rate of heat transfer.

3. What is the purpose of using evaporators in heat transfer systems?

The primary purpose of using evaporators in heat transfer systems is to transfer heat from a liquid to a gas, thus cooling the liquid. This can be useful in a variety of applications, such as air conditioning and refrigeration, where the evaporator helps to remove heat from the air or liquid being cooled. Additionally, evaporators can also be used to concentrate solutions by removing water through the process of evaporation.

4. How do different types of evaporators work?

There are several types of evaporators, including forced circulation evaporators, natural circulation evaporators, and falling film evaporators. Each type works in a slightly different way, but they all essentially use the same principles of heat transfer through evaporation. Forced circulation evaporators use pumps to circulate the liquid through the evaporator, while natural circulation evaporators rely on density differences to circulate the liquid. Falling film evaporators work by allowing the liquid to flow down a vertical tube while hot vapor flows upwards, causing the liquid to evaporate.

5. How can the efficiency of heat transfer through evaporators be improved?

There are several ways to improve the efficiency of heat transfer through evaporators. One method is to increase the surface area of the evaporator, as this allows for more heat to be transferred. Another way is to optimize the flow rate of the liquid, as a higher flow rate can increase the rate of heat transfer. Additionally, using more efficient heat transfer fluids or incorporating heat exchangers can also improve the efficiency of heat transfer through evaporators.

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