# Calculating Rate of Heat Transfer for Refrigerator Evaporators

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In summary, the conversation discusses the complexities of using Newton's law of cooling to calculate heat transfer in a refrigerator evaporator. The equation includes factors such as overall heat transfer coefficient, external surface area, and log mean temperature difference. The question is whether this equation can be applied to refrigerators or if calculations must take into consideration the boiling and condensing of the refrigerant for more accurate results. The person asking for help is trying to understand the heat transfer rate in the evaporator.
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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?

I'm really just trying to get an idea of the heat transfer rate in the evaporator.Thank you for your help.John

## 1. How do you calculate the rate of heat transfer for refrigerator evaporators?

The rate of heat transfer for refrigerator evaporators can be calculated by using the formula Q = U x A x ΔT, where Q is the heat transfer rate in watts, U is the overall heat transfer coefficient in watts per square meter per degree Celsius, A is the surface area of the evaporator in square meters, and ΔT is the temperature difference between the refrigerant and the surrounding air in degrees Celsius.

## 2. What is the overall heat transfer coefficient and how is it determined?

The overall heat transfer coefficient (U) is a measure of how easily heat can be transferred between two mediums, such as the refrigerant and the surrounding air. It is determined by taking into account the individual heat transfer coefficients for convection, conduction, and radiation, and combining them using the appropriate equations.

## 3. How does the surface area of the evaporator affect the rate of heat transfer?

The surface area of the evaporator directly affects the rate of heat transfer, as it determines the amount of surface area available for heat exchange to occur. A larger surface area means more contact between the refrigerant and the surrounding air, resulting in a higher rate of heat transfer.

## 4. What factors can impact the rate of heat transfer for refrigerator evaporators?

The rate of heat transfer for refrigerator evaporators can be impacted by various factors, such as the type of refrigerant being used, the design and size of the evaporator, the surrounding temperature and humidity, and the efficiency of the refrigeration system.

## 5. How can the rate of heat transfer for refrigerator evaporators be improved?

The rate of heat transfer for refrigerator evaporators can be improved by increasing the surface area of the evaporator, using a more efficient refrigerant, and optimizing the design of the evaporator for maximum heat exchange. Regular maintenance and cleaning of the evaporator can also help improve its efficiency and heat transfer rate.

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