# Finding the enthelpy for a sub-cooled working fluid using steam tables

• CuriousQuazim
In summary, to calculate the properties of a subcooled fluid using steam tables, you will need to use specific volume and specific heat data and follow a series of equations to determine the enthalpy and entropy values. Subcooling refers to a state where a fluid is at a lower temperature than its saturation temperature at a given pressure. These steps can help you accurately calculate the values for a given subcooled fluid.
CuriousQuazim
Hi guys, kind of stuck here because I either don't remember or wasn't taught this but...

I'm working with refrigeration and I've come across a problem that involves a state where the fluild has been sub-cooled by 10K - Its easy enough dealing with super heated fluids but when it comes to sub cooled, how can I go about calculating the values using the steam tables? It's easy enough using a P-h diagram but that is neither as accurate as I'd like it to be nor is it a method I'd like to rely on.

So yes, for a given working fluid and its respective steam table (Rogers & Mayhew) how would one go about finding the subcooled enthalpy/Entropy?

Thanks for the help in advance guys ^^, really appreciate what you guys do.

Hi there,

Calculating the properties of a subcooled fluid using steam tables can be a bit tricky, but it is definitely possible. First, let's define what subcooling means. Subcooling refers to a state where a fluid is at a temperature below its saturation temperature at a given pressure. In other words, the fluid is still in liquid form but is at a lower temperature than it would be at that pressure in a saturated state.

To calculate the enthalpy and entropy of a subcooled fluid, you will need to use the specific volume and specific heat data from the steam table. Here are the steps you can follow:

1. Determine the pressure and temperature of the subcooled fluid. This information will be given in the problem or you can measure it using a pressure and temperature sensor.

2. Look up the specific volume (v) and specific heat (Cp) of the fluid at the given pressure and temperature in the steam table.

3. Use the following equation to calculate the enthalpy (h) of the subcooled fluid:
h = v * Cp * (T - Tsat)

Where:
h = enthalpy (kJ/kg)
v = specific volume (m^3/kg)
Cp = specific heat (kJ/kgK)
T = temperature of the subcooled fluid (K)
Tsat = saturation temperature at the given pressure (K)

4. To calculate the entropy (s) of the subcooled fluid, use the following equation:
s = Cp * ln(T/Tsat) - R * ln(v/vsat)

Where:
s = entropy (kJ/kgK)
Cp = specific heat (kJ/kgK)
T = temperature of the subcooled fluid (K)
Tsat = saturation temperature at the given pressure (K)
R = specific gas constant (8.314 kJ/kgK)
v = specific volume (m^3/kg)
vsat = specific volume at saturation at the given pressure (m^3/kg)

5. Now, you have the enthalpy and entropy values for the subcooled fluid. You can use these values to solve any other problems or calculations related to the fluid.

I hope this helps! Let me know if you have any further questions or need clarification on any of the steps. Good luck with your research!

## What is enthalpy and why is it important in thermodynamics?

Enthalpy is a measure of the total energy of a system, including both its internal energy and the work required to change its volume. It is important in thermodynamics because it helps us understand the energy transfers and transformations that occur in physical and chemical processes.

## How do steam tables help in finding the enthalpy of a sub-cooled working fluid?

Steam tables provide a convenient and accurate way to calculate the properties of water and steam at various temperatures and pressures. By using these tables, we can find the enthalpy of a sub-cooled working fluid by looking up the specific enthalpy value at the given temperature and pressure.

## What are the units of enthalpy and how are they represented in steam tables?

The units of enthalpy are typically joules (J) or kilojoules (kJ) per kilogram (kg). In steam tables, enthalpy values are typically represented in kilojoules per kilogram (kJ/kg).

## Can enthalpy values be negative and if so, what does it indicate?

Yes, enthalpy values can be negative. This indicates that the system has lost energy, either through heat transfer or work done by the system. In the case of a sub-cooled working fluid, a negative enthalpy value may indicate that the fluid has lost energy as it was cooled down.

## Are there any limitations to using steam tables for finding the enthalpy of a sub-cooled working fluid?

Yes, there are some limitations to using steam tables. They are only accurate within a certain range of temperature and pressure, and they may not account for all possible variations in the properties of the working fluid. Additionally, steam tables may not be available for all substances, so alternative methods may need to be used in those cases.

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