# The temperature change in a throttling process

• riquelme8
In summary, the conversation discusses the physics behind heat pumps and the process of throttling. The joule-thomson effect is mentioned, which explains the temperature variation during the throttling process and is affected by the joule-thomson coefficient and gas temperature. The conversation also mentions two mechanisms that contribute to the change in temperature during throttling, depending on the gas temperature and pressure. The temperature variation is measured with respect to pressure to determine the joule-thomson coefficient at constant enthalpy.
riquelme8
Hi,

I'm writing a small student-paper on heat pumps, and I'm not at the point where i explain the physics behind it. This is probably a silly question, but there's something I can't seem to understand about the throttling process.

First off I get that the enthalpy is conserved through the throttling process, and that Upotential increases, causing the Ukinetic to decrease, which cause the liquid to cool. But this is my problem with the theory:

Let's say that the throttle is 2 cm long. If you look at the liquid as it goes into the first cm of the throttle, you decrease the volume of the liquid, causing the temperature to increase. And if you then look at the process where it moves on through the last part of the throttle, it'll increase it's volume, thereby decrease it's temperature. So why aren't the liquid at the same temperature as when it went into the throttle?

Last edited:
Ever heard of joule-thomson effect.which describes the temprature variation during throttling process.the reason for getting cooled or heated depends on joule- thomson coefficient and whether the gas temperature is above or below the inversion temperature.As for the physical mechanism I am quoting from wiki
As a gas expands, the average distance between molecules grows. Because of intermolecular attractive forces (see Van der Waals force), expansion causes an increase in the potential energy of the gas. If no external work is extracted in the process and no heat is transferred, the total energy of the gas remains the same because of the conservation of energy. The increase in potential energy thus implies a decrease in kinetic energy and therefore in temperature.
A second mechanism has the opposite effect. During gas molecule collisions, kinetic energy is temporarily converted into potential energy. As the average intermolecular distance increases, there is a drop in the number of collisions per time unit, which causes a decrease in average potential energy. Again, total energy is conserved, so this leads to an increase in kinetic energy (temperature). Below the Joule–Thomson inversion temperature, the former effect (work done internally against intermolecular attractive forces) dominates, and free expansion causes a decrease in temperature. Above the inversion temperature, gas molecules move faster and so collide more often, and the latter effect (reduced collisions causing a decrease in the average potential energy) dominates: Joule–Thomson expansion causes a temperature increase.
moreover the temperature variation is measured with respect to pressure to determine joule- thomson coefficient at constant enthalpy.

## What is the temperature change in a throttling process?

The temperature change in a throttling process is the change in temperature that occurs when a fluid or gas undergoes a rapid expansion or contraction without any external work being done on the system.

## What causes the temperature change in a throttling process?

The temperature change in a throttling process is caused by the decrease in pressure and increase in volume of the fluid or gas. This leads to a decrease in temperature due to the Joule-Thomson effect.

## How is the temperature change in a throttling process calculated?

The temperature change in a throttling process can be calculated using the Joule-Thomson coefficient, which is a measure of how much the temperature changes for a given change in pressure at constant enthalpy.

## What factors can affect the temperature change in a throttling process?

The temperature change in a throttling process can be affected by the type of fluid or gas, the initial pressure and temperature, and the flow rate. Other factors such as the shape and material of the throttling device can also play a role.

## How is the temperature change in a throttling process used in practical applications?

The temperature change in a throttling process is used in various practical applications, such as refrigeration and air conditioning, where it is used to cool down a gas or fluid. It is also used in the oil and gas industry to regulate pressure and temperature in pipelines and equipment.

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