Can a turbo expander convert more heat to work than a piston expander?

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MysticDream
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Trying to get a better understanding of adiabatic expansion
A turbo expander is a turbine moved by high velocity gas hitting it's blades and doing work, reducing it's temperature and pressure. A cylinder-piston expander uses pressurized and/or heated gas to do work on a piston reducing it's temperature and pressure. Let's say in both cases we have the same initial pressure, temperature, and exit pressure. In the case of the turbo, it's pressure and temp is it's stagnation point. The exit velocity is low enough to assume an approximate stagnation condition in both cases.

My question is, in which case can we extract the maximum amount of heat (and do the most amount of work) so that the temperature is lowest for the same exit pressure? So far, in the case of the cylinder-piston expander, the formula for adiabatic expansion seems to give the maximum amount of work that can be done and the lowest temperature that can be reached for a desired exit pressure. If I desired a lower exit temperature, it cannot be done unless I expand to a lower pressure. If I'm mistaken, please correct me.

In the case of the turbo expander, can more work can be done because the heat and pressure can be converted to kinetic energy by increasing the gases' velocity through the nozzle and doing work on the blades? I have yet to work out the formula for that. Any help would be appreciated.
 
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Is this a theoretical question or a practical question?

In theory, if I have gas at temperature T and pressure P going into a black box, with temperature y and pressure p coming out, iit doesn't matter what kind of machinery is in the box - the maximum work is the sane. In practice, it probably depends on more than just piston vs turbine.
 
  • #3
Vanadium 50 said:
Is this a theoretical question or a practical question?

In theory, if I have gas at temperature T and pressure P going into a black box, with temperature y and pressure p coming out, iit doesn't matter what kind of machinery is in the box - the maximum work is the sane. In practice, it probably depends on more than just piston vs turbine.

Well it’s both. I’m trying to get the temperature lower at the exit of an expander for a desired pressure. The pressure can’t be lower because it feeds a compressor that has a specific compression ratio. I want to be able to use as much of the added heat to the gas as possible to do work. In the piston case, the heat has caused a rise in pressure which can then be used to do work. In the turbine case the heat has caused an increase in velocity and kinetic energy which can then be used to do work.
 

1. What is a turbo expander, and how does it differ from a piston expander?

A turbo expander, also known as a turboexpander or expansion turbine, is a type of gas turbine that converts the energy from a high-pressure gas expanding to a lower pressure into mechanical energy, typically used to drive a compressor or generator. Unlike piston expanders, which are based on reciprocating technology where a piston moves within a cylinder, turbo expanders utilize a radial or axial turbine through which the gas expands, converting pressure and thermal energy into rotational energy.

2. Can a turbo expander convert more heat to work than a piston expander?

Generally, whether a turbo expander can convert more heat to work than a piston expander depends on the specific application and operating conditions. Turbo expanders are often more efficient in systems where there are large volumes of gas and high pressure ratios, primarily due to their continuous and smooth operation which minimizes losses. Piston expanders, on the other hand, might be more effective in smaller systems or where precise control over the expansion process is required. The efficiency of converting heat to work also heavily depends on the design and optimization of the expander for its specific application.

3. What are the efficiency considerations for turbo expanders and piston expanders?

The efficiency of both turbo expanders and piston expanders depends on several factors including design, operating conditions, and the thermodynamic properties of the working fluid. Turbo expanders typically have higher isentropic efficiencies due to fewer moving parts and lower friction losses, making them suitable for applications like natural gas processing or cryogenic expansion. Piston expanders may have lower efficiencies due to friction, wear, and tear of more moving components but can be advantageous in applications requiring variable output or direct mechanical drive capabilities.

4. What applications are most suitable for turbo expanders?

Turbo expanders are most suitable for applications involving high flow rates and pressures, such as in air separation plants, natural gas processing, and certain power recovery systems. They are highly effective in cryogenic applications, where they are used to liquefy gases by extracting thermal energy. Their ability to handle large volumes efficiently and their compatibility with continuous operation make them ideal for these settings.

5. Are there any specific maintenance or operational challenges associated with turbo expanders compared to piston expanders?

Yes, there are specific maintenance and operational challenges associated with each type of expander. Turbo expanders generally require less maintenance than piston expanders due to their fewer moving parts and smoother operation. However, they can be sensitive to operational conditions such as inlet temperature and pressure, and require precise control systems to manage these parameters. Piston expanders, while potentially requiring more frequent maintenance due to wear and tear on mechanical parts, are often easier to repair and can be more robust under variable operational conditions. Both systems need regular inspections and maintenance to ensure optimal performance and longevity.

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