Steam is condensed in a closed system at a constant pressure of 75 kPa from a saturated vapor to

Steam is condensed in a closed system at a constant pressure...

Key Concepts

Second-law Efficiency

Second-law efficiency is a measure of how effectively a process or system converts available energy into useful work relative to the maximum (reversible) work possible. It is defined as the ratio of the minimum work required (calculated using thermodynamic limits and exergy considerations) to the actual work or energy exchanged in the process, highlighting the role of irreversibilities and entropy generation.

Exergy Analysis

Exergy analysis involves quantifying the maximum useful work obtainable from a system as it comes into equilibrium with its environment. This concept is central to understanding and quantifying inefficiencies in thermodynamic processes by comparing real operations with ideal reversible processes, thereby determining the lost work potential due to irreversibilities.

Phase Change Process

A phase change process, such as condensation, involves the transition of a substance from one phase to another (in this case, from vapor to liquid) at constant pressure. This process typically involves significant latent heat transfer and requires careful thermodynamic analysis to relate energy and entropy changes during the phase transformation.

Entropy Generation

Entropy generation quantifies the degree of irreversibility within a process. Any departure from an ideal reversible process results in entropy production, reducing the available energy (exergy) that can be converted into work. This measure is fundamental when assessing the second-law performance of a thermal process.

Transcript

00:01 And this problem, we have steam that is being, well, is being compressed, but we're actually not in a, compressing it in kind of the usual way by doing, by, you know, putting some compression on it because we are, we have a constant pressure.

00:24 So what's kind of happening here is that i'll try to draw a schematic of it here and explain what, you know, what this system.

00:31 That might be kind of modeling.

00:35 So it's kind of like if you put hot water or, you know, like boiling water in a two -liter bottle of soda or an empty two -liter butter of soda and cap the top and then let it cool that it then, you know, basically sucks the walls in.

00:51 And so, again, you can actually get these things to like, there's a great demo on youtube of just basically a very old video of a professor putting boiling water or steam into a 55 -gallon drum, a huge steel drum, and then putting it in an ice bath and then waiting and waiting for it to basically, it collapses.

01:17 It just kind of sucks in on itself because the pressure gets so low in there because it's, you can't, the volume needs to, it's trying to, the pressure gets so low that basically it almost becomes a vacuum and then it sucks the walls of the steel drum.

01:35 But so in this case, we have a constant pressure.

01:38 So instead of basically sucking the walls of something in, we're actually basically pulling, we can think of it as a piston here.

01:47 And i drew it upside down because we have a pressure that's below atmospheric pressure.

01:52 So assuming this was sealed and we had a weight here, or this had some weight, then we'd have a lower pressure in here.

02:01 Because we're causing, trying to increase, we're trying to basically increase the volume of this steam by pulling this down.

02:12 So we have this weight here.

02:15 So then what we have initially is we have a bunch of steam in here.

02:18 We have saturated vapor.

02:21 And then at the end of the process, we have saturated liquid.

02:25 And we have heat transfer that causes that.

02:28 So we have some kind of cold bath around here, although it's really not that cold, 37 degrees c.

02:37 But it's cooler, cooler than what temperature in here is.

02:41 So we have some heat transfer out.

02:44 And so as that heat transfers out where then this vapor, we cross over the pv curve or the ts curve from saturated to, from saturated vapor to saturated liquid.

03:02 And in doing that, this thing gets pulled up.

03:05 So we're actually doing, if you look at the pv diagram, we're going from a higher volume to a lower volume.

03:11 So we're actually doing work.

03:13 We're putting work in.

03:15 But that's actually useful work that we have because, you know, it could basically be pulling something up.

03:21 So think about this as being massless and we have a weight here.

03:26 And so now we can actually, you know, we can use that work that's going into the gas to lift something.

03:33 So that's the useful work, is the work in.

03:39 So we have energy balances the work in, minus the heat transfer out, is the change in internal energy, you know, the steam in here of the water.

03:50 So at the initial properties, we have, again, we have surrounding conditions too, they're given.

03:57 We have, again, we have the specific volume of the saturated vapor, the internal energy of the saturated vapor and the entropy of the saturated vapor.

04:07 And these are all in standard units, standard si units.

04:11 So cubic meters per kilogram, kilojoules per kilogram, and kilojoules per kilogram degree c or kelvin, however you want to look at that.

04:24 And then afterwards, we have all the saturated liquid conditions.

04:29 And so you can see immediately that obviously these things change.

04:32 A lot and specifically the specific volume changes a whole lot so you know this thing actually gets you know you have you know what is this like um less than a thousand times the um the volume um so you know this thing basically got you know just like you can imagine when steam condenses you don't have or when when water boils you know their the water of the volume or the densities of water and steam, liquid water and steam is like 1 ,000 difference...

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