A: This is a great question. I stared at this and thought about it for about an hour yesterday. I'm still not totally convinced I have a good answer, but let me just give you a bit of context.

A: If you need the volume flow rate of air to calculate the latent heat gain, but it seems like the latent heat gain is being used to calculate the volume flow rate of air. So I'm not quite following what was being calculated there. What I would say as a general comment for that problem is that the central idea of that problem is that the mass flow rate of water, the evaporation rate of water vapor from the pool into the air that then needs to be removed by the ventilation that we're providing, is equal to the mass flow rate of air.

A: I would have necessarily say it's exactly the same flavor problem It does use the density factor, but it's not about heating systems capacity. It's just about a fans capacity To move air it's about the volume flow rate of the fan and Basically in that problem we're given the volume flow rate at sea level And we're asked to find the volume flow rate at 4,000 feet And there's also some temperature deviation there so there's a temperature density factor and a pressure density factor The pressure ends up being the bigger story of the elevation So but you have to multiply them together in the solution and And I guess the way I ended up I did not sure exactly how I presented it in the video solution, but I was making a bunch of assumptions Some of them explicit some of them may be not stated About what was going on with the fan and I think it's something like this I think the breakhorse power of the fan isn't changing just because it's at a different horse different Elevation and I'm assuming that the efficiency equals one which is to say then that the airhorse power is also not changing and What is a fan power airhorse power is a function of two things The volume flow rate and the delta p And we don't know anything about the delta p in this problem So I further assume that that is also constant Which you I don't know maybe that doesn't sit well not sure if it does but that's kind of what I was going with and it's the product of these two divided by some constant And then what is Q while we know from the continuity equation m dot is density times volume flow rate So if we wanted to substitute for volume flow rate that's mass flow rate over density So If the mass flow rate isn't changing Which if the delta p is in changing then the airhorse powers not changing Then I guess Q would be the same But then I thought We don't have any reason to be sure that the mass flow rate is changing so if the density is lower at elevation Then Q would have to go up So I don't know I kind of looked at it for a while I ended up confusing myself and now that whole question is sort of Looking a bit suspicious to me But I wanted to still bring it on here and talk about it and sort of double down on the conviction that I have on number 80 Which I feel really good about and I hope that that one makes a lot of sense and Sort of just talk about some of the assumptions that I was making on seven and if it's If it's a bit fuzzy then it's bit fuzzy for me too Not sure if anybody has any additional thoughts on that yeah, I think number 80 the first one you explain makes sense to me I am still a little bit confused about the second one because I do it does make sense to me how Overall looking in it from like a higher level that the heating capacity would be less at a higher density or higher elevation That makes sense, but I'm still a little confused why we would divide it in the other one Yeah, well, I have to take a harder look at seven and and Really probably Not make so many assumptions in the solution it may be put a little more meat on the bone or the question itself and like what's staying constant or what's changing Because it might just be an underdefined problem statement where like You take a fan that has a certain CFM At sea level and put it at elevation like is that enough information to answer the question? I'm not sure myself now So yeah, now I'm glad you asked this one and I want to I like to add yeah go ahead to the safe to the problem One way we have a photo and feet elevation factor.

A: No, this is a fair point. This is a case where the rule of thumb may not have been the best choice, probably better to go ahead and adjust for density. Good job for spotting that.

A: For this question, you draw the fan to the right of the cooling and reheat coils. So this was a situation where there's some reheat coil dedicated electric reheat coil. And the fan itself is doing a little bit of reheat as well, driving one degree of temperature increase in addition and there's a cooling coil.

A: So this is some practice exam number 75. And no one's found this. We've been through, I want to say, two major updates to the program in the last couple of years.

A: So this one, I went to the air tables. I see what you're saying. You're saying we know how much he is being added.

A: And we got a pump, a couple of chillers, an air handling unit, and a control valve that has to get through. We know the height of the building. We know this thing's in the basement.

A: This is a great question. And this is a nuanced question. It's one that no one has really asked, but it makes sense that you're asking it.

A: I was thinking it should be 22 years because the life cycle is 20 years from completion, and it took two years to complete. I just want to make sure I understand how to determine the correct time to use in a situation like this. I think I looked back at this one after this question was submitted, and I can't justify using 20 years as the duration for the salvage value.

A: Yeah, yeah, I can, you're right. So I guess we want to kind of, there's going to be different sources. So in this problem statement, I had written that there was a certain mass flow rate of water being added to the air.

A: Yeah, I wrote this problem to take you off autopilot and it worked. So let's draw a simple wall with only one layer to go through. In a lot of problems, who give the inside temperature and the outside temperature and then you draw your thermal network and you say, okay, I've got convection inside to the wall and then conduction through the wall and then more convection.

A: Or we could maintain the existing one. The point is there's going to be optionality down the road, but we need to make a decision today, and that's what the uniform, the equivalent uniform annual cost helps you do. In any economic analysis, you may choose to look at things a couple different ways.

A: Yeah, fair point on this one. I took a look at this one preparing for tonight. And for whatever reason, I didn't include depreciation.

A: So I guess my question to your question is, if you found the density factor the way I did, why also find the elevation corrected pressure? Like those are the same thing. So the density factor accounts for the elevation.

A: Don't the pumps also need to overcome gravity. So this one was about building with pumps in the basement. And I'll just do a quick sketch.

A: So I took a look at this one and I really wanted it to be the case that the only way to do it was to make the ice and traffic assumption. But I think your question is a good one because if I look at the normal shock table and the formula 3.6.3 which you referenced, there's really no way to know for sure which one to use. So you could end up with different answers.

A: Yeah, this was one where I gave you the temperature and I told you how much he was being added and wanted to know the final enthalpy. And the proposal is to use Q equals MC delta t which is interesting. Q equals MC delta t.

A: In this problem, the solution was inches of mercury to inches of water to PSI G to PSI A. I tried to use the reference manual for converting into atmospheres and back, but I seem to never come out of it right, and general should I avoid using conversions to atmospheres, or is there maybe some general knowledge about atmospheres versus atmospheres standard, that I should be keeping in mind. There's nothing wrong with atmospheres.

A: Yeah, if you're looking at the pressure enthalpy diagram in the reference handbook scroll down one page there's a table and the table will allow you to collect some of these values. And that's where I'm pulling most of the values from for that solution. And there are some errors in the solution which I've corrected as many as I could in the video.

A: So this one was about heating air, and the solution roadmap was to use the air table. So going to that table, which I think he'd said, yeah, on the air table. We're looking at the right table.

A: So it's a bit of algebra to show it, but I think it's worth going through just to convince ourselves that the answer is yes. So they are the same. So let's just start with a little exponents review.

A: Okay, so a couple things there. First of all, by using the SHR, that's good, that's fine. And that's fundamentally the same thing as drawing a line, connecting the two state points, the entering air, the room air, and the coil condition.

A: So the problem with that in this problem is air. When we can't assume that the specific capacity of air, C sub P is constant over such a large range. We're getting up to temperatures of around 3600 degrees Fahrenheit.

A: So this is very similar to the one that came up a few problems ago about the pool, this one. So we've got another situation where we're dehumidifying the pool. And you try to use the latent heating and cooling rule of thumb, which is an interesting idea.

A: In the solution we are assuming that in an ideal world, the superheated steam would enter into the saturated phase and we saw for the quality to find the enthalpy. My question is wouldn't the ideal world scenario make more sense if we assume the superheated steam to stay superheated steam, leaving exiting the turbine. Since in an ideal world, there would be no losses that is contributing to phase change of superheated saturated steam.

A: Yes, I'll read that last part for sure. Yeah, it's a good question and good instinct in terms of your kind of planned solution path. So this problem was about a swimming pool and it was a situation where we had evaporation.

A: Yeah, so I think you're just looking for some reassurance that you're okay. I guess, yeah, so the general thing that you're looking for is thing that you should know and maybe you've already noticed this is that there are cipricles. So what you did is totally valid.

A: Well, just the one that's in the, I don't think that in the solution. The one that's right there in the measurement relationships. You, I don't think, unless I'm missing something that you used that one.

A: So we shouldn't interpolate on curves. The whole idea of linear interpolation is it has to be aligned. So that's the reason there.

A: This is actually a double question. The next one is basically the same thing. I see that someone already asked about qc delta t for this problem.

A: So I think we probably shouldn't do it with weplob for the reasons to explain in the problem before last. We should do it with Ancel P because it's a linear scale. Like I said, we might get away with it with weplob.