Sensible and Latent People Loads

[PagoMitch]

Doing some Hospital HVAC in the Tropics, on an island where much of the population are persons of size.

ASHRAE lists values from 245S/105L for sitting, to 250S/250L for standing and walking.
However, I cannot find any references - anywhere - of how these values are affected by persons of size, turn-over of same (people entering/leaving), and external ambient conditions.
In this case, external conditions are 91DB/82WB. Wherever you go outside, hot and sweaty is the norm. And whenever you go indoors, you drag a lot of that heat and moisture with you.

I used to live there and worked at the hospital as the Facility Engineer. During that time, I had always used 300 BTUH Sensible and 300 BTUH Latent for individuals for HVAC design. This was based on my observations that whenever a 250-300 lb person comes in (realistically, 75% of the adult population) and sits down, you can feel the heat radiating off them from literally a foot away, for 15-20 minutes.

If this were a library, where people come in and sit for hours on end, this would not be as critical.
But for a Hospital Waiting Rm, which continually has people entering and leaving, the extra heat/water vapor per person adds up.

So.
Has anybody seen an official reference to increase the individual person BTUH S/L beyond the "usual" ASHRAE values?
I am convinced of the validity of this approach, because I have experienced it; but I am looking for something to substantiate and quantify (higher? lower?) my experience.

TIA.

 

[MintJulep]

Starting here, ASHRAE/ISO sensible/latent heat gains:

https://www.engineeringtoolbox.com/metabolic-heat-persons-d_706.html

Note that there is a column for "Average Metabolic Rate - male adult", with more details here:

https://www.engineeringtoolbox.com/met-metabolic-rate-d_733.html

There you see that the heat released is a function of a person's surface area. That's your adjustment factor for persons of size.

 

[HVAC-Novice]

If you continuously have people coming directly in from outside, you also have infiltration from outdoors.

Those are average values and different for each person. I think the movement in and out of the space also impacts how many people are in the area. that should be accounted for by the people load. if people coma in leave all the time, they also spend some time NOT in the space.

With ever increasing people, you could use a higher value. Look at some other categories and pick a value that is lightly more than the space you use. Same way you should use higher ambient temperature anticipating higher temperature due to climate change.

 

[PagoMitch]

Thanks gents,

To be clear, I can, and have, accommodated my observations in the designs for spaces at the facility.
But... I would have thought that some standards setting organization... ASHRAE for instance, would have recognized that:

1. Individuals of size put out more heat in general, regardless of the metabolic task, and attempted to quantify that value. Engineers everywhere should not have to literally guess at some activity with elevated metabolic rates to be "close" to a correct perceived value.
2. It is an easy assumption to estimate an increased surface area from one individual to another; although probably not very repeatable. My wife is 125#, and she appears about 1/3 the overall "size" of a 250# person of size standing next to her. I can therefore assume that the guy next to her has 3X the surface area, but that does not mean 3x the heat radiated.
3. The environment is hot and very humid. How humid? Enough so that you sweat so much during the day that you take a shower at night, rather than the am. So when a 125# person and a 250# person, both hot and sweaty, walk into an air- conditioned space, the immediate questions are 1) how much heat are they generating, and 2) at what rate does their metabolism - and heat rejection to the space - diminish to "normal". My observation is 15-20 minutes for larger folks, but usually somewhat less for smaller; maybe 10-15 minutes?
4. Movement in and out of the space is at least a more directly solvable issue. The 30-seat waiting room sees about 100 people/day. Packed first thing in the am, and then a mid-day surge until it tapers off at dinnertime. While the average may only per 10 people/hour, in reality there will be 3-4 times a day, for maybe an hour, that the place is packed. The design pretty much has to include 100% occupancy, with the Terminal Unit tapering back as necessary.

I/we'all can make assumptions all day long - and on this topic, apparently, we have to - and probably... if we are lucky and have some experience in our craft... and have some direct experience in the conditions we are guessing about...not be too far off to cause a design problem.
But these values are all measurable. IMHO, this is what standards settings organizations should be doing. This issue (rapidly rising population obesity) is not going to go away.

Anybody from ASHRAE listening?

 

[HVAC-Novice]

People's body temperatures are very consistent - we regulate body temperature. Except for the skin, even a person coming from outside doesn't have an elevated core temperature. if they do, that is a state of illness already. So that 300# person sitting inside will have at 38°C, and the 300# person coming in will may be at 38.5°C on average (if we assume the skin and clothing being much warmer.

Metabolic rate probably rises more with mass than surface. And muscle mass likely matters more than fat. Muscles use up a lot of energy even if you do nothing. that is why the body reduced muscle mass if it is not used - to save energy.

the activity matters more. A 300# person sitting likely has less heat dissipation than a skinny person walking around. There probably is information on that. but unless we talk about a specific overweight clinic, the design values may be OK. You should have a good idea of person density. Maybe add a few people for safety?
Alternatively use one of the other activities.

You bring up good points. but ultimately we design a conservatize system that will work for most scenarios. Unless the client specifically requires so, we don't design for all those people being 300# football players (muscles!) that start to hectically exercise in the lobby.

Our relatively small brain uses 20% of the energy even if we do nothing. but that doesn't mean you have to design for 30 Albert Einsteins (and I'm not even sure his brain was larger or used more energy)

 

[MintJulep]

My wife is 125#, and she appears about 1/3 the overall "size" of a 250# person of size standing next to her. I can therefore assume that the guy next to her has 3X the surface area, but that does not mean 3x the heat radiated.

That assumption is not valid. If you consider a person as a cylinder, you see that weight increases in proportion to D[sup]2[/sup], while surface area increases in proportion to D.

If treating an entire person as a cylinder isn't to your liking, then you can consider legs, arms and torso as individual cylinders to get a bit closer.

ASHRAE funds research into topics that go into the ASHRAE Handbooks. Research is initiated from Technical Committees.

Load calculations are under the control of ASHRAE Technical Committee 4.1.

You can find contact info for the TC 4.1 chair here:

https://tpc.ashrae.org/?cmtKey=ad90e53c-cbc6-46d2-95d1-a63e373601cf

 

[HVAC-Novice]

I just saw that ASHRAE fundamentals chapter 18 table 1 shows a table of heat loads. they show a male and an adjusted male/female column.

If you suspect (or know) the population there is on the heavier side, I would choose a category that is one step higher. For example, if the work is categorized as "very light work", use the "Moderate active work".
You may also adjust people density because that way you also increase ventilation (more metabolism needs more fresh air). You need to use some engineering judgment. All people re different and have different activity. just using a standing desk vs. sitting makes a small difference. People density is even harder to guess.

I don't have proof, but I suspect heavier people will move around less. So that will close the gap in metabolic rate compared to regular sized people.