Gas fired water heater - sizing

Click For Summary

Discussion Overview

The discussion revolves around the sizing of a gas-fired water heater, particularly in the context of a hospital setting. Participants explore the necessary calculations for determining flow rates and temperature rises, as well as considerations for peak demand and real-world applications.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant discusses the formula for calculating flow rate (Q = c x dt) but notes the absence of a defined dt when sizing a gas-fired water heater.
  • Another participant questions the flow rate and temperature rise assumptions, seeking clarification on the intended application (e.g., household vs. hospital use).
  • A scenario is presented where a hospital requires 35 liters of hot water per person for 75 people, leading to a total capacity requirement of 2625 liters and a calculated heating input of 153 kW.
  • Concerns are raised about the validity of the flow rate assumptions, particularly regarding whether the 35 liters per person is per hour or per day.
  • Participants discuss the importance of considering peak demand and the potential need for a worst-case scenario approach in calculations.
  • Flow rates for various fixtures (e.g., basin, sink, bath, shower) are shared, with a request for confirmation on their accuracy and relevance to code requirements.
  • There is a discussion about the implications of gradual water usage versus full heating, including considerations of tank insulation and energy efficiency.
  • One participant suggests that larger buildings, such as hospitals, may not require a tank at all, while another emphasizes the need for adherence to plumbing codes in real applications.

Areas of Agreement / Disagreement

Participants express differing views on the assumptions regarding water usage rates and the appropriate methods for sizing the water heater. There is no consensus on the best approach or the accuracy of the initial calculations, indicating ongoing debate and exploration of the topic.

Contextual Notes

Limitations include unclear definitions of flow rates and temperature rises, as well as the dependence on specific applications (hospital vs. residential). The discussion also highlights unresolved questions about the impact of gradual versus full heating on efficiency and energy use.

Who May Find This Useful

This discussion may be of interest to professionals in building services, plumbing, and HVAC design, as well as students studying thermodynamics and fluid mechanics.

lewis581
Messages
5
Reaction score
0
Hi Guys,

I am aware how to calculate flow rate Q (c x dt), but when sizing a direct gas fired water heater there is no dt but of course need allow for temp rise, but what parts of information can be known and unknown and how would i even start to put a formula together? Ultimately i would like to be able to size the flow rate required?

Any help would be really appreciated!

Regards
Lewis
 
Engineering news on Phys.org
Flow rate of water? Of gas? Without a temperature rise? For what purpose? Household hot water? Restaurant dishwashers?
 
Hi Bystander

Thanks for the reply to my question, i think i have moved on a little since my post, but would appreciate any feedback.

I have mocked up a scenario where a gas fired water heater is being used in a hospital for 75 people.

From cold to heat the hot water the following has been considered.

35 litres per person (based on a hospital) x people 75 = 2625 litre capacity requirement

2625 x spc (specific heat capacity) of 4.2
x 50 (temp rise) = 551250/3600 (seconds to hour) = 153 kw input to heat 2625 litres of water.

Do you feel i could be missing anything from this? Or am i right in thinking that this would be enough info to size accordingly?
 
If I'm reading this correctly, you're heating 35 liters/person/hour. That much hot water consumption for a hospital? Or is it per day? Which would cut the heating load somewhat.
 
Without having a lot of experience in sizing/thermodynamics i would have thought that my investigation would be based on peak demand? I have also done a calculation on re-heat over an hour and a temp rise of 30 needing an kw input of 35kw?
 
Is this for a real application, homework or just curiosity? For real applications, domestic water flows are determined by codes.
 
Good afternoon Russ,

I work in sales and would look like to further my understanding as i also find it very interesting!

I have identified average flow rates from a building services book i brought, (i am also waiting to take delivery of a physics thermodynamics book too!), anyways the flw rates I've obtained are as follows:

Basin 0.08 l/s
sink 0.15 l/s
bath 0.15 l/s
shower 0.6 l/s

Would you agree or is this not what you are referring to in the sense of codes??
 
lewis581 said:
Good afternoon Russ,

I work in sales and would look like to further my understanding as i also find it very interesting!
Ok, so a hypothetical...
I have identified average flow rates from a building services book i brought, (i am also waiting to take delivery of a physics thermodynamics book too!), anyways the flw rates I've obtained are as follows:

Basin 0.08 l/s
sink 0.15 l/s
bath 0.15 l/s
shower 0.6 l/s

Would you agree or is this not what you are referring to in the sense of codes??
Yes, but they also specify diversity, which is how many you can expect/assume to be in use at once, expressed as a percent. Obviously, a person wouldn't tend to use his sink and shower at the same time and not everyone will be using one or the other at the same time either.

In any case, just for the thermodynamics:
35 litres per person (based on a hospital) x people 75 = 2625 litre capacity requirement

2625 x spc (specific heat capacity) of 4.2
x 50 (temp rise) = 551250/3600 (seconds to hour) = 153 kw input to heat 2625 litres of water.
You didn't specity the time -- Bystander asked. Since you divided by 3600 seconds, that implies you meant 35 liters per person per hour, but that doesn't come anywhere close to any of the usage point flow rates, so I have no idea where you got that number. However, yes, 153 kW will heat 2625 liters by 50 C in one hour.
 
Hi Russ,

Appreciate your time and and feedback!

So does this mean rather then me being so rigid with my calculations i should base them in worst case scenario?

In the eye of design and not hyperthetical, what should it be based on?

My understanding was a gas water heater was sized on heating a volume of water from cold (in an hour) and also a re-heat (in an hour). But yes from what you have stated... what does happen if that water is being used gradually?? Would the insulation of the tank stop the water from cooling too much, so therefore avoiding heatloss and the energy,time and money to re-heat all the time? Or is a gradual reheat throughout the day better than a full one?

What would you suggest in a way of formula to achieve the correct method?I honestly would be grateful for your help, hope you don't feel bombarded with all this, i have tried to help myself but just can't get it right!
 
  • #10
lewis581 said:
So does this mean rather then me being so rigid with my calculations i should base them in worst case scenario?

In the eye of design and not hyperthetical, what should it be based on?
To be real, it should be based on whatever the code says, but since it is hypothetical, it is probably fine the way it is.
My understanding was a gas water heater was sized on heating a volume of water from cold (in an hour) and also a re-heat (in an hour).
Re-heat? You mean from re-circulation? It may or may not have re-circulation, but if so, sure. But the re-circulation will be a small flow rate and low delta-T. [edit] Or, if you mean keeping a tank warm, that's a pretty small number too. I don't think that is taken into account into the calculations because it is a lot smaller than any safety factor would be.
But yes from what you have stated... what does happen if that water is being used gradually?? Would the insulation of the tank stop the water from cooling too much, so therefore avoiding heatloss and the energy,time and money to re-heat all the time? Or is a gradual reheat throughout the day better than a full one?
For a hospital, I'm not sure you need a tank at all. Larger buildings tend not to have them, though 75 is a pretty small hospital.
What would you suggest in a way of formula to achieve the correct method?
The formula you used worked fine, it is just the method of selecting the flow rates that is suspect. The solution to that is to find a copy of the international plumbing code.
I honestly would be grateful for your help, hope you don't feel bombarded with all this, i have tried to help myself but just can't get it right!
You're welcome.
 

Similar threads

Increasing pipe size and then decreasing
  • · Replies 2 ·
Replies
2
Views
4K
Cleaning of the finned tubes on gas cooler without disassembling
  • · Replies 7 ·
Replies
7
Views
2K
High School Why does hot water rise to the top of a water heater?
  • · Replies 66 ·
3
Replies
66
Views
5K
Airflow thru 6 mesh, .035 gauge screen
  • · Replies 6 ·
Replies
6
Views
2K
How does a pump trade head for flow rate?
  • · Replies 3 ·
Replies
3
Views
3K
Time to empty gas cylinder (problem worked, but would like feedback)
  • · Replies 7 ·
Replies
7
Views
4K
Flow discharge between 2 open water bodies connected by a concrete sluice
  • · Replies 2 ·
Replies
2
Views
1K
What is the Cooling Time for Steel Tubes with Ambient Nitrogen Gas Flow?
  • · Replies 7 ·
Replies
7
Views
3K
Calculating Valve Size With a Hydraulic Accumulator
  • · Replies 8 ·
Replies
8
Views
3K
Need help with a research project for firefighting
  • · Replies 7 ·
Replies
7
Views
2K