Heating water without accumulation for 120 appartments?

[TSN79]

Does it sound at all reasonable that a heat-exchanger of 250 kW can heat water from 5 to 55 degrees C for about 120 "normal" appartments directly without any accumulation when probability is taken into account? If not, how many liters should be accumulated?

 

[russ_watters]

What are we talking about here - is this a domestic hot water system with an indirect water heater (heating boiler heating the domestic water)? I see there is another similar thread about this... Are you actually trying to design a domestic water system? Don't you have help? Anyway, this is all handled by the codes - it tells you what flow rates and diversities to use.

 

[TSN79]

An external source brings in hot water (about 120 degrees supply) which is to heat the domestic water through this heat-exchanger, which the manufacturer claims to be 250 kW. And so the question becomes; Can average sized appartments which have a normal daily cyclus of hot water use, be heated by this without having to accumulate water?

The equations I learned of from the other thread didn't help me out that much, but I do have help with this, so don't worry about it :) I just like to ask several places to compare answers afterwards :)

 

[Artman]

The water delivered by the external system is 120 deg? I assume this is 120 deg C, high pressure heating hot water. Is the 250 KW heat exchanger rating based on this input heating temperature, or is it a nominal rating based on a temperature below 100 deg C?

 

[TSN79]

I believe the 250 kW rating is based on 90 degrees C supply.

 

[russ_watters]

I don't have time to look up the required flowrates right now, but doing an indirect water heating system without a storage tank is an exceedingly bad idea - it isn't just about instantaneous capacity. Nevermind that at 4:00 in the morning, you'll boil the water in the heat exchanger because you don't have any flow - if some drunk or old person steps into the shower without checking the temperature, you could kill someone.

My boss has an indirect water heater tied to his boiler and doesn't have a tank on it - everyone who has used the shower or a faucet has been burned at least once. And his boiler "only" puts out 180F water...

edit: wait - does this 120C water have a control valve on it...?

 

[TSN79]

Of course one would have to mount valves to blend the output water with cold water to reach a more "usable" temp around 55-60 deg C, otherwise as you say it would be dangerous. I was thinking of running a circulation pipe around the building and letting the exchanger keep the temp of this water to around 55-60 C, only letting it pass through the exchanger as it dropped below a specific value, otherwise keeping it separated from the exchanger so that it doesn't boil at 0400...

 

[Artman]

I don't have time to look up the required flowrates right now, but doing an indirect water heating system without a storage tank is an exceedingly bad idea - it isn't just about instantaneous capacity. Nevermind that at 4:00 in the morning, you'll boil the water in the heat exchanger because you don't have any flow - if some drunk or old person steps into the shower without checking the temperature, you could kill someone.

My boss has an indirect water heater tied to his boiler and doesn't have a tank on it - everyone who has used the shower or a faucet has been burned at least once. And his boiler "only" puts out 180F water...

You will need a master mixing valve with Hi/Low capacity temperature control that will only allow a maximum of approximately 120 deg F (check your local codes for the actual maximum) to go to the fixtures at varying flows with an acceptable pressure drop at your peak flow.

Something like this: http://www.leonardvalve.com/products.asp?l=105&ind=-99"

 

[Artman]

According to a quick calculation, 120 apartments figuring 1.5 bath each and 120 clothing washers, would require about 1,000,000, btuh at peak demand with 40 deg F incoming domestic water figuring 100 deg rise.

250kw is 853,000 btuh at 90 deg C entering heating water, it will deliver more at higher entering heating water as the temperature difference between the heating and domestic water will be higher. 250 KW at this rating should be adequate, but it depends on actual fixtures in the apartments, actual incoming domestic water temperature and actual rise required.