Rule of thumb for hot water buffer vessel 1

[paduk]

Hi guys and gals,
I can't remember this rule of thumb.
Would you say 30 litres per each KW correct?
Thank you

 

[paduk]

Sorry I forgot to add the followings:
the system is a CHP producing Hot water for an heating system whose peak is 490KW.
Water temperature is 90C.
thanks again

 

[quark]

Don't know about the capacity vs kW. The basic idea is to provide some volume for the water expansion due to increase in temperarature. You can use the following method.

First calculate total hold up volume. check the specific volume of the water at makeup temperature and at 90C. The difference gives you the expansion per kg of water. Multiply this with the total hold up volume and you have your buffer capacity.

For ex., say, you fill the system at 20C and the corresponding specific volume is 1.00184 L/kg and at 90C it is 1.03595 L/kg. So, the difference is 0.034 L/kg. Suppose, your hold up volume is 1000 L then the buffer volume is .034x1000 = 34 liters approximately (for atmospheric buffer tanks).

 

[ChrisConley]

What is the buffer for? For expansion, quark's answer is spot on. I'd use an expansion tank sizing software to get the required volume though (initial fill pressure and operating pressure are factors in non-atmospheric tanks)

If it is to ride the peak of the system, then you should be able to compare: output of CHP compared to the peak of the system x duration of peak, with the extra required heat being in storage.

 

[paduk]

the buffer is to ride the peak of the system.
The CHP output is 233 KW heat and 200 KW electricity.
I am not sure about the duration of the peak and its frequency, lets assume, 10 hours.
thanks

 

[ChrisConley]

I'm going to move the units over to Btu/hr (I'll move them back to metric when I'm done).

490kW Peak Load = 1,672,355 Btu/hr

CHP Heat Output = 233 KW = 795,221 Btu/hr

Shortfall = 877,133 Btu/hr * 10 hours

Buffer tank would be required to supply 8.77 MBtu. By definition a Btu = lb water * deltaT (F)

I'm going to make an assumption that the water is being used for heating. As such, I'm going to assume that the working temp of the fluid can be from 190F (90C) to 160F (70C) which gives us a tank delta T of 30F. Feel free to modify this number to suit your actual operation.

8.77M Btu / 30 = 292333 lb water ~ 35,000 gallons of water.

So to turn everything back to metric. Based on the assumptions above we'd require about 133,000 L, which is closer to 271 L / kw.

Shorter peak time and/or greater deltaT will reduce that number in a pretty linear fashion. It looks like your rule of thumb would survive a 1 hour duration of peak load.

 

[quark]

Chris' calculation is the way to go than depending upon some thumb rules. However, there are two issues I would like to rise here.

One is that temperature rise of the fluid across a utility heater is pretty ok at 20C but temperature drop of 20C across the control space heater requires larger coil area and may not be economic.

Secondly, the heater capacity selection is very bad. The selection was done for 50% load and for 50% of time.

The SI calculation is not bad. Assuming a 20C drop of hot water temperature across the use equipment (a control space heater) and the peak load duration of 10 hours,

m kg x 4.182 kJ/kg C x 20 C = (490-233)kJ/s x 10 hrs x 3600 s/hrs

or m kg = 257*10*3600/(4.182*20) = 110616 kgs (or liters)

 

[quark]

One more issue is that the peak load duration in your case can't be user defined but fixed. As you have only 33 kW excess capacity (that is what I understood from what you mentioned), you pile up heat during non peak hours and utilize it during peak hours.

Suppose, x is the time period of normal load in hours then,

33*x = 290 (24-x), here (24-x) gives you the maximum peak load duration your heater can take care of, and 290 kW is the extra heat energy required during peak loads.

So, x = 290*24/323 = 21.55 hours and you can cater to a maximum peak load duration of 2 hours, approximately. So, your buffer tank capacity is limited to 1/5th of what we calculated i.e 22123 liters.

 

[quark]

OK, I overlooked on the CHP part, so ignore my second point of today's first post. However, you can use the method we both provided for getting the buffer capacity, if you know the average demand and extra capacity of the CHP.

 

[pretendfarmer]

Paduk,

If you have electric boilers you can disregard the following. But if your boilers are gas fired, please read on:

Is the reason you want a buffer tank is to keep your boiler(s) from excessive cycling? Do you have gas fired copper finned tube boiler(s)? If so, copper finned boilers are notorious for requiring a buffer tank because of the low water volume in the boiler. Without additional water volume the copper finned boiler will cycle on and off every few minutes. Is your system serving 2 way control valves? 2 way valves make the cycling even worse at low load. Typically copper finned boilers require primary-secondary piping when serving 2 way valves. On low system load requirements the boilers will cycle off on high limit because the heat has nowhere to go but back to the boilers. Is the boiler control single stage (on-off), two stage (high-low), or more? Typically copper finned boilers can be as high as 4 stages of fire.

The amount of buffer tank volume to keep the boilers from cycling is drastically different from the hot water storage system tank volumes calculated above. Depending on the number of boilers and the number of stages of fire of those boilers, the tank size can be reduced significantly. A tank as small as a few hundred gallons will reduce the cycling of a boiler plant significantly because you only have to provide enough volume to correspond with the lowest fire stage of the boiler plant. Example: If you have a boiler plant that is a total of (3) 2,000,000 BTUH output boilers and each boiler is a 4 stage boiler, then the buffer tank only has to be sized for 500,000 BTUH (the lowest fire situation). Then all you have to decide is how long you want the boilers off for in between cycles (10-15-20-30 minutes, whatever) and size your tank accordingly.

I have attached a pdf for a buffer tank that you may find helpful if you are working with copper finned (or any other low water volume) boilers. It walks you through the entire sizing procedure. Good luck.

 

[ChrisConley]

My understanding was that the system was a CHP (Combined Heat and Power) system.