Sizing According To Continuous Output; Determining The Required Dhw Cylinder, Example 1 (With Constant Flow Temperatures) - Viessmann Vitocell 100-H Technical Manual

Sizing
(cont.)
V = Required minimum volume of the heating water buffer
P
cylinder
´ = Calculated peak flow rate of the freshwater module
t = Time, during which the peak flow rate is required – the
value can be based on e.g. the time taken to fill the bath,
information provided by the user, or the standard value to
DIN 4708 (10 min).
(T /T ) = for the temperature spread between heating water buffer
P WW
cylinder and DHW:
0.5 = when temperature spread is high (e.g. 90/45 °C)
0.7 = when temperature spread is medium
(e.g. 70/45 °C)
1.0 = when temperature spread is low (e.g. 55/45 °C)
s = Safety factor for consideration of user behaviour:
N
1 = normal draw-off pauses
2 = short draw-off pauses
3 ... 4 = very short draw-off pauses
Example:
A buffer cylinder is to be selected for the detached house in the exam-
ple on page 21 (chapter "Determining the DHW demand").

4.3 Sizing according to continuous output

Sizing according to continuous output is employed in cases where hot
water is to be continuously drawn off from the DHW cylinder. This cal-
culation method is therefore mainly used for commercial applica-
tions.
4

Determining the required DHW cylinder, example 1 (with constant flow temperatures)

Preconditions:
■ Continuous output in l/h or kW
■ DHW outlet temperature in ºC
■ Cold water inlet temperature in ºC
■ Heating water flow temperature in ºC
The required DHW cylinders (capacity and number), the heating water
flow rate and the head of the circulation pump for cylinder heating are
calculated using the DHW cylinder "specification".
The DHW cylinder is sized in the same way.
The following example illustrates the calculating process.
See
Example:
For production purposes, a factory requires 4100 l/h DHW at 60 ºC.
Boilers deliver a heating water flow temperature of 90 ºC. The cold
water inlet temperature is 10 ºC.
■ Continuous output = 4100 l/h
■ DHW outlet temperature = 60 ºC
■ Cold water inlet temperature = 10 ºC
■ Heating water flow temperature = 90 ºC
■ Required cylinder type: Stainless steel, vertical
Calculating the number and size of the DHW cylinders
Procedure:
1. Select Vitocell 300-V, type EVI
2. Refer to the specification for cylinder banks in the Vitocell 300-V
datasheet.
3. In the table, find the line for "Continuous output from 10 to 60 ºC"
and Heating water flow temperature "90 ºC".
4. In the column Cylinder capacity = 500 l and Number of cylinders =
3, a continuous output of 4179 l/h is specified.
Selected DHW cylinders:
3 × Vitocell 300-V (type EVI), each with 500 l capacity.
The continuous output of the selected DHW cylinders must be at least
equal to the required continuous output.
VIESMANN
22
The peak flow rate is 29 l/min.
The future system user has indicated that he "enjoys long showers".
He has indicated a demand duration of 15 min.
For reasons of energy efficiency, the storage temperature in the buffer
cylinder should be no more than 70 °C.
The draw-off temperature is 60 °C.
This results in a low temperature spread of 70/60 °C, giving a correc-
tion factor of 1.
As the future system user has indicated that he "enjoys long showers",
short draw-off pauses have been assumed. Therefore, the
safety factor s is 2.
N
The minimum buffer volume V
P
V = ´ x t x (T /T ) x s
P P WW N
= 29 l/min x 15 min x 1 x 2
= 870 l
According to the datasheet, a Vitocell 100-E with a volume of
950 litres is selected.
Calculating heating water flow rate
A heating output of 243 kW must be made available for the calculated
continuous output (see "Specification", table "Specification" in the
datasheet for the DHW cylinder). The required heating water flow rate
is stated in the table column for the selected DHW cylinder - heating
water flow rate = 19.5 m 3 /h, i.e. size the circulation pump for cylinder
heating for a heating water flow rate of 19.5 m
Calculating the pressure drop on the heating water side
The total flow rate of 19.5 m 3 /h must be taken into account for the
heating water flow and return lines (valves, bends etc.) as well as the
boiler when calculating the pressure drop in the complete system.
Where several cylinders are connected in parallel, the total pressure
drop is equal to the pressure drop of an individual cylinder. The pres-
sure drop of the DHW cylinder on the heating water side for the head
of the circulation pump for cylinder heating is calculated as follows:
As the 3 cylinders are connected in parallel, each cylinder has a heat-
3
ing water flow rate of 6.5 m /h (see following diagram). Refer to dia-
gram Pressure drop on the heating water side in the datasheet for
Vitocell 300-V (type EVI). For a heating water throughput of 6500 l/h,
a pressure drop of 400 mbar (40 kPa) can be read off the straight line
of the cylinder with a capacity of 500 l.
is therefore:
3
/h.
DHW heating