Danfoss VFS 2 Gegevensblad - Pagina 6

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Data sheet

Control valve sizing diagram

for steam

(continued)

6 | VD.HC.L3.02

VFS 2

1 For saturated steam

Design data:

Flow rate: 1000 kg/h

Absolute inlet pressure: 5 bar (500 kPa)

- follow dashed line -

The absolute inlet pressure is 500 kPa. 40% of

this is 200 kPa.

Locate the diagonal line corresponding to the

pressure drop of 200 kPa (line A-A).

Read the absolute inlet pressure on the lower left

hand scale (point B), and draw a horizontal line

across until it meets the pressure drop diagonal

(A-A) at point C.

From this point extend a vertical line upwards

until it meets the horizontal line representing

the steam flow of 1000 kg/h from point D. The

intersection of this is point E.

The nearest diagonal kvs line above this is line

F-F with a k

of 25 (point E'). If the ideal valve size

is not available the next largest size should be

selected to ensure design flow.

The pressure drop through valve at the flow rate

is found by the intersection of the 1000 kg/h

line with F-F (point E') and dropping a vertical;

this actually hits the horizontal line for 500 kPa

(point E'') inlet pressure at a pressure drop

diagonal of 70 kPa. This is only 14 % of the inlet

pressure and the control quality will not be

good until the valve has partially closed. As with

all steam valves this compromise is necessary

since the next smaller valve would not pass the

required flow (maximum flow would have been

about 600 kg/h).

The maximum flow for same inlet pressure

is found by extending the vertical line (C-E)

through point E until it crosses the k

(point E''') and reading off the flow (1700 kg/h).

2 For superheated steam

Design data:

Flow rate: 500 kg/h

Absolute inlet pressure: 5 bar (500 kPa)

Steam temperature: 190 °C

The procedure for superheated steam is much

the same as for saturated steam, but uses a

different flow scale which slightly elevates the

readings according to the degree of superheat.

- follow dotted line -

As before, the diagonal pressure drop line

A-A is located as before for 40 % of 500

(200 kg/h). The horizontal inlet pressure line

through point B is now extended to the left to

read off the corresponding saturated steam

temperature at point G (150 °C). The difference

between the saturated steam temperature and

the superheated steam temperature is

190 °C - 150 °C = 40 °C.

The superheated steam flow is found on the

upper right hand scale, point H, and the diagonal

line is followed down from here until it meets

a vertical line from the steam temperature

elevation (40 °C) at point J.

As before, the horizontal line through point B is

drawn to cut line A-A at point C and the point

where the vertical line from this point meets the

horizontal line from point J is the operating point

(point K). This horizontal line, J-K, is the corrected

flow line. The nearest diagonal line above this is

for k

10 (point K'). A vertical line dropped from

the intersection of J-K with the 10 k

the 500 kPa inlet pressure line (point K'') at a

pressure drop diagonal of about 150 kPa. This

is about 30% of the inlet pressure which will

give reasonable control quality (compared to

recommended ratio of 40%).

25 line F-F

line intersects