Data Sheet for Product
Technical Instructions MVS661..N Series Modulating Refrigerant Valves
Document Number 155-404 for Ammonia (R717) and Safety Refrigerants
May 11, 2016
Page 16 Siemens Industry, Inc.
MVS661…
as a hot-gas valve
The control valve throttles the capacity of a compressor stage. The hot gas passes
directly to the evaporator, and allows capacity control in the range from 100% down to
approximately 0%.
Indirect hot-gas bypass
application
Figure 9. Indirect Hot-Gas Bypass
Application.
Suitable for use in large refrigeration
systems in air conditioning plant, to
prevent unacceptable temperature
fluctuations between the compressor
stages.
Application example
With low loads, the evaporating and condensing pressures can fluctuate depending on
the type of pressure control. In such cases, evaporating pressure increases and
condensing pressure decreases. Due to the reduction in differential pressure across the
fully open valve, the volumetric flow rate will drop – the valve is undersized. This is why
the effective pressures must be taken into account when sizing the valve for low loads.
Refrigerant R507; 3 compressor stages; Q
0
= 75 kW; t
0
= 39°F (4°C); t
c
= 104°F (40°C).
Part load Q
0
per stage = 28 kW; t
0
= 39°F (4°C); t
c
= 73.4°F (23°C).
KH-R507
t
0
= 32°F
(0°C)
t
0
= 50°F
(10°C)
Interpolation at
t
c
= 73.4°F
(23°C)
t
c
= 68°F (20°C)
14.4
9.0
14.4 + [(22.4 – 14.4) × (23 - 20)/(40 - 20)]
15.6
t
c
= 73°F (23°C)
15.6
11.0
t
c
= 104°F (40°C)
22.4
22.0
9.0 + [(22.0 – 9.0) × (23 - 20)/(40 - 20)]
11.0
Interpolation at
t
0
= 39°F
(4°C)
15.6 + [(11.0 – 15.6) × (4 - 0) (10 - 0)]
13.8
k
vs
theoretical = 28 kW/13.8 = 2.03 m
3
/h (C
v
= k
vs
× 1.156 = 2.03 × 1.156 = 2.35)
Valve MVS661.25-2.5 is suitable, since: C
V
= k
vs
× 1.156 = 2.03 × 1.56 = 2.35
(2.03 m
3
/h / 2.5 m
3
/h × 100% = 81% [> 50%])
Direct hot-gas bypass
application
The control valve throttles the capacity of one compressor stage. The gas is fed to the
suction side of the compressor and then cooled using a reinjection valve. Capacity
control ranges from 100% down to approximately 10%.
Figure 10. Direct Hot-Gas Bypass
Application.
Suitable for large refrigeration systems in
air conditioning applications with several
compressors or compressor stages, and
where the evaporator and compressor are
some distance apart (attention must be
paid to the oil return).