14
D6.3.4/0715-0716/E
3.2
Pressure safety controls
3.2.1 High-pressure control
A high-pressure control with a maximum cut-out setting of 28.8 bar(g) is required.
The high-pressure cut-out should have a manual reset feature for the highest level of system
protection.
3.2.2 Low-pressure control
The normal minimum is 0.1 bar(g) for R404A.
The low-pressure cut-out should have a manual reset feature for the highest level of system
protection.
3.2.3 Internal pressure relief valve
Single-stage compressors (50 Hz) with a displacement
≥ 50m
3
/hr are equipped with an internal
pressure relief valve placed between suction chamber and discharge chamber. This valve
protects the compressor against bursting if the discharge shut-off valve is accidentally fully
closed.
On two-stage compressors this valve is placed between inter-stage and low-pressure side and
opens at ≈ 15 bar (see
Figure 9
).
NOTE:
The valve will not protect the installation against dangerous system pressures!
Before operating the compressors the pressure switches and other safety devices must be
installed correctly. Maximum allowable pressures must not be exceeded.
For connecting a high-
pressure switch there is a tapped hole with plug in 1/8” – 27 NPTF on
each cylinder head.
Figure 9
Figure 10
3.2.4 Oil pressure switch
The oil pressure switch breaks the control circuit when the pressure difference between the oil
pump outlet and the crankcase is too low. The switch must be properly adjusted and tamper
proof. If the oil differential pressure drops below the minimum acceptable value the compressor
will be stopped after a 120-second delay. After having solved the problem the control has to be
reset manually.
NOTE:
Proper oil pressure safety control with an approved switch is a condition of
warranty!
Specifications for electro-mechanical oil pressure switches are as follows:
Cut-out pressure:
0.63 ± 0.14 bar
Cut-in pressure:
0.9 ± 0.1 bar
Time delay:
120 ± 15 sec.
The Alco Control FD-113-ZU mechanical oil pressure switch operates with the above set points.
