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Chapter 11: Functional description
11.1.4.2 Description of the hydraulic unit
11-30
As soon as the water inlet valve V41 opens, the water flows through the
pressure reducing valve A61 into the water inlet chamber H66a of the
heater block H66 and across the heat exchanger H77 into the heater
rod chamber H66b.
The concentrate pump P23 admixes the concentrate to the inflowing
water per balancing chamber phase.
The vent tubing prevents pressure from building up in chambers H66b
and H66c. In the HOT RINSE mode, the developing vapor can escape
through the vent tubing.
While it is rising, the fluid is warmed up to the preset temperature by the
heater H54. The heater is controlled by the temperature sensor PT2.
From chamber H66b, the dialysate flows into the chamber H66c.
Incorporated in this chamber is the float switch S5, which controls the
solenoid valve V41, thus ensuring the correct fluid level.
The degassing pump P29 draws in the dialysate via the degassing
orifice H89. This generates a negative pressure of approx. 0.8 bar.
In the lines and the following degassing chamber H88a, the dialysate is
degassed to a level which is sufficient for hemodialysis.
Via the degassing pump P29, dialysate and released air are directed
tangentially into the primary air separator H88c, where air bubbles and
the airless dialysate are separated. The air accumulates at the top of the
chamber H88c. Then, together with the recirculation flow and via the
loading pressure valve A65 as well as the chamber H66c, the air
escapes into the atmosphere.
Chamber H88c is provided with a separating disc (standard hydraulics
only), which serves to prevent bicarbonate, if added, from being
recirculated via the heater rod chamber H66b.
At the bottom of chamber H88c, the degassed dialysate is pressed out
and into the balancing chamber H68 by means of the loading pressure.
Together with the eight solenoid valves V31 to V38, the balancing
chamber H68 constitutes the balancing system. Each of the two
sections of the balancing chamber comprises of two compartments
separated by an elastic membrane each. Hence, there are two
chambers with four spaces:
– F1 and F2: fresh solution
– A1 and A2: used solution (waste)
As soon as one of the chambers (A1 or A2) is filled with dialysate, the
solenoid valves are reversed in groups of four. The valves are reversed
by the electronic evaluation of the current rise pulse of the drive motor
of the flow pump P21; this pulse is created upon membrane abutment.
Within the filling phase, F1 or F2 is filled with fresh dialysate by means
of the loading pressure. In order to obtain a continuous flow, a second
chamber is switched parallel to the first chamber. The second chamber
is operated at an inverse sequence.
Each time the chamber is changed over (maximum deflection of the
membrane), all valves are closed for approx. 100 ms (dead time).
Fresenius Medical Care 4008 S (Version V10) SM-EN 7/01.13
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