MAJOR BLOCKS OF A DIALYSIS MACHINE

MAJOR BLOCKS WITH VITAL AND COMPLEX FUNCTIONS IN THE HYDRAULIC CIRCUIT.

INTRODUCTION.

The blocks in a hydraulic circuit with different functions helps in formulating a dialysate and delivery of the same to the dialyzer. These blocks are very essential to the working of a hydraulic circuit, without the blocks the dialysis treatment is rendered almost too impossible to undertake.

There are SIX main blocks in a hydraulic circuit with various multi-functions that ensure the easy flow of the dialysate to and from the dialyzer. These include:

      1.      HEATER BLOCK  #66a #66b #66c (please refer the above flow diagram)

      2.      MULTIFUNCTION BLOCK #88a #88b #88c

      3.      MIXING CHAMBER #205

      4.      BALANCING CHAMBER #68

      5.      HEAT EXCHANGER #77

1. HEATER BLOCK #66

The heater block just like any other heater allows pressure regulated RO water to be preheated before flowing to the multifunction block.

The heater block is divided into three main chambers:

                    i. Water Inflow chamber #66a

                  ii.  Heater rod chamber #66b

                iii.   Float switch chamber #66c

Diagram showing heater block Fresenius 2008k AND 4008S machine.

Heater Block (Top View)

Heater block (bottom view)

OPERATION.

As soon as the water inlet valve #41 opens, the water from RO flows through the pressure reducing valve #61. This regulator protects incoming water valve #41 from excess incoming water pressure approx. 20psi/1.0bars 0.9 to 1.4 Bars of pressure. The water then proceeds into the water inflow chamber #66a of the heater block and across the heat exchanger.

HEAT EXCHANGER-The heat exchanger is the energy regenerative part of this machine in that it conserves energy in this case heat from the spent fluid and exhumes off/dissipates off the heat to the incoming water from the water inflow chamber. This is so as to not overwork the heater rod chamber. The heat exchanger consists of two compartments the incoming waterside and the spent dialysate side. The heated spent fluid on its way to the drain passes through the heat exchanger and warms the conductive plates which warm the incoming water as it passes through.

The water with some heat then flows into the heater rod chamber #66b to be preheated further. In the chamber is a heater rod#54 of 240V. While it is rising, the fluid is warmed up to the pre-set physiological temperature near/equal 37-degree Celcius. The heater rod is controlled by a temperature sensor #2.

From the heater rod chamber #66b, the fluid flows to the float switch chamber #66c of the heater block. The float switch #5 controls the solenoid valve #41 thus ensuring the correct fluid levels. The float switch consists of a bob that moves vertically up and down on a shaft. A magnet is embedded in the bob and a reed switch is embedded near the bottom of the shaft .As the water level decreases, the bob drops until it reaches the reed switch. The signal monitored by the sensor board informs the actuator board to open valve #41.

Still housed in the heater block is a vent tubing #92 that prevents pressure from building up in chambers #66b and 66#c by creating a vacuum. In the hot rinse mode, the developing vapor escapes through the vent tubing #92. It also helps in removing the dissolved gases released at the primary air separator.

Diagram showing heat exchanger and conducting plates Fresenius 4008s.

Heat Exchanger

COMMON FAULTS YOU MAY EXPERIENCE WITH THE HEATER BLOCK.

1. Float switch faulty corroded.

2. Solenoid valves 41 and 61 clogged/blocked.

3. Reed switch not making contacts.

4. Heater rod corroded

5. Faulty thermostat #2

ALARMS

No flow alarm

Low/high temperature alarm

Cyclic error

2. MULTIFUNCTION BLOCK #88

The purpose of the multifunction is to remove air/dissolved gases first in the water and then secondly to the used dialysate. Water heated to physiological temperatures is subjected to a negative pressure created by the degassing pump to remove any air. Like the heater block, the multifunction block has three chambers as well:

1. Water Inflow chamber #88a

2. Primary air separator chamber #88b

3. Secondary air separator chamber #88c

OPERATION

The Degassing pump #29 draws the heated water via a degassing orifice #89...the orifice restricts water so that water flows in a streamline. The orifice will allow water to be pressurized and removal of air. This generates a negative pressure of - 0.8bars.

The degassing orifice #89, degassing pump#29, the moderate temperature of the water, water inflow chamber#88a, and the loop helps in degassing the water to required levels suitable for dialysis.

Via the degassing pump #29, the water and released air are tangentially/indirectly directed into the primary air separator #88b where the air bubbles and the airless water are separated. The air accumulated at the top of the chamber 88b then together with the recirculation flow and via the loading pressure valve #65 as well as chambers 66c all help air escape into the atmosphere through a vent#92.

At the bottom of the chamber 88c, the degassed heated water is pressed out into the mixing chamber #205…

NB;  The secondary air separator#88c in the multifunction block removes air from the spent dialysate before being drained to the balancing chamber. Here we have the deaerator pump and level sensor #6. The deaerator pump removes air from the spent dialysate/fluid before it’s fed to the balancing chamber. The level sensor 6 will activate the deaerator pump to eliminate any air and raise the level of fluid. Only airless dialysate is always delivered into the balancing chamber #68 any presence of air bubbles in the balancing chamber would cause balancing errors, flow alarms, cyclic pht error.

Diagram showing the Multifunction block in  Fresenius 4008s machine.

3. MIXING CHAMBER #205.

The degassed heated water is now pressed out into the mixing chamber #205. The mixing chamber inputs degassed water, air-free acid concentrate and air free bicarbonate solution with a stipulated ratio for correct proportioning 1 part of A 1.83 part of B and 34 of water.

#94 and #95 concentrate suction tubes…#71 and #72 are their filters.202 and 204 Level sensors, 201 and 203 air separators for both A and B parts 23 and 25 concentrate pumps acid and bicarb pumps respectively. All these with the help of their respective pumps help to draw the concentrate from the carboy jugs.

A solution called dialysate is hence formulated and is ready for volumetric control in the balancing chamber.

Diagram showing the mixing chamber in Fresenius 20008s machine.

4. BALANCING CHAMBER #35

The balancing chamber system ensures that equal amounts of fluid enter and leave the dialyzer 60ml to be precise. This leads to an exact balancing of the dialysate and in conjunction with the ultrafiltration pump #22 a controlled volumetric ultrafiltration. The Balancing chamber has two chambers each having a membrane separating the spent dialysate A1 A2 and the fresh dialysate F1 F2.These work with a specific valve cycle command that ensures exact delivery of dialysate to and from the dialyzer.

The balancing chamber has 8valves #31-#38 that cycle twice opening and closing 4 valves apart to achieve volumetric control and in turn correct ultrafiltration.  Each time the chamber is changed over, all valves are closed to approx. 150ms(dead time).

If any of the diaphragm from balancing chamber ruptures machine automatically gives this alarm.

a) Cyclic PHT error

b) Balancing error

The main advantage of having a closed hydraulic system is to get very precise volumetric ultrafiltration control.

PRINCIPLE OF OPERATION OF A BALANCING CHAMBER THE VALVES CYCLE.

Diagram: shows cycle 1 and cycle 2

1^st Cycle CLOSED VALVES: 31, 34, 36 and 37

  Ø  F1 is filled with fresh dialysate

  Ø  A1 used dialysate is discharged into the drain.

  Ø  A2 is filled with used dialysate.

  Ø  F2   fresh dialysate is forced into the dialyzer.

2^nd Cycle CLOSED VALVES: 32, 33, 35 and 38

  Ø  A1 is filled with used dialysate

  Ø  F1 solution is forced into the dialyzer

  Ø  F2 is filled with fresh dialysate

  Ø  A2 used dialysate is discharged into the drain

NB: This system ensures that equal amounts of the dialysate enter and exit the dialyzer. This leads to an exact balancing of the dialysate and in conjunction with the ultrafiltration pump, a controlled volumetric ultrafiltration. The Balancing Chamber best describes efficient volumetric control relied on to perform dialysate fluid delivery to the patient and from the patient.

Diagram of a real balancing chamber Fresenius 2008k machine.

SHUNT DOOR INTERLOCK SWITCHES.

These are three micro switches located under the shunt door that sense the position of dialyzer Hansen connectors. If the shunt door is open, machine goes into bypass (valve 26 opens and valves 24a and 24b closes)

Both dialyzer Hansen connectors must be in the shunt door and the door closed to enter a cleaning program in order to complete the circuit.

Diagram of shunt interlock switches Fresenius 4008s machine.

DRAIN VALVE 87.

After all is said and done, our spent/used dialysate with substances often removed from patients’ blood goes to the heat exchanger to exchange heat to the incoming water and then to the drain where it is dissipated off via a conduit pipe color-coded black and valve #87 which is a drain valve. The drain pipe should be a full grade material and should have a free space just at the end to avoid any bacteria rebound.

BRIEF SUMMARY OF THE HYDRAULIC CIRCUIT.

Description of the hydraulic unit

As soon as the inlet valve (41) opens, the water flows through the pressure reducing valve (61) into the chamber (66a) of the heater block and across the heat exchanger (77) into the heater rod chamber (66b).

The concentrate pump (23) admixes concentrate to the inflowing water per balancing chamber phase.

The vent tubing prevents pressure from building up in chambers b and c. 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 (54). The heater is controlled by the temperature sensor (2).

From chamber b, the dialysate flows into the chamber (66c). Incorporated in this chamber is a float switch (5), which controls the solenoid valve (41), thus ensuring the correct fluid level.

The degassing pump (29) draws in the dialysate via the degassing orifice (89). This generates a negative pressure of approx. 0.8 bar.

In the lines and the following chamber (88a), the dialysate is degassed to a level which is sufficient for hemodialysis.

Via the degassing pump (29), dialysate and released air are directed tangentially into the primary air separator (88c), where air bubbles and the airless dialysate are separated. The air accumulates at the top of the chamber (88c). Then, together with the recirculation flow and via the loading pressure valve (65) as well as the chamber (66c), the air escapes into the atmosphere.

Chamber 88c is provided with a separating disc (standard hydraulics only), which serves to prevent bicarbonate, if added, from being recirculated via the heater rod chamber (66b).

At the bottom of chamber 88c, the degassed dialysate is pressed out and into the balancing chamber (68) by means of the loading pressure.

Together with the eight solenoid valves (31 to 38), the balancing chamber (68) constitutes the balancing system. Each of the two sections of the balancing chamber comprises two compartments separated by an elastic membrane each. Hence, there are two chambers with four spaces:

– F1 and F2:  fresh fluid

– A1 and A2: waste fluid (used)

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 pump (21), which receives this pulse 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).

From the balancing chamber, the dialysate flows through the conductivity cell (7) with integrated temperature sensor (3). The measured conductivity values are indicated on the monitor in ms/cm, related to 25 °C.

The temperature sensor (3) has the following functions:

– Temperature compensation of the conductivity display,

– Indication of the dialysate temperature.

Should the actual values (temperature or conductivity) of the dialysate exceed or fall below the limit settings, the bypass valve (26) opens, and the dialyzer valve (24) is closed. The system is now in the bypass mode. The dialysate is discharged into the drain not via the dialyzer, but via the secondary air separator (88b) and the balancing chamber (68).

If the actual conductivity and temperature values of the dialysate are within the set limits, the dialyzer valve (24) opens. The valve (26) is closed. The dialysate flows to the dialyzer.

After the dialyzer, the dialysate which is now loaded with the substances usually eliminated with the urine flows into the secondary air separator (88b) via a filter (73), the valve (24b) and the blood leak detector (8). The secondary air separator (88b) comprises the pressure transducer (9) and the level sensor (6).

With a hematocrit of 0.25, blood losses of 0.5 ml per minute are recognized in the dialysate by the blood leak detector.

Together with the venous back pressure, the signal of the pressure transducer (9) is evaluated and indicated on the monitor as TMP. The fluid level in the secondary air separator (88b) is monitored by the level sensor (6). Due to the secondary air separator (88b), only airless dialysate is always delivered into the balancing chamber (68). Any presence of air bubbles in the balancing chamber (68) would cause balancing errors.

The dialysate is pressed into the balancing chamber (68) by the flow pump (21). As mentioned above, the balancing chamber valves are reversed by the current rise pulses of the drive motor of the flow pump. Using the speed of this pump, the dialysate flow can be adjusted in the dialysis program: 300, 500, and 800 ml/min. In the cleaning programs, the flow of the dialysate is fixed.  The relief valve (78) is used to limit the pressure of the flow pump before the balancing chamber to approx. 2 bar.

After the balancing chamber, the dialysate flows through the valve (30), the heat exchanger (77) and the valve (87) into the drain.

The valves (86) and (87) serve to recirculate fluid during the hot rinsing and disinfection programs.