Hospital air. Air purification and pressure gradient in hospital rooms


It is required that proper air purity is maintained in hospital rooms. Such rooms are classified by allowable impurities level. How to ensure the appropriate cleanliness class? Filters in ventilation units and pressure gradation in relation to adjacent rooms are responsible for air purification in hospitals.

Hospital air, i.e. pressure gradient in clean and dirty rooms

Increasing pressure in clean rooms – where very high air quality is of utmost importance – means that after the door is open, the air does not flow directly inside but is pushed out by the higher pressure. Additionally, higher pressure protects from dirty air inflow through leaks. The air supplied to the room must first pass through filters.

Polluted air in dirty rooms

The opposite operation is required in dirty rooms. This means that after decreasing the pressure contaminated air does not flow out of the room, but is directed to filters by the exhaust device. An air lock may be used between the corridor and the hospital room with corrected pressure to additionally increase air tightness.

How to ensure proper pressure in a room?

There are two ways to ensure proper pressure in a room. The first one is based on pressure regulators. The disadvantage of such a solution is the fact that you cannot control the number of air exchange cycles. Hence, it is possible that in order to fulfil the condition of proper pressure in the room it will be necessary to provide less than the required number of air exchange cycles. Additionally, with varied ventilation system operation, the system becomes more susceptible to maladjustment.

Better system performance thanks to VAV regulators

In order to provide better performance parameters, it is best to use a system based on VAV regulators. Differential pressure in rooms is achieved by correcting the balance of air supply relative to air exhaust or air exhaust relative to air supply. To this end, you can apply a theoretical method or use a PID regulator.

Pressure correction dependence

Theoretical corrections can be divided into those based on percentage or fixed values. Fixed value correction consists in selecting fixed flow difference between the air supply and air exhaust balance. If night reduction of pressure is applied in a room, it is preferable to use the percentage correction, because with lower flow it is necessary to decrease the balance difference to achieve the same differential pressure. Therefore, it is a better solution to make the pressure correction conditional on flow setting.

How to ensure stable system operation?

The last adjustment method is based on a PID regulator. Such a solution ensures stable system operation regardless of pressure changes in the duct. It is a definite advantage compared to theoretical flow control. If the flow to adjacent rooms is increased, it may be necessary to widen the angle of VAV regulators’ opening. The recommended differential pressure between two adjacent rooms should be from 10 Pa to 20 Pa.

Pressure control

In order to control the pressure you may use light or acoustic signals triggered after exceeding the dead zone of the set point. Information about incorrect pressure is sent to a relay which may turn on the information beacon. In ventilation systems, air turbulences that interfere with pressure readings may occur. The current pressure value may fluctuate around the set point, provided it falls into the dead zone. Usually, it is 10% of the set point. Such a control method will ensure that the alarm will not be triggered unnecessarily.

Pressure readout

In ventilation systems, air turbulences that interfere with pressure readings may occur. The current pressure value may fluctuate around the set point, provided it falls into the dead zone. Usually, it is 10% of the set point. Such a control method will ensure that the alarm will not be triggered unnecessarily.

Zbigniew Kowalski

Automation Engineer

Clean air in hospitals and things to bear in mind during installation

In clean rooms, ventilation pipes and ducts should be installed in such a way that they do not hinder access to elements that require cleaning or replacement. Clearances must be kept to allow service hatch opening and filter replacement. In rooms with air gradient, attention must be paid to proper quality of construction works. There should be no openings between the rooms which would allow the flow of significant amounts of unfiltered air. A properly sealed door with a small clearance (or a seal) between the door and the floor play a vital role. Apart from proper room workmanship, appropriate operation is essential. Users must remember not lo leave the door or windows ajar.

How to avoid a sudden drop in pressure decrease?

Opening the room door will cause sudden pressure decrease, because the air will begin to flow from the room with higher pressure to the space with lower pressure. Because of this, the air from the protected atmosphere room will push out the contaminated air, preventing its flow into the room. The pressure gradient system should provide for prompt recovery of the set point. For theoretical control method, this will be the time resulting from the fact that the correction size ensures mainly compensation of air that flows out of the room, and now it must additionally increase the pressure from a value close to 0 Pa to about 20 Pa. In the case of a PID regulator, the flow will be higher before adjustment.

Which regulator to choose for flow correction?

Pressure adjustment may be executed on air exhaust or air supply. The decision which regulator to choose for flow correction in order to obtain a certain pressure value should depend on whether negative pressure or overpressure is required in the room. From the technical point of view, the most advantageous solution is to achieve overpressure with air supply increase and negative pressure with air exhaust increase. With such a control method, regulators always have to increase flows in order to achieve the required pressure value, thus fulfilling the condition of minimal number of air exchange cycles. The opposite control method is also possible, where the operation of the air handling unit is less intensive, however in this case the flow may be too small. In less complex systems, one VAV regulator and one CAV regulator are used instead of two VAV regulators.

Quick response to pressure changes

The speed of response to pressure change is an important parameter. Regulators may be equipped with quick actuators (run time of about 3 s) or slow actuators (run time of about 150 s). If you wish to ensure appropriate room protection with high air flow speed, it is best to use quick actuators at the door. Additionally, if there are other devices equipped with air exhausts in the rooms, the ventilation system should monitor flow in these devices. This will further increase the adjustment speed, because additional amount of exhausted air will be automatically included in the balance. Lack of such monitoring will make the system begin adjusting the flow on the regulator only after the pressure fall is detected by the relay. In the case of slow actuators, the adjustment time will be longer, which may force installation of a quarantine airlock at the room entrance.

How to shorten the time of adjusting the flow?

The time needed for air flow adjustment may be shortened by using a door reed contact. After opening the door, the actuator will be paused until it is closed again. After closing the door the regulator will have the same set point as before opening. It may only slightly increase the flow to restore the balance – it does not need to adjust the pressure starting from zero. It is also possible to use a virtual reed contact. In such a solution, when the controller detects sudden large pressure drop, it will also pause the actuator as a reed contact would. Actuator pause time should be set during system commissioning.

Three air filters that ensure air quality in hospitals and laboratories

Another element that plays an important role in maintaining cleanliness are air filters. Non-woven fabric filters are used very frequently. Three basic filter groups can be distinguished, starting from the least effective: EPA, HEPA and ULPA.

The role of electrostatic purifiers

Electrostatic purifiers constitute the second type of air filters. They use electric current to neutralize contaminants. Air supplied to such a device is passed through the preliminary filter which removes larger dirt particles. In the electrostatic cartridge, the high-voltage current causes ionization of dirt particles. Negatively charged particles are then attracted to collector plates and neutralized. The resultant deposit is biologically neutral. Because of their properties, filters of this type cause low flow resistance. Apart from the benefits of lower energy loss, lower air resistance results in less noise generated by the installation.

Dirt in the filter reduces the quality of filtration

To maintain high purification parameters, the non-woven filter elements must be replaced or electrostatic filter cartridges must be cleaned. A dirty non-woven filter element will reduce filtration effectiveness and, at the same time, increase flow resistance and thus noise and loss of energy. If a pressure switch with relay output is installed, it is possible to check if the filter requires replacement. For electrostatic filters, the soiling of the filter is monitored by an electronic circuit.

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