Laws of vaporization

Molecules of a liquid have a mutual attraction for each other (a phenomenon called cohesion), which is sufficiently great for them to remain in close proximity. But they also possess varying degrees of kinetic energy and are in constant motion, colliding with each other. If the liquid has a surface exposed to air or other gasses, or to a vacuum, some molecules with a high kinetic energy will escape from this surface, resulting in the process of evaporation or vaporization. The molecules from the liquid, which exist in the gaseous phase, are known collectively as a vapour. This vapour exerts a pressure on its surroundings, which is referred to as vapour pressure. If the space above the liquid is enclosed, some of the molecules that have escaped while moving freely in the gaseous state will collide with the surface of the liquid and re-enter it. Eventually, there will occur an equilibrium in which the number of molecules re-entering the liquid equals the number leaving it. At this stage the vapour pressure is at a maximum for the temperature of the liquid and so is called the saturated vapour pressure (SVP).

Factors affecting vaporization of a liquid

Temperature

Vaporization is increased if the temperature of the liquid is raised, since more molecules will have been given sufficient kinetic energy to escape. Fig. 3.1 shows the vapour pressure curves of volatile anaesthetic agents (as well as water) and shows how they vary with temperature. If the liquid is heated, a point is reached at which vaporization now occurs not only at the surface of the liquid, but also in vapour bubbles that develop within its substance. The liquid is now boiling and this temperature is its boiling point. At this temperature, the SVP of the liquid is equal to the ambient atmospheric pressure.

Figure 3.1 Vapour pressure curves for anaesthetic agents.

The boiling point of a liquid may therefore vary with atmospheric pressure. At high altitudes (where the air is thinner, has a lower ambient pressure and therefore exerts less pressure on the surface of a liquid) there is a significant depression of the boiling point. This may render the administration of agents with low boiling points, such as ether, difficult. Fig. 3.2 shows the depression of the boiling point for water with change in atmospheric pressure.

Figure 3.2 Variation of boiling point of water with atmospheric pressure or altitude.

Vaporizing systems

The various organic liquids that possess anaesthetic properties are too potent to be used as pure vapours and so are diluted in a carrier gas such as air and/or oxygen, or, nitrous oxide and oxygen. The device that allows vaporization of the liquid anaesthetic agent and its subsequent admixture with a carrier gas for administration to a patient is called a vaporizer.

A modern vaporizer needs to be constructed so that it provides a suitable, stable and predictable concentration of anaesthetic vapour for admixture with other patient gasses. The delivered amount of vapour must not be affected by changes in temperature, and flow rates of other gasses.

Types of vaporizer

Appropriate vaporization may be achieved by either:

• splitting the patient gas flow so that only a portion passes through the vaporizer. This picks up saturated vapour and then leaves to mix with the remainder of the gas that has gone through a bypass. The final concentration may be altered by varying the splitting ratio between bypass gas flow and vaporizer gas flow, using an adjustable valve. This type is often referred to as a variable bypass vaporizer (Fig. 3.3); or

• alternatively, the vaporizer can be constructed so that it heats the anaesthetic agent to a temperature above its boiling point (in order that it may behave as a gas) and which can then be metered into the fresh gas flow (Fig. 3.4A). Similarly, a vaporizer may contain a fine metal sieve that is submerged in the anaesthetic agent and through which a small independent and metered gas supply (normally oxygen) can be made to pass. The minute bubbles produced have a very large surface area and produce a saturated vapour at ambient pressure, which can then be passed into the fresh gas flow (Fig. 3.4B). These types of vaporizer are often referred to as measured flow vaporizers.

Figure 3.3 A schematic diagram of a variable bypass vaporizer. A flow splitting valve that can be rotated to alter the relative diameters of the vaporizer and bypass channels and so vary the flows through them. L, liquid; C, vaporizer chamber.

Figure 3.4 Schematic diagrams of measured flow vaporizers. A. H, electric heater; L, liquid anaesthetic agent; C, vaporizer chamber; V₁, V₂, flow control valves; E, electronic valve controller to proportion flows. B. L, liquid anaesthetic agent; C, vaporizer chamber; V₁, V₂, flow control valves.

It should also be noted that the various anaesthetic inhalational agents currently available have widely differing potencies and physical properties and hence require devices constructed specifically for each agent. Very potent agents (halothane, enflurane, sevoflurane, isoflurane and desflurane) require vaporizers that can accurately control the concentration of vapour leaving the vaporizer. However, agents such as diethyl ether, with a lower potency, may be used safely with simpler apparatus (if necessary), in which the vapour concentration is not accurately known, since there is less risk of over-dosage (see Chapter 27).

Variable bypass vaporizers

Design features

Surface area of contact between carrier gas and the liquid

Vaporizers, which are required to be very accurate, should always present a saturated vapour to the bypass gas across a wide range of flows. To ensure sufficient vaporization at the highest planned flow, a sufficiently large surface area of liquid should be present. This size is also governed by the volatility of the agent used. A highly volatile liquid will require a smaller surface area. The surface area for vaporization of a liquid can be increased by causing it to spread (by capillarity) over a large sheet of porous material which may be folded in such a way that the carrier gas passes across its entire surface (Fig. 3.5).

Figure 3.5 Vapour pick-up in vaporizers. A. Carrier gas passing over a small surface area of liquid anaesthetic agent. B. The surface area for vaporization is increased by porous wicks dipped into the liquid. The carrier gas is also made to pass close to the surface of the liquid by the baffles, thus increasing vapour pick-up.

Temperature

As vaporization progresses, the vaporizing liquid as well as the vaporizer cools and the quantity of vapour produced decreases. In an attempt to retain the performance of the device, the temperature drop is minimized or prevented by the incorporation of a heat source (heat sink). This normally takes the form of a water bath or substantial metal jacket or even a heating element surrounding the vaporizing liquid. Metal jackets and water baths, however, can only transfer a finite quantity of heat and so only minimize the inevitable fall in temperature.

In order to maintain the expected output of the vaporizer when this occurs, a greater proportion of carrier gas is required to pass through the vaporizing chamber in order to collect sufficient vapour molecules. This is achieved by using devices that are sensitive to changes in temperature (temperature-compensating devices, Fig. 3.6) and which then increase the flow through the vaporizing chamber.

Figure 3.6 Temperature-compensating devices. A. The bi-metallic strip in a vaporizer bypass operating at ambient temperature. B. The same device operating at a cooler temperature; the bi-metallic strip has moved closer to the inflow increasing the resistance in the bypass and increasing the amount of gas passing through the vaporizing chamber and therefore increasing vapour pick-up. C. A bi-metallic arrangement that works on a similar principle. The inner rod is made of Invar, a relatively non-expansile metal. The outer jacket is in contact with the vaporizing liquid and is made of an expansile metal (brass). D. When this contracts (with cooling) it drags the choke on the inner rod into the bypass, increasing the resistance to flow through it.

Two types are commonly used:

1. The first (Figs 3.6A and B) consists of two dissimilar metals or alloys placed back to back (i.e. a bi-metallic strip). As the two metals have different rates of expansion and contraction with temperature, the device has the ability to ‘bend’. It can, therefore, be used to vary the degree of occlusion in the aperture of a gas channel (usually the bypass) and thus alter the flow of carrier gasses through it.

2. In the second arrangement (Figs 3.6C and D), the bi-metallic device consists of a central rod made of Invar, a metal alloy with a low coefficient of expansion, sitting inside a brass jacket, the top part of which is attached to the roof of the vaporizing chamber. The rod is attached only at the base of the brass jacket, which has a higher coefficient of expansion. The outer surface of the jacket is immersed in liquid anaesthetic agent in the vaporizing chamber. As the aforementioned liquid cools, the brass jacket contracts more than the Invar, which is pushed upwards into the bypass, restricting the flow of bypass gas and increasing the flow of carrier gas through the vaporizing chamber.