Physical Principles of Vaporizers PART -1.pptx

Physical principles

1. 2024. 3. 13. 1 PHYSICS FOR - vaporizers

2. 3/13/24 2 Dr. Rakesh Chintalapudi Associate Professor – Anaesthesia

3. 2024. 3. 13. 3 Physical Principles for Vaporizers

4. 2024. 3. 13. 4 Physical principles Anesthesiologists should understand the basic physical principles and how they will influence the design of the vaporizer. This will help the anesthesiologists to select the vaporizer well-suited in his practice and to use them safely, economically and to maintain them in optimum working condition. 

5. 2024. 3. 13. 5 1 Vapor & Vapor Pressure 2 Saturated Vapor Pressure 3 Critical Temperature 4 Latent Heat of Vaporization 5 Specific Heat 6 Thermal Conductivity

6. 2024. 3. 13. 6 Naakenti ? Naakenti ?

7. 2024. 3. 13. 7 7 Heat Capacity – Thermal Cap 8 Boiling Point 9 Partial Pressure 10 Volume percent

8. Vapor & Vapor Pressure 2024. 3. 13. 8 1

9. 2024. 3. 13. 9 Physical Principles for Vaporizers Vapor Pressure In the range of pressures and temperatures in which a substance may exist in both liquid and gaseous forms, there is a constant flux of molecules between these two phases. Molecules with high kinetic energy will overcome the tight inter-molecular forces of the liquid phase and enter the gaseous phase, i.e. evaporation. Molecules in the gaseous phase have high kinetic energy and exert a pressure if confined in a closed container, the vapor pressure. At equilibrium (i.e. constant temperature and volume) this is the Saturated Vapor Pressure (SVP) at that temperature. Vapor Pressure or Saturated Vapor Pressure

10. 2024. 3. 13. 10 Physical Principles for Vaporizers SVP is dependent on temperature and independent of the ambient pressure. If SVP equals ambient pressure, the substance will rapidly vaporize and “boil”. The temperature at which this occurs is the boiling point. The boiling point is thus dependent on ambient pressure and will decrease with decreasing pressure (e.g. high altitude). The closer the boiling point to room temperature, the more Volatile the substance Example –Desflurane -- Vapor Pressure & Boiling Point

11. 2024. 3. 13. 11 Physical Principles for Vaporizers A gas is a substance above its critical temperature whilst a vapor refers to the gaseous state of a substance below the critical temperature. Strictly speaking, Nitrous Oxide (N2O), Carbon Dioxide (CO2) and Cyclopropane (C3H6) (all delivered in “gas” cylinders) are vapors; but commonly referred to as gases as they are in the gaseous phase at room temperature and ambient pressure, but may be a liquid in the cylinder under pressure. Gases & Vapors  :

12. 2024. 3. 13. 12 Vapor pressure  Molecules escape from a volatile liquid to the vapor phase, creating a "saturated vapor pressure" at equilibrium.  Vapor pressure (VP) increases with temperature . VP is independent of atmospheric pressure, it depends  only  on the physical characteristics of the liquid, and its temperature. So, even although evaporation proceeds at a rate governed by liquid temperature and is independent of altitude (barometric pressure), individual vaporizer types may or may not function the same at altitude. Physical Principles for Vaporizers

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14. 2024. 3. 13. 14 Physical Principles for Vaporizers Gases & Vapors  : The pressure exerted by a vapor on a liquid when they are in equilibrium Vapor Pressure

15. 2024. 3. 13. 15 Physical Principles for Vaporizers If you take a gas, and compress it really hard, the particles that compose it are brought ever so close to each other. As you keep compressing the particles will at some point coalesce and convert the gas into liquid.   However, if the gas is above a certain temperature, called a “critical temperature ”, whatever amount of pressure you apply, that gas will not become a liquid.   This temperature is called “ critical temperature” and every gas has its particular critical temperature.

16. Saturated Vapor Pressure 2024. 3. 13. 16 2

17. 2024. 3. 13. 17 Physical Principles for Vaporizers A substance with a high vapor pressure is said to be  volatile . Examples of volatile substances include gasoline and rubbing alcohol (liquids) and paradichlorobenzene (solid). In an open container, molecules of a liquid that escape as vapor do not strike a container and reach an equilibrium pressure. Instead, vapor molecules evaporate. A nonvolatile liquid has a vapor pressure lower than that of water and only slowly evaporates. A volatile liquid has a high vapor pressure and quickly evaporates.

18. 2024. 3. 13. 18 Physical Principles for Vaporizers AGENT S V P @ 20 deg. C Halothane 243 Enflurane 175 Isoflurane 238 Sevoflurane 157 Desflurane 669 Methoxyflurane 20.3 S V P @ 20 C Physical properties of some gases

19. Critical Temperature 2024. 3. 13. 19 3

20. 2024. 3. 13. 20 Physical Principles for Vaporizers A gas that is currently below its critical temperature is called a “vapor “ If compressed with enough pressure, it will condense into a liquid. A gas that is currently above its critical temperature remains a gas.  However hard you compress it, it will not condense into a liquid. Let us take isoflurane as a example. The critical temperature of isoflurane is about 200 degrees centigrade. Therefore, at room temperature (e.g. 21 degrees centigrade), the gaseous phase of isoflurane would be called “isoflurane vapor “ Now for a moment, let us imagine that you worked on the planet Venus. The surface temperature on Venus is about 500 degrees centigrade.

21. 2024. 3. 13. 21 Physical Principles for Vaporizers Now, because the “˜room temperature on Venus’ (500 C) is higher than the critical temperature of isoflurane (200 C), the gaseous phase of isoflurane would be called “isoflurane gas “ Let us come back to Earth. If all this is confusing you, just remember, on Earth, at room temperature, all the gaseous forms of common anesthetic  agents exist as vapors.

22. 2024. 3. 13. 22 Physical Principles for Vaporizers G A S V A P O R

23. Latent Heat of Vaporization 2024. 3. 13. 23 4

24. 2024. 3. 13. 24 Physical Principles for Vaporizers Specific Latent Heat: This term refers to energy per unit mass that is either absorbed or evolved when a substance changes phases.  Phase change is relevant in anesthesia because the volatile agents transition from the liquid phase to the gas phase.  Phase change

25. 2024. 3. 13. 25 Physical Principles for Vaporizers There are two types of Specific L atent H eat to consider: (a) Latent heat of fusion which is heat given off when a liquid changes into a solid and (b) Latent heat of vaporization which involves heat absorption when a liquid turns into the gas.  The general idea is that a liquid is in a higher energy state compared to solid and that the vapor is and a higher energy state compared to the liquid.  Accordingly, energy (heat) must be provided to induce phase change from solid to liquid as well as from liquid to vapor.

26. 2024. 3. 13. 26 Physical Principles for Vaporizers Evaporation Causes Cooling

27. 2024. 3. 13. 27 Physical Principles for Vaporizers Vaporizers: Output Regulation:  Because the S tandard Vapor P ressures (SVP) of commonly used inhaled anesthetics are in substantial excess compared to anesthetic requirements a dilutional step is needed. H alothane -- SVP: 243 mm Hg; Isoflurane -- SVP: 238 mm Hg; Enflurane ----SVP: 175 mm Hg; Sevoflurane -- SVP: 160 mm Hg . The dilutional step is accomplished by the vaporizer in accord with mixing gas that has bypassed the chamber containing volatile agent with gas that contains saturated vapor 

28. 2024. 3. 13. 28 Physical Principles for Vaporizers Latent Heat of Vaporization.  Energy (heat) is required to promote the vaporization process.  The molecular basis of this requirement is probably be intermolecular forces in the liquid phase that must be overcome in order to promote  molecular transfer to the gas phase.  Formally, the latent heat of vaporization would be the amount of calories (heat) needed to convert a unit mass (e.g. one gram) of the liquid into vapor.   Given the molecular basis that underlies the energy required, that is it's easy to imagine that different molecules might have different degrees of intermolecular forces that need to be overcome, the heat to vaporization volatile anesthetics differ one to the next. 

29. 2024. 3. 13. 29 Physical Principles for Vaporizers Specifically, Halothane, requires 35 cal /g; Isoflurane and Enflurane require 41 cal /g where as methoxyflurane requires 58 cal /g.  Since the vaporization process requires heat, it is not surprising that the vaporization process, which draws heat from the liquid anesthetic itself as well as the container, is a cooling process.  So, as vaporization proceeds the cooling of the liquid anesthetic would tend to retard the vaporization process.  Since vaporization is a temperature dependent process, if no heat is added, vaporizer output would decline.  Accordingly, many vaporizers have temperature compensatory systems to ensure that heat loss due to vaporization is compensa - - ted and therefore does not reduce output.

30. Specific Heat 2024. 3. 13. 30 5

31. 2024. 3. 13. 31 Physical Principles for Vaporizers The specific heat is another parameter of interest and is defined began as calories required to raise the temperature of the unit mass (e.g. one gram) by 1  o C--it is a distinct value--only for one gram going one degree. The technical definition for specific heat is that it is a ratio of the heat capacity of a substance to the capacity of the reference material, typically water.  The heat capacity of water is defined as one calorie per gram per degree Celsius. Here are some examples of specific  heats

32. 2024. 3. 13. 32 Physical Principles for Vaporizers G old: 0.0312  cal / (g deg   C) (at 18  o C) C opper: 0.092 cal / (g  o C) (at 20  o C) or 0.389 J/gm  o C L ead: 0.031 cal /gm  o C or 0.13 J/gm  o C Important in vaporizer design and construction an important characteristic has to do with heat conduction to the  liquid anesthetic.  Since we know that will be a tendency during the vaporization process for a reduction in temperature and that constant outflow would require compensation for this heat loss, thermal conductivity is an important factor. 

33. Thermal Conductivity 2024. 3. 13. 33 6

34. 2024. 3. 13. 34 Physical Principles for Vaporizers Substances with both a high specific heat and high thermal conductivity are thought  best for vaporizer construction.  Copper approximates an ideal substance and more currently stainless-steel and bronze are also used in vaporizer fabrication.  Eg. "Copper Kettle ®" Substances which have large (thermoconductivity values) promote good heat conduction. 

35. 2024. 3. 13. 35 Physical Principles for Vaporizers Material Specific Heat Relative Thermal Conductivity (glass=1) Copper 385 403 Aluminum 897 236 Brass 383 53 Steel 466 25 Glass 840 1 Water 4181 0.6 Air 1012 0.02

36. 2024. 3. 13. 36 Physical Principles for Vaporizers T he ease with which heat transfers through a substance. This is important in vaporizers to allow ambient heat to flow to the vaporizing chamber and maintain a constant temperature. Glass, as used in the Boyle's Bottle, has a higher specific heat than copper, but does not maintain the temperature as well as it has poor thermal conductivity, even with the use of a water bath. Physical properties of materials used in vaporizers

37. Thermal Capacity – Heat Capacity 2024. 3. 13. 37 7

38. 2024. 3. 13. 38 Physical Principles for Vaporizers Thermal capacity is defined as the quantity of heat necessary to produce a unit change of temperature in a unit mass of a material. Thermal conductivity describes the ability of a material to conduct heat, and the specific heat capacity tells how much heat energy is absorbed or released depending on the temperature difference and mass. 

39. 2024. 3. 13. 39 Physical Principles for Vaporizers Specific Heat Capacity The amount of heat energy (Joules) required to raise a kilogram of substance by a 1 kcal kg-1 ºC . The heat capacity of an object is the amount of heat required to raise its temperature by 1 ºC. The heat capacity of a vaporiser is important in maintaining a constant temperature to allow a constant saturated vapor pressure. Vaporizers constructed of material with a high specific heat capacity will be more thermostable than those made of a low specific heat capacity. Physical properties of materials used in vaporizers

40. 2024. 3. 13. 40 Physical Principles for Vaporizers Material Specific Heat Relative Thermal Conductivity (glass=1) Copper 385 403 Aluminum 897 236 Brass 383 53 Steel 466 25 Glass 840 1 Water 4181 0.6 Air 1012 0.02

41. Boiling Point 2024. 3. 13. 41 8

42. 2024. 3. 13. 42 Physical Principles for Vaporizers

43. 2024. 3. 13. 43 Physical Principles for Vaporizers Boiling point is defined as the temperature at which vapor pressure equals atmospheric pressure (760mmHg). Desflurane SVP is almost one atm(760 mm Hg) 1 atm, and and atmospheric pressure is 760 mm Hg, and the Boiling temperature is 23 degrees centigrade, ie . room temperature.

44. 2024. 3. 13. 44 Physical Principles for Vaporizers

45. 2024. 3. 13. 45 Physical Principles for Vaporizers AGENT Boiling point @S L Deg.C Halothane 50.2 Enflurane 56.5 Isoflurane 48.5 Sevoflurane 58.5 Desflurane 23.5 Methoxyflurane 104.8 Boiling Point Physical properties of some gases

46. 2024. 3. 13. 46 Physical Principles for Vaporizers The Critical Temperature of a substance is the maximum temperature above which it cannot be liquefied, irrespective of the pressure exert ed. Critical Pressure is the pressure required to liquefy it (i.e. the vapor pressure) at the critical temperature. Below the Critical Temperature , a substance may exist as a vapor, a liquid or both; depending on the temperature and pressure; as illustr - ated below.

47. 2024. 3. 13. 47 Physical Principles for Vaporizers AGENT Blood Gas partition Coefficient @ 20 C M A C Halothane 3.6 0.75 Enflurane 1.9 1.7 Isoflurane 1.4 1.2 Sevoflurane 0.6 2 Desflurane 0.4 6 Methoxyflurane 12 0.16 Physical properties of some gases Blood Gas Partition Coefficient & MAC values

48. 2024. 3. 13. 48 B G P C Salt =Ether Sand = Sevoflurane

49. 2024. 3. 13. 49 Physical Principles for Vaporizers Substance Critical Temperature ºC Critical Pressure Bar = 100 kPa 02 -119 49.7 N2O 36.5 73 CO2 31.2 73 Xe 16.6 58.4 H2O 374 217 Physical properties of some gases

50. Partial pressure 2024. 3. 13. 50 9

51. Volume Percentage 2024. 3. 13. 51 10

52. 2024. 3. 13. 52 Physical Principles for Vaporizers Traditionally, vapor concentration is expressed in volume percent (vol. %), i.e. unit volumes of vapour in 100 unit volumes of carrier gas Modern vaporizers reflect this on the dial. However, clinical effect is dependent on partial pressure (kPa or mm 'Hg') of the vapour, not the vol. % concentration. Partial pressure is independent of ambient pressure and this assumes importance with vaporizers at altitude and the method used to calibrate them.

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