Heat capacity and Latent Heat

Question 1

Define specific heat capacity

vs.

Define heat capacity

Answer 1

specific heat capacity is the amount of heat required to raise the temperature of 1 kg of a substance 1 kelvin

SI unit of specific heat capacity is:

J/kg.K

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Heat capacity is the amount of heat required to raise the temperature of an object by 1 K (SI unit –> J/K

Heat capacity applies to an object as a whole rather than per kg

Question 2

How can the heat capacity of a vaporizer (made up of multiple different materials) be calculated

Answer 2

Add the capacities of each of the components

Question 3

What is the specific heat capacity of human tissue and what is the heat capacity of a 70 kg person

Answer 3

3.5 kJ/kg.deg C

(multiple by mass for heat capacity) = 245 kJ/deg C

Question 4

A 70 kg patient needs to increase his body temperature by 1 deg C. Shivering increases heat production up to fourfold to 320 W from basal level of 80W. The heat capacity of a 70 kg patient is 245 J/K. How long will it take the patient to increase the body temperature by 1 deg C

Answer 4

245 000 J/deg C (Heat capacity of 70 kg human)
_____________
240 J/s (additional power from shivering W = J/s)

= 1020 seconds = 17 minutes

Question 5

What is the specific heat of water in J and in calories

Answer 5

4.18kJ/kg.Deg C

1 calorie = 4.18 J

So specific heat of water is 1 kcal/kg.Deg C

1 kilocalorie = 1 Calorie (Capital C)

Question 6

Relative to other substances is the specific heat of water high or low? How is this applied practically in anaesthetic practice

Answer 6

Very high

Used in vaporizers to provide a reservoir of heat and so maintain a steadier temperature (Boyle’s bottle vaporizer)

Question 7

The specific heat capacity of blood is 3.6 kJ/kg.Deg C. If 2 kg of blood is transfused at 5 deg C and warmed to 35 deg C inside the patient, how much will this lower the patients body temperature?

Answer 7

3.6 kJ x 2kg x 30 deg C (temp change)
__
kg.deg C

Units cancel leaving kJ only
= 216 kJ

And Heat capacity of human (7oKg) 245 kJ/degC

216 is close to 245 so body temperature can fall by 1 deg C if 2 L of blood at 5 deg C are infused.

Question 8

How and why do the specific heat capacities of gases differ from the other phases? What are the anaesthetic applications of this

Answer 8

Thousands of times smaller. This is because the specific heat is expressed in terms of volume (L) rather than mass (kg). Since gas has minimal mass the energy required to alter its temperature is minimal.

E.g. specific heat air (LITRES NOT KG) = 1.2 J/L.Deg C

vs water specific heat (IN KG) is 4.18 kJ/kg/Deg C

Thousands of times smaller

Minimal energy required to change the temperature of anaesthetic gases

Question 9

Define Specific latent heat

Answer 9

The heat required to convert 1 kg of a substance from one phase to another at a given temperature

SI: J/Kg

Eg 1 kg of water requires 2.26 Megajoules of heat energy to produce 1 kg of steam at 100 deg C

Question 10

Differentiate latent heat of vaporization from latent heat of fusion

Answer 10

Latent heat of vaporisation is the heat required to convert a liquid into a gas

Latent heat of fusion is the heat required to convert a solid into a liquid

Question 11

What is the latent heat of vaporisation for Nitrous Oxide at 36.5 degrees. How does this relate to critical temperature of N2O

Answer 11

0 Mj/Kg
36.5 degrees C is thus the critical temperature of nitrous oxide. Above this temperature, Nitrous Oxide cannot be compressed into a liquid regardless of the amount of pressure applied.

The temperature at which the latent heat of vaporization becomes 0 for N2O corresponds to its critical temperature

Question 12

How does ethyl chloride cool the skin

Answer 12

Ethyl chloride is stored in a pressurised glass vial which allows the ethyl chloride to be stored in its liquid form as at room temperature it is below its critical temperature (187 deg C). A fine jet of the substance is released via a tap onto the skin where it is no longer compressed and quickly vaporises. The latent heat of vaporisation required for this process is taken from the skin’s surface resulting in cooling of the skin as the substance vaporises.

Question 13

How does latent heat of vaporisation effect volatile vaporiser design and function?

Answer 13

Volatile liquids vaporise util they reach equilibrium. At a specific temperature, the partial pressure in the adjacent air at equilibrium is known as the saturated vapour pressure (SVP). However, as the vaporisation occurs, latent heat of vaporisation is consumed by this process resulting in the remaining volatile liquid and the vaporiser container’s temperature to decrease. With decreased temperature, the SVP decreases. Volatile agent vaporisers are designed and calibrated to keep the concentration of vapour constant. These systems are based on thermostatic devices.

Question 14

What happens to the cylinder itself and the gauge pressure of a nitrous oxide cylinder if it is rapidly emptied?

Answer 14

Cylinder

• N2O is stored as a liquid and a vapour inside french blue cylinders (filling ratio 0.75 or 0.67 tropical regions)
• Rapid emptying will cause rapid vaporization of the liquid component inside the cylinder. this process requires latent heat of vaporization which is taken from the cylinder walls and the liquid itself.
• The temperature of the cylinder will cool rapidly and might condense or freeze water vapour in the surrounding air on the outside of the cylinder

Gauge pressure
- As the cylinder is opened the gauge pressure will fall linearly with a constant flow rate. However, once the flow is discontinued, vaporization in the cylinder will continue until the original gauge pressure is observed.
Therefore, until the liquid component in the N2O cylinder is depleted, the gauge pressure does not reflect the quantity of this substance left inside the cylinder. Cylinder’s should be weighed and this subtracted from the Tare (empty) weight.

Question 15

What is a Vacuum Insulated Evaporator (VIE) and how does it work?

Answer 15

its like a “gigantic vacuum flask”

Used for economical storage of large quantities of hospital O2 (versus multiple small cylinders)

O2 must be stored below its critical temperature of -119 deg C. In a VIE, it is stored at -160 deg C with a SVP at this temperature of 7 bar (safety valve).

O2 taken from top must pass through a superheater coil, then through a pressure regulator into the pipeline at 4.1 bar.

However, as O2 vaporises it cools the system (latent heat vaporization consumed). A supplementary source of heat is needed: Pressure-raising vaporizer senses the pressure of O2 leaving the top of the flask and regulates the amount of liquid O2 permitted to enter the pressure raising vaporizer. Low pressure –> more liquid oxygen present outside the vacuum flask in the pressure raising vaporizer. Hence low pressures prompt automatic provision of supplemental heat to the system and high pressures reduces this. This is calibrated and the SVP of O2 is kept at 7 bar at a temp of -160 deg C

Page 112 Kenny and Davis

Question 16

Is a refrigeration unit required for a Vacuum Insulated Evaporator

Answer 16

No. The liquid O2 keeps itself cold due to the regulated loss of the latent heat of vaporisation by the system.