Mine Ventilation

Psychrometry & Mine Climate

Dry-bulb, wet-bulb, relative humidity and moisture content — quantifying the hot, humid deep-mine atmosphere and the body's cooling power.

PART 1

Topic Breakdown & Traps

The Engineering Principle

Deep workings are hot and humid because of auto-compression, strata heat and machinery. The thermal stress on a miner depends not on temperature alone but on the air's ability to remove body heat — driven by dry-bulb temperature, humidity and air velocity. Psychrometry is the bookkeeping of moist air: it links the dry-bulb temperature, the wet-bulb temperature (which falls as evaporative cooling occurs), the relative humidity (how close the air is to saturation) and the moisture (humidity) content (mass of water vapour per unit mass of dry air). Saturated air can hold no more vapour, so evaporative cooling — the body's main defence — stops, which is why high humidity is so dangerous underground.

The Core Formula Matrix

Relative humidity (ratio of actual to saturation vapour pressure at the dry-bulb temperature):

R H = \frac{p _{v}}{p _{s}} \times 100%

p_{v}

= actual vapour pressure,

p_{s}

= saturation vapour pressure at dry-bulb

[kPa]

.

Moisture (humidity) content — mass of water vapour per kg of dry air:

W = 0.622 \frac{p _{v}}{P - p _{v}} [kg/kg dry air]

P

= barometric pressure

[kPa]

. The factor 0.622 is the ratio of molar masses (water 18 / dry air 29).

From RH back to vapour pressure:

p_{v} = \frac{R H}{100} p_{s}

.

Multiply

W

by 1000 to quote in g/kg.

Schematic psychrometric relationship: at a fixed dry-bulb temperature, the actual vapour pressure pᵥ sits below the saturation pressure pₛ; their ratio is the relative humidity.

The ‘IIT Traps’

⚠
RH uses pressures, not temperatures. $R H = p_{v} / p_{s}$ at the *same* (dry-bulb) temperature. Dividing wet-bulb by dry-bulb temperature is meaningless.
⚠
**Use $P - p_{v}$ in the denominator, not $P$ .** Moisture content is referenced to *dry* air, so the partial pressure of vapour is subtracted from the total. Using $P$ alone slightly under-estimates $W$ .
⚠
Keep pressures in the same unit. Mixing kPa and Pa (a factor of 1000) wrecks both $R H$ and $W$ . Convert everything first.
⚠
Saturation ≠ 100 % comfort. At RH = 100 % evaporative (latent) cooling stops entirely; a modest dry-bulb temperature can still be hazardous — temperature alone is not the criterion.

PART 2

Progressive 3-Tier Question Suite

Q1BASIC1 Mark · MCQ

At a certain dry-bulb temperature the saturation vapour pressure is

p_{s} = 2.8 kPa

and the actual vapour pressure is

p_{v} = 1.4 kPa

. The relative humidity is:

Q2MEDIUM2 Marks · NAT

Mine air at a barometric pressure of

P = 100 kPa

has an actual vapour pressure

p_{v} = 2.0 kPa

. The moisture content of the air is ______ g of water per kg of dry air. (Round off to two decimal places.)

g/kg

Q3HARD2 Marks · NAT

At a working face the dry-bulb temperature gives a saturation vapour pressure

p_{s} = 4.24 kPa

, the relative humidity is

70%

and the barometric pressure is

P = 101 kPa

. The moisture content of this air is ______ g/kg of dry air. (Round off to two decimal places.)

g/kg