Psychrometry

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Process Engineering Guide:
GBHE-PEG-DRY-003

Psychrometry

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Process Engineering Guide:

Psychrometry

CONTENTS

SECTION

0

INTRODUCTION / PURPOSE

2

1

SCOPE

2

2

FIELD OF APPLICATION

2

3

DEFINITIONS

2

4

PSYCHROMETRIC CHARTS

3

5

EXAMPLE CALCULATION

3

6

CHARTS FOR SPECIFIC SYSTEMS

4

7

BIBLIOGRAPHY

4

FIGURES
1

2

GROSVENOR CHART (Humidity vs. Temperature)
FOR AIR-WATER VAPOR AT 1.0133 bar

5

MOLLIER CHART (Enthalpy vs. Humidity) FOR
NITROGEN-TOLUENE VAPOUR AT 100 kPa

6


0

INTRODUCTION/PURPOSE

Psychrometry is concerned with the science of solvent vapor/gas mixtures.
Where the solvent is water, psychrometry is often called hygrometry. The
usefulness of psychrometry in the area of drying is that it gives the relationship
between the Enthalpy of a vapor/gas mixture and the composition of that mixture,
where the maximum concentration of the vapor in the gas is limited by its
saturation vapor pressure at a given temperature.
Psychrometry therefore permits calculation of the change in solvent vapor
concentration, gas temperature, limiting gas volume, heat input etc. during a
drying operation, essential for the proper design of a drying system.
The science is normally used in the form of two types of psychrometric chart (see
Clause 4).

1

SCOPE

This Guide explains the use of psychrometric charts and provides an example
calculation.

2

FIELD OF APPLICATION

This Guide applies to process engineers in GBH Enterprises worldwide.

3

DEFINITIONS

For the purposes of this Guide, the following definitions apply:
SPS

The Separation Processes Service (SPS) is a research and
consultancy organization, based in the UK. It is active in the
main operations related to separation, including
comprehensive coverage of drying

Mollier Chart

A plot of Enthalpy versus solvent vapor concentration (i.e.,
humidity for water).


Grosvenor
Chart

A plot of solvent vapor concentration
(humidity for water) versus temperature.

Dry Bulb
Temperature

The temperature of a gas.

Wet Bulb

The steady state temperature reached by a small amount of
liquid evaporating into a large amount of unsaturated
vapor/gas mixture.

Dew-point
Temperature

The temperature at which the vapor pressure of the liquid
vapor in the gas is equal to the saturation vapor pressure
of the liquid. Taken in practice to be the temperature at
which condensation of the liquid begins to occur on cooling
the gas mixture at constant pressure.

Absolute
Humidity

The mass of liquid vapor per unit volume of the gas
mixture, measured e.g. in kg/m3.

Relative
Humidity

The ratio of the actual vapor pressure of the liquid to the
saturation vapor pressure over a plane surface of the liquid
at the same temperature, expressed as a percentage.

Enthalpy

The heat content of the material on a relative basis, usually
to a reference temperature of 0°C, and equal to the product
of the temperature, T, and mean specific heat at constant
pressure of the material over the temperature range 0 to
T°C, normally evaluated at T/2. Since the specific heat of dry
air is approximately 1 kJ/kg/K, its Enthalpy in kJ/kg is
numerically equal to its temperature in °C.

With the exception of terms used as proper nouns or titles, those terms with initial
capital letters which appear in this document and are not defined above are
defined in the Glossary of Engineering Terms.


4

PSYCHROMETRIC CHARTS

The two types of psychrometric charts are the Grosvenor Chart, shown in Figure
1 for air-water, and the Mollier Chart, shown in Figure 2 for nitrogen-toluene.
In the Grosvenor Chart the sloping dotted lines join points of equal Wet Bulb
Temperature and also Enthalpy, the slopes of which happen to coincide for the
air-water system. The value of the Wet Bulb Temperature is given by that
temperature at which the dotted line intersects the curve of 100% Relative
Humidity.
The Mollier Chart is more complex. In the example shown in Figure 2 the Y-axis
is humidity. Temperature is shown along the X-axis, but the isotherms slope
slightly to the right of vertical. The Enthalpy is shown by a series of solid lines
sloping downwards from left to right, the values being shown on the right hand Yaxis, continuing on to the X-axis below the temperature values. The dotted lines
extending down from left to right are points of equal Wet Bulb Temperature,
their values again being those temperatures at which the intersect with the 100%
Relative Humidity curve.

Note:
In this system the wet bulb isotherms have a different slope from the lines of
constant Enthalpy.
These charts can be used to calculate one of the following, given any two of the
others:
(a)

Dry Bulb Temperature.

(b)

Wet Bulb Temperature.

(c)

Dew-point Temperature.

(d)

Absolute Humidity.

(e)

Relative Humidity.

(f)

Enthalpy.


5

EXAMPLE CALCULATION

Take the Take the case where air at 27°C and 40% Relative Humidity is heated
indirectly to 130°C. This hot air is then contacted with a water-wet product and
exits the dryer at 50°C. To avoid the risk of condensation immediately
downstream of the dryer, it was decided that for this example the exit air should
have a Relative Humidity no greater than 60% at 50°C, and that the maximum
weight of water that could be evaporated per unit weight of air was to be
determined.
Examining the Grosvenor Chart in Figure 1, start at point A and heat the air to
point B. The air's Relative Humidity is now very low, much less than 2%. The air
now loses heat in evaporating water and its Relative Humidity increases.
Assuming the cooling is adiabatic, the changing conditions of the air are shown
by a line parallel with the sloping dotted lines shown in the Figure. It is seen that
at point C, the temperature of the air is 323 K (50°C) and its Relative Humidity is
about 50%. The water that can be evaporated per kg of air can be read off the
X-axis, being the difference between points C and B, i.e. 0.042 - 0.009, = 0.033
kg water per kg air. If the moist air now loses heat to its surroundings,
condensation of water will occur at the dew point, D, at a temperature of 311 K
(38°C).
In this way, the minimum quantity of hot air required to dry a charge of wet
material, or the minimum flow rate of hot air in a continuous dryer, can be
estimated.
The effect of mixing of gases with different temperatures and Relative Humidifies
can also be found from the charts by joining the points corresponding to the
conditions of the two gases by a straight line and finding the conditions for the
mixture on that line using the lever rule.

6

CHARTS FOR SPECIFIC SYSTEMS

The charts for any particular solvent/gas system vary with atmospheric pressure,
and appropriately modified charts have to be used for those cases where the
prevailing pressure is different from normal, e.g. in designing for plants at high
altitude. Grosvenor Charts and Mollier


For further details on psychrometry and the use of psychrometric charts see
Refs. [1] and [2]. Ref. [1] in particular contains numerous worked examples. Ref.
[3] is a users guide to the SPS software.

7

BIBLIOGRAPHY

[1]

Perry R H, Green D W; Perry's Chemical Engineers Handbook (6th
Edition), Chapter 12 (1984)

[2]

Coulson J M, Richardson J F; Chemical Engineering (4th Edition) Vol 1:
Fluid Flow, Heat Transfer and Mass Transfer, Chapter 11 (1990)


FIGURE 1

GROSVENOR CHART (Humidity vs. Temperature)
FOR AIR-WATER VAPOR AT 1.0133 bar


FIGURE 2

MOLLIER CHART (Enthalpy vs. Humidity) FOR
NITROGEN-TOLUENE VAPOUR AT 100 kPa


Psychrometry

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