Thermal Comfort(2)

Comfort his the absence of discomfort. People usually feel discomfort when they are too cold or too hot or when the air is odorous or stale

Thermal Comfort varies with;

i. Activities engaged in,
ii. The body’s ability to adjust to range of ambient and iii.Individual metabolism,

Thermal condition in an enclosed space are usually controlled to ensure;

Health and comfort of occupants (comfort conditioning)

The proper functioning of sensitive electronic equipment(Process Conditioning)

The conditions required for human comfort and the machineries varies that is why you may feel discomfort in a computer lab.
The specifications are available from the manufacturers and in ASHRAE Handbook.

After setting the conditions for the machineries, acceptable comfort should be provided for the occupants
Humans have the capacity to adapt but our productivity does vary.

Benefits of thermal Comfort in a Thermal Environment

i. Fewer Accidents Lower rates of absenteeism and employee turnover ii.Reduced health hazards such as respiratory illness
iii. attentiveness and fewer errors
iv. Increased productivity and improved quality of Products and Services



A refrigerant is a substance or mixture, usually a fluid, used in a heat pump and refrigeration cycle. In most cycles it undergoes phase transitions from a liquid to a gas and back again. It is a Refrigerating  fluid used for heat transfer in a refrigerating system which absorbs heat at a low temperature and low pressure of the fluid and transfers heat at a higher temperature and higher pressure of the fluid, usually involving changes of state of the fluid

Properties of Refrigerant.

1) The refrigerant should have low boiling point and low freezing point.

2) It must have low specific heat and high latent heat. Because high specific heat decreases the refrigerating effect per kg of refrigerant and high latent heat at low temperature increases the refrigerating effect per kg of refrigerant.

3) The pressures required to be maintained in the evaporator and condenser should be low enough to reduce the material cost and must be positive to avoid leakage of air into the system.

4) It must have high critical pressure and temperature to avoid large power requirements.

5) It should have low specific volume to reduce the size of the compressor.


Work is the energy transferred to or from an object as the result of the action of a force.

Positive work is a transfer of energy to the object, and negative work is a transfer of energy from the object.

Work = Force x Displacement

The unit of work is the joule*, J, which is defined as the amount of work done when a force of 1 newton acts for a distance of 1 m in the direction of the force. Thus, 1 J = 1 N m

NOTE: If the body moves in the same direction as the force the angle is 0 degree, then;

                    Workdone = F x d

Where d is the distance.

When the angle is 90 then the work done is zero.


If a graph is plotted of experimental values of force(on the y-axis) against distance moved (on the x-axis), the graph is called force-distance graph or work diagram.

The area under the graph represents the work done.

A constant force F, used to raised  a load of distance d, can be represented by a force – distance graph shown below;

Force-distance graph 

Similarly, a spring extended by distance, d by a force F, can be represented by a force – distance graph as shown below;

force-distance graph

Dimensions 1

Any physical quantity can be characterized by dimensions. The magnitudes assigned to the dimensions are called units.

The system of units used in Engineering is the Systeme International d’Unites also known as International System of Units and is based on the metric system.

A unit has to have a special name and symbol. The usefulness of a unit is  as a means of communicating to everyone who does science how it was particularly defined.

Are classified into two types:

  • Basic Units
  • Derived Units

BASIC UNITS:  Basic units are those units from which other units are derived. Some basic dimensions such as mass m, length L, time t, and temperature T are referred to as Basic dimensions.. Basic units are also known as primary or fundalmental units.

NOTE: Length, Mass, Time are basic quantities, while their units(meter, kilogram, seconds) are basic units.





The Force transmitted across any section, divided by the area of that section, is called intensity of stress or stress. Forces acting on a material cause a change in dimensions and the material is said to be in a state of stress.


Stresses which are normal to the plane on which they act are called direct stresses and  either tensile or compressive

stress for

The symbol  σ is called SIGMA


Strain is a measure of the measure of the deformation produced in the member by the load. The fractional change in a dimension of a material produced by a force is called the strain. For a tensile or compressive force, the strain is the ratio of the change of length to the original length. The symbol used for the strain is ε (Greek epsilon).  Tensile strain will be positive While compressive strain will be negative.

NOTE: Strain is dimensionless and is often expressed as a percentage

strain for


Humidity Control(2)


A humidifier is a device that increases humidity (moisture) in a single room or an entire building.  A humidifier works by adding water vapor to the indoor air. Humidifiers accomplish this by one of the following four methods or variations/combinations of producing vapor from the water.

NOTE: Humidifiers can also be attached to the ductwork.


Some types of humidifiers are:

1.Central Humidifiers;

2.Portable Humidifiers.


These types o humidifiers are attached to the ductwork and add moisture to the airspace before it leaves the supply grill to the space to be conditioned.

They can be installed ;

1.Via bypass ductwork

2.Directly into the return or supply ductwork

Some types of central humidifiers are;

BYPASS HUMIDIFIER: Just as the name suggests, it is used to bypass air flowing from the supply air plenum. Not all the conditioned air follows this bypass route. The air that is bypass is humidified.


EVAPORATIVE HUMIDIFIERS: This type of humidifier add moisture to the air through evaporation. It contains an element that is kept damp.

Some of the moisture in the element  evaporates as air is forced through the humidifiers thereby increasing the humidity of the air.

The types of evaporative humidifiers are;

Plate humidifiers

Fixed filter humidifiers

Rotating drum humidifiers(Used for restricted spaces)

Rotating disk humidifiers


Portable humidifiers are standalone units that are placed in the spaces where the humidity level is to be maintained. They are used for room control humidity and not building humidity control.

Many portable humidifiers add moisture by circulating air across a wet filter, though some units ultrasonic humidification.

Portable Humidifiers

Portable humidifiers are easy to maintain. The water tank and the wet filter can be removed and cleaning or replaced depending on the conditioned.

It uses a float-operated switch that turns on an indicator light when the water level in the water pan gets too low. Most of the units must be refilled manually.