Refrigeration & Air-Conditioning

Lesson 29. Air conditioning systems and numerical

Module 7. Psychrometry

Lesson 29
AIR CONDITIONING SYSTEM AND NUMERICAL

29.1 Introduction

As discussed earlier, air conditioning refers the control of environmental condition of the air in terms of temperature, humidity, distribution of air and purity of air depending on the use of the air conditioning.

The design of air conditioning system is very difficult task as it involves the knowledge of variation of environmental conditions in different seasons, psychrometric processes, design of air handling system, load calculations, economic considerations etc. One type of system designed for specific place may not be suitable at some another place.

29.2 Requirement for Comfort Air Conditioning

The following are the important requirements for the comfort feeling of occupant.

29.2.1 Oxygen level

The human body requires about 0.65 m³ of O₂ per hour under normal conditions and produces 0.2 m³ of CO₂. The concentration of CO₂ in atmospheric air is nearly 0.6%. The partial pressure of oxygen gets reduced when CO₂ in air exceeds above 2% creates breathing problem. The CO₂ level around 6% creates extreme discomfort and unconsciousness occurs at 10% CO₂ level. Therefore, it is necessary to maintain the level of CO₂ by replacement of air from the air conditioning system.

29.2.2 Heat removal

The human body converts thermal energy into mechanical work with 20% efficiency and remaining amount of heat appears as heat to the atmosphere. Therefore, it is necessary provide sufficient air circulation to carry away the heat dissipated by the occupant to prevent rise in temperature of the space.

29.2.3 Moisture control

The loss of moisture from human body is nearly 50 g per hour under rest condition. The removal of vapour from the air is important to provide comfortable conditions in the air conditioned room. The majority of people feel comfortable at 60-65% relative humidity in the air. As the humidity increases, the rate of evaporation will also decrease. The control of humidity can be achieved by installing dehumidifier in the system.

29.2.4 Air distribution

The velocity of air affects the heat transfer from the body. If air temperature is lower than body temperature, increased velocity improves the comfort feeling but reverse action takes place when air temperature is higher than body temperature. It is recommended that the air velocity should not be more than 6 to 9 m/min at 20 °C and 9 to 15 m/min at 22 °C. Uniform air distribution at recommended air velocity is also very essential for comfort requirement in the air conditioned space.

29.2.5 Quality of air

The air supplied in the air conditioned space should be free from odour, dust, toxic gases, smoke and bacteria to prevent harmful effect on the human health. These can be eliminated by using various types of air purification equipment. Air filters are being used to remove dust, dirt, lint etc. from the air. Supply of quality air in the air conditioned space is the main requirement of centralized air conditioning system. High Efficient Particulate Air Filters (HEPA), Ultra Low Penetration Air Filters (ULPA), Activated Carbon Filters and Plasma Air Purifiers are employed in air conditioning system of advanced countries.

29.3 Design of Air Conditioning System

Air conditioning system is defined as an assembly of different components (heating, humidifier, dehumidifier, cooling etc) used to produce required condition of air within a required space or building.

The air conditioning systems are mainly classified as (i) Central air conditioning system (ii) Unitary air conditioning system.

The central air conditioning system consists of several components grouped together in one central room and conditioned air is distributed in air conditioned rooms using air carrying duct. The central air conditioning system requires the following components.

(i) Sensible cooling coil

(ii) Cooling & dehumidifying coil

(iii) Heating coil

(iv) Air cleaning equipment

(v) Humidifier

(vi) Blower and motor

(vii) Control devices

A block diagram indicating the principle of air conditioning system is shown in Fig. 29.1.

Fig. 29.1 Block diagram of air conditioning system

The condition of air passing through the room follows path along the room SHR (or SHF) line depending on the Q_s (Sensible heat gain) and Q_l (latent heat gain). The condition of air entering and leaving the conditioned room is shown as ‘a’ and ‘b’ respectively on the psychrometric chart in Fig. 29.2.

Fig. 29.2 SHR of air conditioning system

The system air conditioning system, total heat removed is equal to sensible heat removed and latent heat removed. Thus,

Q_s+Q_l = m (h_b-h_a)

Where, m= flow rate of air, kg dry air/h

SHR = Q_s / (Q_s - Q_l)

High value of SHR (or SHF) indicates high sensible heat load. The capacity of various components of the system can be estimated based on the initial quality of air, rate of air recirculation, SHF etc. Psychrometric chart gives clear idea for the selection of different components of the air conditioning system after locating initial air quality and required comfort condition.

An air conditioning system with cooling and dehumidifying is shown in the Fig. 29.2 and corresponding psychrometric chart indicating the conditions of the air is shown in Fig. 29.3.

Fig. 29.3 Air conditioning system with mixing air

Fig. 29.4 Air conditioning system on psychrometic chart

In the system, fresh air is mixed with the exhaust air. The proportion of the mixing depends on the quality of the air streams. The condition 4 is the mixing of air at condition 2 and 3. The condition 5 is the condition of air leaving the cooling and dehumidifying coil and 5-1 represents the heating of air passing through blower due to friction. The process 1-2 represents the condition of air passing through the air conditioned room depending on the RSHF of the process.

The year round air conditioning system consists of cooling, heating, humidifying, dehumidifying components. The operation of these components is regulated to achieve required conform conditions of air.

29.4 Numerical

29.4.1 A psychrometer reads dbt of 40 °C and wbt of 28 °C. Calculate the following.

1. Specific humidity

2. Relative humidity

3. Dew point temperature

4. Enthalpy of the air

Solution

Using the Apjohn equation

[Note: Atmospheric pressure = 1.01325 bar, 1 bar= 1.0197 kg/cm²]

29.4.2 Atmospheric air at 15 °C dbt and 80% RH is heated to 22 °C dbt in steam coil heater at the rate of 100 m_­³ per minute. Find

1. Heat added per minute

2. RH of the heated air

Solution

29.4.3 Ambient air at the rate of 50 m³/min at 25 °C dbt and 50% RH is mixed with another air stream having dbt of 40 °C and wbt of 29 °C at the rate of 100 m³/min. Find the dbt and wbt of the mixture (Use psychrometric chart).

Fig. 29.5 Psychrometric chart

29.4.4 Ambient air at 35 °C dbt and 24 °C wbt is heated to 200 °C in an indirect steam coil

heater. The flow rate of air is 10 m³/s. Find the capacity of heating oil in kW (use psychrometric chart).

Solution:

29.4.5 Ambient air at dbt of 35 °C and wbt of 24 °C is passed through an adiabatic humidifier having efficiency of 90%. Find the dbt and specific humidity of resultant air.

Solution

From psychrometric chart, air at 35 °C dbt and wbt= 24 °C

Fig. 29.6 Pdychrometric chart

29.4.6 A class room is to be air-conditioned for 60 students when outdoor conditions are 30 °C dbt and 75% RH. The quantity of air supply is 0.5 m³/min/person. Find the following.

a) Capacity of the cooling coil in TR

b) Capacity of heating Coil in kW

c) Amount of water removed by dehumidifier

d) By-pass factor of the heating coil, if the surface temperature of the heating coil is 25 °C.

The required comfort conditions are 20 °C dbt and 60% RH. The air is conditioned first by cooling & dehumidifying and then by sensible heating.

Solution

The points 1 and 2 are located on the psychrometric chart as their conditions are known as shown in figure below.

The process 1-2-3 represents sensible cooling and dehumidifying and process 3-2 represents sensible heating. Point 3 is located by drawing horizontal line through point 2 which cuts the saturation line at point 3.

From the psychrometric chart,

Fig. 29.7 Pdychrometric chart

29.4.7 A class room of 60 seating capacity is to be air conditioned. The out door conditions are 32 °C dbt and 22 °C dbt. The required comfort conditions are 22 °C dbt and 55% RH. The quality of air required is 0.5 m³/min/student. The required conditions are achieved first by chemical dehumidifying and then by sensible cooling of the air. Find

1. dbt of the air leaving chemical dehumidifier

2. Capacity of dehumidifier

3. Capacity of cooling coil in ton.

4. Surface temperature of the cooling coil, if the by pass factor of the cooling coil is 0.3.

Fig. 29.8 Pdychrometric chart