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1.3 Solar Radiation

Solar radiation outside the earth’s atmosphere:

The sun is a large sphere of very hot gases, the heat being generated by various kinds of fusion reactions. Its diameter is 1.39 x 10 ⁶ km while that of the earth is 1.27 x 10 4 km. The mean distance between the two is 1.50 x 10 ⁸ km. Although the sun is large, it subtends an angle of only 32 minutes at the earth’s surface. This is because of the very large distances of separation of the earth and sun. Thus, the beam radiation received from the sun on the earth is almost parallel. The brightness of the sun varies from its center to its edge. However, for engineering calculation, it is customary to assume that the brightness all over the solar disc is uniform.

The “ solar constant” is the rate at which energy is received from the sun on a unit area perpendicular to the rays of the sun at the mean distance of the earth from the sun. The value of the solar constant has been subject of extensive investigations and its standard value is 1353 W/m².

Solar radiation at the earth’s surface:

Solar radiation is received at the earth’s surface after being subjected to the mechanism of attenuation, reflection, and scattering in the earth’s atmosphere. The radiation received without change of direction is called “ Beam Radiation’. The sum of the beam and diffused radiation flux is referred to as “ Total Radiation” or “ Global Radiation”.

Instruments for measuring total radiation and sunshine:

PYRANOMETER:

It is an instrument which measures either global or diffuse radiation over a hemispherical field of view.

Basically the pyranometer consists of a black surface, which heats up when exposed to solar radiation. Its temperature increases until its rate of heat gain by solar radiation equals its rate of heat loss by convection, conduction or re-radiation. The hot junctions of a thermopile are attached to the black surface, while the cold junctions are located in such a way that they do not receive the radiation. As a result, an emf is generated. This emf, which is usually in range of 0 to 10 mV can be read, recorded or integrated over a period of time and is a measure of the global radiation.

SUNSHINE RECORDER:

The duration of bright sunshine in a day is measured by means of sunshine recorder (refer fig: 1.3.2). The sunrays are focused by a glass sphere to a point on a card strip held in a groove in a spherical bowl mounted concentrically with the sphere. Whenever there is a bright sunshine, the image formed is intense enough to burn a spot on the card strip. Through out the day, as the sun moves across the sky, the image moves along the strip. Thus, a burnt trace whose length is proportional to the duration of sunshine is obtained on the strip.

1 Black Surface

2 Glass Domes

3 Guard plate

4 Three leveling screws

5 Mounting Plates

6 Grouted Bolts

7 Platform

2. Refrigeration

2.1 Refrigeration:

Refrigeration is defined as the branch of science that deals with the process of reducing and maintaining the temperature of an s space or material below the temperature of the surrounding.

2.2 Refrigerating load:

The rate at which heat must be removed from the refrigerated space or material in order to produce and maintain the desired temperature condition is called the refrigeration load. It is also called cooling load or heat load.

2.3 Refrigerating effect

The quantity of heat that each unit mass of refrigerant absorbs from the refrigerated space is known as refrigerating effect

2.4 Unit of refrigeration:

The unit of refrigeration is expressed in tons of refrigeration. a ton of refrigeration is the heat absorbed from water at zero degrees Celsius to convert it to ice at the same temperature in 24 hours. It is equivalent to 12560 Kj/hr.

2.5 Refrigeration:

A refrigerator is a device which, operating in a cycle maintains a body at a temperature lower that the temperature of the surrounding.

Let the body A be maintained at temperature ‘ t2 ’ which is lower than ambient temperature ‘ t1 ’. Even though A is insulated there will always be heat leakage ‘ Q2 ‘ into the body from the surrounding by virtue of the temperature difference. In order to maintain the body at A at a constant temperature ‘ t2 ‘, heat is to be removed from the body at the same rate at which heat is leaking into the body. This heat is discharged into the atmosphere, which is a t temperature ‘ t1 ‘ with the expenditure of work ‘ W ‘ in a device operation g a cycle. The device is then called as refrigerator.

Coefficient of performance

[ COP } ref = Designed effect

Work Input

Desired effect = Q2

Work input = W = Q1 – Q2

Therefore, [ COP ] ref = Q2 / W= Q2 / ( Q1 – Q2)

2.6 Types of Refrigerator:

There are mainly two types of refrigeration system. They are:

Vapor compression refrigeration system.
Absorption refrigeration system

I. Vapor Compression Refrigeration System:

A vapor refrigeration compression system plant flow diagram is shown in fig 2.6.1

The vapor compression cycle consists of the following processes. Refer fig: 2.6.2 ( a &b)

Compression:

A reversible adiabatic process 1-2 or 1’ – 2’ either starting with saturated vapor with saturated vapor ( state 1) called dry compression or starting with wet vapor ( state 1’) called wet compression.

Cooling and condensing:A reversible constant pressure process 2-3 first Desuperheater and then condensed ending with saturated liquid. Heat Q1 is transferred out.

Expansion:

An adiabatic throttling process 3-4 for which enthalpy remains unchanged. States 3 and 4 are equilibrium points. Process 3-4 is adiabatic but not isentropic.

Tds = dh – vdp

OR _P2

S4 – S3 = - ∫_P1v dp/T

vapor compression refrigeration plant flow diagram

Evaporation:

A constant pressure reversible process 4–1 which completes the cycle. The refrigerant is throttled by the expansion valve to a pressure, the saturation temperature at this pressure being below the temperature of the surroundings. Heat then flows by virtue of temperature difference from the surrounding, which gets cooled or refrigerated; the evaporator thus produces the cooling or the refrigerating effect.

II. Absorption Refrigerating System.

The absorption refrigeration system is heat-operated unit, which uses a refrigerant that is alternatively absorbed and liberated from the absorbent. Refer fig 2.6.3

Ammonia vapor is vigorously absorbed in water. So when low pressure ammonia vapor from the evaporator comes in contact with the weak solution in the absorber coming from the generator, it is readily absorbed, releasing the latent heat of condensation. The temperature of the solution tends to rise, while the circulating water, absorbing the heat of solution ‘Qa’ and maintaining a constant temperature, cools the absorber. Strong solution rich in ammonia is pumped to the generator when heat ‘ Qg ‘ is supplied from an external source. Since the boiling point of ammonia less that that of water, the ammonia vapor is given off from the aqua ammonia solution at high pressure and the weak solution returns to the absorber through a pressure reducing valve. The heat exchanger pre heats the strong solution and pre cools the weak solution, reducing both the heat to be supplied in the generator ‘Qg ‘ and the heat to be removed in the absorber ‘Qa’ respectively. The NH3 vapor condenses in the condenser, absorbing the heat of evaporation from the surroundings or the brine to be chilled. Thus refrigeration is achieved.

Absorption Refrigeration System

2.7 SIMPLE INTERMITTENTR ABSORPTION REFRIGERATION SYSTEM.

A simple absorption refrigeration unit consists of two containers connected by a pipeline (refer Fig 2.7.1). Container A contains ammonia and container B contains water. A valve V is provided between the two containers.

OPERATION:

When valve V is open, ammonia in container A evaporates and gets absorbed in water in container B. evaporating ammonia takes away the latent heat of evaporation from container A, there by cooling it and ice may be formed around this container by immersing it in a drum of water. Container B, on the other hand must be cooled to remove the heat of absorption. Either air or water may be used for this cooling. After some tine the water in container B gets saturated with ammonia and no more ammonia can be absorbed in it. The valve V is closed. The above process must now be reversed. Container B is heated possibly by focusing sun ready on its blackened bottom using a parabolic reflector to drive out the absorbed ammonia. The de-absorbed ammonia is condensed in container A by cooling the container with water or air passed around it. The valve V is closed after this process of refrigeration of the refrigerant. The water in the container B is now weak in ammonia and is ready to re-absorb ammonia from container A.

2.8 Continuous absorption refrigeration system:

In continuous absorption refrigeration system, the vapor is drawn from the evaporator by absorption into a liquid having high affinity for the refrigerant. The refrigerant is expelled from the solution by the application of heat and its temperature is also increased. This refrigerant in the vapor form is passed to the condenser, where heat is rejected and the refrigerant gets liquefied. This liquid again flows to the evaporator at the reduced pressure and the cycle is completed.( refer Fig 2.8.1)

2.8 Continuous absorption refrigeration system:

Application	Freezer (space required liters)	Volume required (liters)	Critical/ non-critical
Vaccine refrigeration	0-5 l	30 liters	Yes
Institutional & commercial food preservation	50-550 liters	200-1000 liters	Yes
Institutional & commercial drinks, etc. cooling	N/A	200-300 liters	No
Dedicated freezing (institutional or commercial)	500-1000 liters	N/A	Yes
Household refrigeration/freezing	15-250 liters	200-550 liters	No




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