Liquid propellants

Monopropellant and bipropellant are the two basic types in this category. Monopropellants do not require an oxidizer and produce thermal energy by decomposition either by ignition or by a catalyst. Since monopropellants cause hazards, they are not preferred. Its use is limited. It is used as a source of power to the turbine in a pump feed system. Examples of monopropellants are hydrogen peroxide, hydrazine and nitromethane. Biopropellant is a combination of an oxidizer and a fuel. On combining, this produces thermal energy. Some important combinations are hydrogen peroxide and gasoline, nitric acid and gasoline, liquid oxygen and alcohol, liquid oxygen and liquid hydrogen, liquid oxygen and hydrazine, liquid fluorine and ammonia, liquid fluorine and liquid hydtrogen, nitric acid and aniline, nitrogen tetroxide and hydrazine, hydrogen peroxide and ethyl diamene, ozone and cyanogens etc. Some combinations are hypergolic. The latest in this category is tripropellant which is a combination of metallic fuels and oxygen. The characteristics of liquid propellants are high calorific value, less toxicity, smooth ignition, high density, high reliability, negligible viscosity change with temperature, longer life span, low corrosion characteristics, easy to handle, easy to store and chemical stability.

Hydrologic cycle

The space on earth where water exists is called hydrosphere. Hydrosphere is spread over 15 km in earth’s atmosphere and down 1 km into the earth’s crust. There exists a hydrologic cycle through which water circulation is maintained in the hydrosphere. Hydrologic cycle is continuous and without any beginning or end. This cycle starts with the water evaporation from ocean surface and lands. This vapour becomes a part of the atmosphere. After some time condensation of water vapour occurs and results in precipitation, interception by vegetation, overland flow, subsurface soil flow, surface runoff etc. In the earth water is stored in oceans, groundwater (fresh and saline), ice, soil, snow, lakes, rivers, marshes and in atmosphere. Many physical parameters influence the hydrology of a region. Some examples are vegetation, topography, geology, human activities etc.

Comparators

A comparator compares two operating parameters- operating quantity and restraining quantity. The two types of comparators are amplitude comparators and phase comparators. Regardless of the value of the phase angle, amplitude comparators compare the magnitudes of operating and the restraining quantity. When the magnitude of the operating quantity exceeds that of the restraining quantity, the relay operates and sends the trip signal to the circuit-breaker. Three varieties of amplitude comparators exist. They are integrating amplitude comparator, sampling amplitude comparator and instantaneous amplitude comparator. The two types of integrated amplitude comparators are circulating current type and the voltage opposed type. In sampling comparators, signals are sampled either at the same instant or different instants and then compared. The averaging type and the phase splitting type are the two types of instantaneous amplitude comparators. Phase comparators compare the phase relation between two input signals. Coincidence type phase comparators and vector product type phase comparators are examples of phase comparators. The different techniques employed in coincidence type phase comparators are block spike phase comparison, phase splitting technique, integrating phase comparison and rectifier bridge phase comparison. The two varieties of vector type phase comparators are Hall Effect type and magneto-resistivity type phase comparators.

Underground cable

In a transmission system, cables play a significant role. The various types of cables exist in industry are low tension cables, high tension cables, super tension cable, extra high voltage solid type cables, extra high voltage oil-filled cables and extra high voltage gas pressure cables. The main parts of a cable design point of view are core, insulation, metallic sheath, bedding, armouring and serving. Entry of moisture to insulating material is prevented by metallic sheath. Lead, lead alloy and aluminium can be used as metallic sheath. Bedding is essential for providing a layer of paper tape compounded with a fibrous material or jute strands over the metallic sheath. Both bedding and armouring protects cables from mechanical injury. In the armouring process, cables are provided with layers of galvanized steel wires or layers of steel tape. The insulating materials used for cable are polyvinyl chloride, polyethylene, vulcanized rubber, cross-linked polyethylene cable, varnished cambric, vulcanized bitumen, gutta percha, silk, cotton, enamel or asbestos.