Central Heating Systems/Air Conditioning /Refrigeration

A central heating system provides warmth to the whole interior of a building (or portion of a building) from one point to multiple rooms .

When combined with other systems in order to control the building climate, the whole system may be a HVAC (heating, ventilation and air

conditioning) system.

Central Heating System… How does it work?

Most modern central heating systems use water as the medium to get heat from the central source (the boiler) to all of the areas to be heated. Usually the main components of central heating systems consist of:

a boiler

a hot water cylinder

a programmer a pump

a radiator

a microprocessor and a touch screen

..A programmer which contains two switches (channels) is what would turn the hot water and central heating services on and off at preselected times of the day and the night.

You can set periods when the heating can be ON and times when the heating will be OFF. Some switches have separate controls (left) for hot water and heating. Timers usually incorporate an 'override' - flick a switch and the heating and/or hot water will be on constantly!

..In a microprocessor-controlled central heating system, users press the touch screen to select the temperature they want.

The microprocessor reads the data from a temperature sensor on the wall and compares it with the value the user has requested.

Temperature sensor is continually monitored by microprocessor If it is lower, then the microprocessor switches the heater on. If it is higher the

microprocessor switches the heater off.

In order for a microprocessor to understand the data from the temperature sensor, it uses an ADC to convert this data to a digital form that it can process.

The microprocessor controls separate actuators: one opens the gas

valves in the boiler and the other switches the pump on.

Microprocessor sends data to LEDs indicating the current room temperature

AS Exam past papers question: >Many workers have a microprocessor controlled central heating system at home .

(i ) , Other than the microprocessor itself identify three items of hardware connected to the microprocessor.

(ii ) Describe how such a system works.

Hardware

Touch screen to input required temperature Sensor to input temperature ADC to convert analogue data to digital Actuator switches radiator on LED screen to display actual temperature

and required temperature

Advantages of central heating

Central heating can provide more effective and energy-efficient convective space heating when:

…rooms have individual thermostatic control

…there are a number of rooms with similar heating requirements regarding temperature and times that heat is needed

…one central heating unit can be used instead of a number of room heaters

…energy-efficient heat sources can be used to power the central unit (for example, solar panels or  heat pumps).

Drawbacks of central heating include:

…installation costs

…limited control over heat output within each individual space without suitable feedback from each space (for example, separate thermostats)

…a time delay before heat output is changed to meet heating demand

…limited flexibility (in terms of moving or resizing components) to meet changing heating requirements

…the need for space to house the central heating unit

…the need for suitable routes for pipes and ducts.

Air Conditioning & Refrigeration

How Air-conditioning WorksCool low-pressure gas is compressed into hot high-pressure

gas by the compressor. When the gas moves through the condenser, it cools down into a liquid. This liquid passes through an expansion valve and it evaporates into a cold low-pressure gas. When the liquid is changing, it absorbs heat from the inside of the building. A fan blows air across metal fins as a result of the heat being absorbed. This causes the room to cool down.

How Air-conditioning Works

1 )condensing coil, 2) expansion valve, 3) evaporator coil, 4) compressor

Microprocessor Control The system controls the amount of heat being let out of the system The valve is controlled by microprocessor to reduce or increase the flow

of refrigerant Microprocessor compares the pressure of the refrigerant to the preset

value If pressure of refrigerant is above preset value microprocessor opens

valve wider If pressure of refrigerant is below preset value microprocessor makes

valve opening narrower Touch screen is used to input the required temperature Microprocessor compares the temperature of the room to the preset value If temperature of the room is above the pre-set value the fans remain

on/are switched on by the microprocessor or microprocessor increases their speed

If temperature of the room is below the pre-set value the fans remain/switched off by the microprocessor

Sensors used in Air-conditioning

Two temperature sensors to control the amount of heat being let out of the system.

Pressure sensor to control the valve to reduce or increase the flow of refrigerant.

Temperature sensor in the rooms where the system operates to control the heat loss from individual rooms.

REFRIGERATION

MISSION IMPOSSIBLE?

Your new task is to move the heat from the inside of a refrigerator to the outside to keep your food fresh. How can you move something you can't see?

 Not only that, but you can't open the door to get at the heat inside, or you'll let the heat straight back in again. Your mission is to remove the heat, continually, without opening the door even once. Tricky problem, eh? But it's not impossible—

Refrigeration Simplified (kind of…)

By compressing gases, we make them hotter; by letting them expand, we make them cooler. How can we use this handy bit of physics to shift heat from the inside of a refrigerator?

Suppose we made a pipe that was partly inside a refrigerator and partly outside it, and sealed so it was a continuous loop.

And suppose we filled the pipe with a gas. Inside the refrigerator, we could make the pipe gradually get wider, so the gas would expand and cool as it flowed through it. Outside the refrigerator, we could compress (squeeze) the gas and release its heat.

The compressor is attached to a grill-like device called a condenser (a kind of thin radiator behind the refrigerator) that expels the unwanted heat. Finally, the gas that circulates round the pipe is actually a specially designed chemical that alternates between being a cool liquid and a hot gas. This chemical is known as the coolant or refrigerant.

If the gas flowed round and round the loop, expanding when it was inside the refrigerator and compressing when it was outside, it would constantly pick up heat from the inside and carry it to the outside like a heat conveyor belt.

Recap… Condensing unit

Allows refrigerant to cool/cools gas to form liquid

Is situated at rear of refrigerator Evaporation unit

Absorbs heat from surrounds thereby cooling it

Is situated inside refrigerator

Refrigeration (Hardware)

Sensor to monitor temperature inside refrigerator Contact switch or pressure sensor to check if the door is

closed Number pad/touch pad/touch screen to input required

temperature LED display/screen to indicate current temperature inside the

refrigerator Actuator to activate buzzer Buzzer if door is left open Actuator to activate compressor Actuator to activate valve

Microprocessor Control The required temperature is input/set Then the microprocessor reads temperature from sensor Data from the sensor is converted to digital using an ADC The temperature of refrigerator/value read by sensor/current temperature is

compared to pre-set value if greater than preset value microprocessor activates/sends a signal to actuator if greater than preset value microprocessor/actuator switches compressor on if greater than preset value microprocessor/actuator opens valve if it is below the preset value microprocessor activates/sends a signal to actuator if below the preset value actuator/microprocessor switches compressor off if below the preset value actuator/microprocessor switches valve off microprocessor sends data to LEDs indicating the current refrigerator temperature microprocessor compares data from pressure sensor with zero/checks contact

switch if it is zero/contact broken microprocessor sends a signal to the actuator if it is zero/contact broken actuator/microprocessor switches the buzzer on