A little Background
When attempting to understand how your refrigerator or any air-conditioner works just think of it as a transfer of heat process. When you want to make something cold you remove its heat or vice versa when you want to make something hot (heat pump).
The two different types of heat are called sensible and latent.
Sensible Heat: Any heat that can change the temperature on a thermometer.
Latent Heat: Latent heat is the amount of energy in the form of heat released or absorbed by a substance during a change of phase (i.e. solid, liquid, or gas), – also called a phase transition, such as the melting of ice or the boiling of water.
During the cooling cycling, condensation forms within the unit due to the removal of latent heat from the air. Sensible capacity is the capacity required to lower the temperature and latent capacity is the capacity to remove the moisture from the air.
You may hear the term Latent Heat of Vaporation and this is what must be absorbed by the substance in the liquid state in order for it to change to a gas or what must be removed from the gas for it to return to a liquid state.
Heat Quantity is measured in British Thermal Units (BTU's) which is the amount of heat required to raise the temperature of a pound of water 1 degree F.
How it works:
We know that at 32 degrees F water freezes and at 212 degrees it boils. However, it is possible to boil or freeze water at different temperatures depending on atmospheric pressure. The HIGHER the PRESSURE the HIGHER the BOILING POINT Temperature. Its is through control of pressure that refrigeration temperatures are controlled. Since we know that Heat flow from the WARMER to COOLER and happens faster as the disparity greatens.
When a liquid changes to a gas it absorbs heat as it flows to it from its warmer surroundings. So, if these surroundings happen to be food in a refrigerator box, heat is withdrawn from it. Heat travels between the food and the cooling unit wall by radiation (through direct waves) and convection (through air circulation), the heat passes through the air through the cooling unit wall to the liquid by conduction.
The substance which soaks up heat during evaporation to create coolness is called a refrigerant. It picks up heat from one place as a gas then gets rid of this heat by turning back to a liquid.
Methods (Refrigeration Cycle):
Compressor: Its the two side beast the is the heart of the refrigeration cycle. The refrigerant, in the form of a gas under low pressure, is drawn from a suction line into the compressor. The gas enters the compressor while the piston is at its downward stroke. On its upward stroke the pressure and temperature of the gas rises and the gas is compressed through the discharge valve to the Condenser. Remember: the reason we do this is because when the pressure raises so does the temperature allowing the warmer temp of the refrigerant to loose its heat to the cooler surrounding air or water of the condenser!
Condenser: The Refrigerant enters the Condenser as a High Pressure Gas and its temperature is higher than that of the air or water cooling the Condenser. As heat flows form the gas to the cooling agent the gas changes from from a gas to a high pressure liquid and passes into the liquid receiver or storage tank. Note: If its air cooled than a Condenser fan will blow outdoor air over the condenser coils allowing more heat transfer to take place.
From the storage tank the high pressure liquid refrigerant passes through a liquid line on its way to the cooling unit.
The high pressure refrigerant must be changed to a low pressure liquid so that it can evaporate at a low temperature. So it must pass through an expansion valve.
Expansion Valve. Takes the high pressure liquid and changes it to a low pressure liquid by controlling the release of the high pressure liquid into the evaporator so the suction stroke of the compressor can reduce the pressure which allows the liquid to evaporate at a low temperature.
Evaporator: As the low pressure liquid refrigerant goes through the evaporator coils it evaporates as it draws heat from the room or food in a refrigerator. It evaporates, absorbing the surrounding heat which is now a low pressure gas.
Now its drawn back to the compressor through the suction line completing the cycle! This is the cylcle of changes as it moves through the system!
If this were reverse through the use of a heat pump than you can heat a house instead of cooling it.
So that is why in your apartment/house you will have an air intake and a fan coil because the air gets pulled in than the heat gets taken out of it. Then then with the heat taken out of it it gets pushed into your room as cool air. If this were reverse in the case of a heat pump then it would be the opposite.
Quick note on Heat Pumps:
There is a pilot valve connected to a solenoid plunger that normally changes between heating and cooling. It is a four-way reversing valve that sits outside of the outbound compressed gas from the compressor.
Since the evaporator and condenser coils are virtually the same this will reverse the two having the condenser be the evaporator and vice versa. Therefore, the four way reversing valve essentially controls where the flow goes from the compressor. Either it will go to the Condenser or the Evaporator on the outbound of the compressor. If it goes to the evaporator it will be in the heating condition and if it goes to the Condenser it will be in the cooling condition.
As far as De-Humidifying: There are essentially two way to accomplish this without using N2.
1. Dehydrate the air using Chemicals
2. Cool the air down to the saturation curve on a Pyschrometric Chart at C. Then remove moisture by condensing it on a cool surface. See curve D. The distance from C to D is the drop in vapor pressure or gains of moisture removed.
The way this chart reads is that if you start at 60%RH at 26 degree C and want to end up at 10 degree C with 30% RH than you will need a coil surface temp of -5 degrees C
From here all you would have to do is look at the manufacturers ratings to find the system that’s right for you!
FYI: When servicing you will see two lines. One line will lead to the evaporator coil inside the unit/house and one line will lead to the the Compressor Suction Inlet Valve.
Chiller
A chiller is a mechanical refrigeration device, like an air conditioner, except that it cools a fluid (usually water) instead of cooling air.
- A chiller consists of a few major components. A compressor, an evaporator heat exchanger, a condenser heat exchanger, an expansion valve or two and some piping and controls are the basics.
- Compressors are usually reciprocating, scroll, centrifugal, or rotary screw types.
The condenser heat exchanger can be either air-cooled, a coil or coils and a fan or fans, or water-cooled, another shell & tube heat exchanger cooled by cooling tower or other water
A typical chiller would for example chill 55°F EWT, (entering water temp.), down to 45°F LWT, (leaving water temp.).
What is the difference between a Chiller and a Cooling Tower?
A cooling tower is used to cool water instead of chilling it during an evaporative process that transfers the heat to the outside environment.
Generally water is dropped through a fill material while air is moved across the fill, transferring heat from the water to the air.
The major component of a cooling tower is a fan to move the air across the fill material. Cooling towers work for cooling higher temperature water or fluids.
A typical cooling tower would for example cool 95°F EWT down to 85°F LWT.
Cooling towers are often used in conjunction with water-cooled chillers, but have many stand-alone uses as well.
Cooling towers are most effective at LWT (Leaving Water Temp) temperatures of 75°F and higher, while you would most likely need a chiller for LWT of 75°F or less.
For LWT temperatures of 65°F and lower, cooling towers would likely not be an option, only a chiller can produce LWT of 65°F and lower.
The interesting thing is that the Delta T (change of temperature) is normally about the same of 10 degrees depending on the systems efficiencies.
So most often large buildings, hospitals, gyms, use a cooling tower in conjuction with a water-cooled chiller.
HVAC use of a cooling tower pairs the cooling tower with a water-cooled chiller or water-cooled condenser. A ton of air-conditioning is the removal of 12,000 Btu/hour (3500 W). The equivalent ton on the cooling tower side actually rejects about 15,000 Btu/hour (4400 W) due to the heat-equivalent of the energy needed to drive the chiller's compressor. This equivalent ton is defined as the heat rejection in cooling 3 U.S. gallons/minute (1,500 pound/hour) of water 10 °F (6 °C), which amounts to 15,000 Btu/hour, or a chiller coefficient of performance (COP) of 4.0. This COP is equivalent to an energy efficiency ratio (EER) of 14.
I know this is a lot of information to take it but I am posting this as more of a reference to you in your future HVAC Kaizen/Kaikaku Projects!
Ja Mata Ne!
