Frequently asked Questions |
Q: How does a airconditioner work? A: An air conditioner uses a condensable working fluid--a chemical that easily converts from a gas to a liquid and vice versa--to transfer heat from the air inside of a home to the outside air. This process involves three major components and at least one fan. The three major components are a compressor, a condenser and an evaporator. The compressor and condenser are usually located on the outside air portion of the air conditioner while the evaporator is located on the inside air portion. The working fluid passes through the insides of these three components in order, over and over again, so I'll start examining what happens to the working fluid as it enters the compressor. The working fluid arrives at the compressor as a cool, low pressure gas. The compressor squeezes this working fluid, packing its molecules more tightly together so that their density and pressure increase. The squeezing process also does work on the working fluid, increasing its energy and therefore its temperature. The working fluid leaves the compressor as a hot, high pressure gas and flows into the condenser. The condenser has metal fins all around it that assist the working fluid in transferring heat to the surrounding outdoor air. As this transfer takes place, the closely spaced molecules of the working fluid begin to stick to one another, releasing additional thermal energy into the surrounding air and causing the working fluid to transform into a liquid. By the time the working fluid leaves the condenser, its temperature has almost dropped back down to the outdoor temperature but it is now a liquid rather than a gas. This high pressure liquid then flows into the evaporator through a narrow orifice. This orifice allows the liquid's pressure to drop so that it begins to evaporate into a gas. As it evaporates, it extracts heat from the air around the evaporator because that heat is needed to separate the molecules of the working fluid. Like the condenser, the evaporator has metal fins to assist it in exchanging thermal energy with the surrounding air. By the time the working fluid leaves the evaporator, it is a cool, low pressure gas. It then returns to the compressor to begin its trip all over again. Overall, the working fluid releases heat into the outside air and absorbs heat from the inside air. The direction of heat transfer, from a cooler region to a hotter region, is the reverse of normal and requires an input of ordered energy so that it doesn't violate the second law of thermo- dynamics (the disorder of an isolated system can never decrease). This ordered energy is used to operate the compressor and is converted into thermal energy in the process. This additional disordered thermal energy enters the outside air and makes up for the additional order that's given to the indoor air as that air is cooled. Q: How does a refrigerator work? A: A refrigerator uses a material called a "working fluid" to transfer heat from the food inside the refrigerator to the air around the refrigerator. This working fluid moves through the refrigerator's three main components--the compressor, the condenser, and the evaporator--over and over again, in a continuous cycle. I'll begin as the fluid enters the refrigerator's compressor, which is usually located on the bottom of the refrigerator where it's exposed to the room air. The working fluid enters the compressor as a low-pressure gas at roughly room temperature. The compressor squeezes the molecules of that gas closer together, increasing the gas's density and pressure. Since squeezing a gas involves physical work (a force exerted on an object as that object moves in the direction of the force), the compressor transfers energy to the working fluid and that fluid becomes hotter as a result.. The working fluid leaves the compressor as a high- pressure gas that's well above room temperature. The working fluid then enters the condenser, which is typically a snake-like pipe on the back of the refrigerator. Since the fluid is hotter than the room air, heat flows out of the fluid and into the room air. The fluid then begins to condense into a liquid and it gives up additional thermal energy as it condenses. This additional thermal energy also flows as heat into the room air. The working fluid leaves the condenser as a high-pressure liquid at roughly room temperature. It then flows into the refrigerator, then through a narrowing in the pipe, and then into the evaporator, which is another snake-like pipe that's wrapped around the freezing compartment (in a non-frost free refrigerator) or hidden in the back of the food compartment (in a frost free refrigerator). When the fluid goes through the narrowing in the pipe, it's pressure drops and it enters the evaporator as a low-pressure liquid at roughly room temperature. It immediately begins to evaporate and expands into a gas. In doing so, it uses its thermal energy to separate its molecules from one another and it becomes very cold. Heat flows from the food to this cold gas. The working fluid leaves the evaporator as a low-pressure gas a little below room temperature and heads off toward the compressor to begin the cycle again. Overall, heat has been extracted from the food and delivered to the room air. The compressor consumed electric energy during this process and that energy has become thermal energy in the room air. Q: Who invented the first refrigerator? A: In 1749, Jean-Jacques d' Ortous de Marian described the Chinese refrigerator which recognized the cooling effect of evaporation. However, the first practical refrigerator was built in 1876 by Karl Paul Gottfried von Linde (German). He used liquid ammonia as a coolant. Ammonia & Water ( mixture ) A propane refrigerator works much the same way, except it uses an absorption cycle instead of a compressor. This is where it gets a little tricky to understand. Heat is removed from the cool chamber by mixing ammonia gas with hydrogen gas (endothermic because the hydrogen gas breaks up hydrogen-bonded clusters of ammonia molecules in ammonia gas), and dumped outside the chamber by dissolving the ammonia from the mixture in water. The water is then refluxed with the gas flame to distil off the ammonia which is recycled to the cool chamber. It is reabsorbed by the water and the cycle starts over. Refrigeration A brief explanation of the operation of the gas/absorption refrigerator could be beneficial in understanding how it works and why it sometimes doesn't work. The gas refrigerator operates on a principle of physics. When certain chemicals combine, heat is given off. When other chemicals combine, heat is absorbed. Therefore, if we combine the proper chemicals within the refrigerator, we can absorb the heat, cooling the refrigerator and by combining different chemicals outside the refrigerator, we can expel this heat. The chemicals used in a gas refrigerator are distilled water, ammonia and hydrogen. By mixing ammonia with hydrogen, heat is absorbed providing cooling. When ammonia is mixed with water, heat is expelled. To move these chemicals through the cooling unit, heat from a gas flame or electric heating element is used.
 The system is set in motion in the generator and pump tube, which works similar to a percolator. A water and ammonia solution is heated, boiling the ammonia out of the solution and carrying the mixture up the pump tube. The ammonia gas then continues up and into the refrigerator where it mixes with the hydrogen gas, absorbing heat and cooling the refrigerator. This ammonia and hydrogen mixture continues the circuit back out of the refrigerator and down where it meets the water which is flowing back down to the generator (percolator).
The mixture is now flowing down the tubes on the back of the refrigerator. Since water has a greater affinity for ammonia than hydrogen, ammonia is absorbed into the water, giving off heat and releasing the hydrogen gas. This hydrogen gas will then flow back up into the refrigerator to mix with more ammonia gas and absorb more heat. The water and ammonia solution returns to the generator, where the heating element or gas flame boils the ammonia out of the water, and completes the cycle.
The result is ammonia mixing with hydrogen inside the refrigerator, absorbing heat, and ammonia mixing with water outside of the refrigerator, expelling heat. For the system to function properly, however, it must have good ventilation to dispel heat, be comfortably level so that the solutions can flow and have sufficient heat to boil the ammonia out of the solution and create the pumping action.
 Items of particular concern which may cause a refrigerator to malfunction are:
1.Insufficient heat due to a blocked or failed gas burner, or a faulty or disconnected electric heating element.
 2. Insufficient ventilation, which will cause poor cooling in all modes.
3. A poorly sealed door gasket, which will allow the cold air to escape, causing poor cooling in all modes.
4. Off-level operation or a failed cooling unit, which will cause no cooling in any mode.
While most refrigerator service should be accomplished by a qualified technician, the RV owner can help care for the refrigerator by taking some preventive measures.
1. Make sure nothing is stored in the ventilation area on the back of the refrigerator.
2. Check the back door seals and make sure they are kept clean and fit tight. (Note: To check a refrigerator door seal, close the door on a dollar bill and pull on the bill. If it pulls out easily, the seal is too loose.)
3. When parked, always keep the RV comfortably level.
I hope this very simplified discussion of gas/absorption refrigeration helps you better understand this very important appliance. While your refrigerator may have many automatic and electronic features, they all operate under these same basic principles. The most common form of refrigeration uses an evaporation/condensation cycle for heat pumping. The refrigerant is contained in a closed loop. It enters the cold compartment as a liquid, and when it evaporates there it draws in heat from the cold compartment. The gas produced then travels to the outside of the fridge, where is is compressed to re-form a liquid. This is usually done with a small motor, but I think your small gas flame is used with a clever system of valves for the gas fridge. When the liquid reforms, a lot of heat is dumped outside the fridge. The next most common form uses evaporation only, with water as the refrigerant, and no recycling. We are talking here of a coolgardie safe or an evaporative airconditioner, where warm air is chilled as it evaporates water from a wet cloth it is blown through, and the water is replaced by capillary action from a tank. This works really well in Melbourne or Adelaide or the inland, but is often not so good in Sydney or Brisbane where the summer heat can be associated with high humidity. I suspect that there would be a lot of engineering difficulties in trying to scale up the gas fridge closed loop refrigeration technology for house cooling. But it might be possible. The coolgardie safe idea is certainly possible. One way to work it is as follows: Put insulating, heat absorbing blankets in your roof space between ceiling and tiles, and introduce several large vents. Have a well-shaded fernery outside an open door or window of your house, with a fountain or a continual fine water spray. If you can get your roof space decently hot, you should get a really good through-draft of cool air from your fernery via the house to replace very hot air rising through your tiles. | 
