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Hi Andrea, Below is a reprint of small article I wrote a couple years back that explains why the transfer of liquid R290 happens without the use of a pump/compressor. R290 = propane (another type of refrigerant) As the temperature warms the bulk tank in the afternoon sun, the aux tank will refill quicker Vs comparring refilling at dawn. Gravity would have no benefit to the refilling system other than under lab conditions. T&P is what is responsible for fluid flow as Jim pointed out with the aux tank vent open.
Refrigeration 101:
To have a complete understanding of the refrigeration process we need to define some rules to follow as it matters to use the correct terminology when designing or servicing heating, ventalating or refrigeration systems. 1) Cold only occurs at minus 460ºR or absolute zero. Above that we only have a absence of heat or can be stated as lack of heat. This is very important to understand as it helps in understanding all refrigeration or heating processes. 2) The accepted slang term AC does not mean "cold" as the true meaning stands for "air conditioning", that is to heat, cool, clean, humidifiy or dehumidifiy common air. 3) The second Law of Thermodymanics states:
"heat flows from a higher temperature source to a lower temperature source" This is really important to understand when dealing with refrigeration or heating as one can quickly figure direction of heat transfer at any point in a process. 4) There's several types of common heat exchangers:
a) air to air b) air to liquid
c) liquid to liquid
5) All refrigerants have a direct relationship between temperature and pressure (called it's T & P) that is to say that a refrigerant at 100º may have a pressure of 196psig and at 0º a pressure of 24psig. With knowing the T & P of the refrigerant I would know I'm dealing with refrigerant 22 (R22) using the above numbers.
Example #1:
A simple refrigeration process can be explained as where liquid refrigerant (from the bottom of a condensor) is fed into a small diameter orifice (capillary tube) that runs into a larger tube (evaporator). As the refrigerant enters the evaporator the refrigerant expands into saturated vapor (vaporized fluid) thus dropping pressure thus lowering it's temperature, hence the refrigerating effect has just completed.
Using the above T & P already discussed above, the condensor would have a temperature of 100º and the evaporator would be at 0º thus our evaporator would try to gain heat from it's surrounding area as stated by the 2nd Law of Thermodymanics. Notice we never talked about the use of a compressor in the above example to obtain the refrigerating effect as only T & P was responsible for the cause of refrigeration effect as explained by the 2nd law of Thermodynamics. Some refrigerated semi trailers and rail cars use a expendable refrigerant based upon the above example. A typical outdoor HP uses the same above therory of refrigerating effect. A HP just adds a reversing valve for direction of refrigerant flow and a compressor for reusing the refrigerant with-in a closed loop refrigerant system. 0n a typical HP we want to use the evaporator as a condensor sometimes and the condensor as a evaporator sometimes and a reversing valve accomplishs this. The compressor pumps refrigerant from the evaporator to the condensor. To keep this short as possible we will not address Latent heat of compression, the effect that a compressor adds to the refrigeration cycle. Notice we never mention using any fans for the above HP unit to complete it's refrigeration cycle nor to obtain the refrigerating effect. Depending on the desired results would we add a fan. During the time that our outdoor HP is in the heating mode, the condensor is used as a evaporator. That means were trying to refrigerate the ambient air and thus obtain heat, (see the begining of example #1). As the HP continues to run the outside eveporator begins to ice from trying to absorb heat from the surrounding air as well gathers moisture. To defrost this ice build up we momentary reverse the refrigerant flow thus melting the ice and the "swoosh" sound our HP makes when it reverses refrigerant flow. Now to increase the heat flow to our HP evaporator during heating mode we can add a liquid to liquid heat exchanger with taking heat from a ground source hence the name geothermal. Now that ground source heat could be obtained form well water or a secondary coil located with-in water with a ground source heat sink. There's many different ways to accomplish a ground source heat gain but a liquid to liquid heat exchanger has the highest efficiency ratio. T_Bone
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