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The RV Fridge Wiki page will use drawings to share the physics governing the process used for common RV refrigerators, discuss safety, and tie together the science for this widely misunderstood refrigeration process.
Overview
Following is a block diagram and a simplified drawing of a typical absorption type RV Fridge. The block diagram is a simplification of the fridge cycle which shows the key components combined with the fluid flow within the circuit. The simplified fluids schematic more closely resembles the actual refrigeration cycle so that the reader can draw a connection between the components on their fridge and what is occurring within the tubing.
Brief History of Absorption Refrigeration
The first absorption refrigeration system was patented by Ferdinand Carre in 1859. This refrigeration system had mechanical pumps and throttling valves which change the pressures in the refrigeration cycle[1]. This page will focus on the single pressure absorption refrigerator due to the fact that it is easy to sustain these fridges in an RV as long as propane (LP gas) is available. The RV Fridge is considered a single pressure absorption refrigeration (SPAR) system. The SPAR was patented by Baltzar von Platen and Carl Munters in 1923[2].
Block Diagram
The block diagram shows the flow of fluids (Qx) along with the flow direction arrows. Also, the heat input (endothermic process) and heat rejection (exothermic process) for the respective components has been identified. The fluid flow is as follows:
Q1: The yellow line is the holding tank mixture of liquid water, ammonia, and a rust inhibitor.
Q2: The blue line is the weak solution. The weak solution preferably consists of only water and the rust inhibitor as a result of the distillation process.
Q3: The pink line is pure ammonia gas (anhydrous ammonia). The heat input in the fired boiler causes the liquid ammonia within Q1 to boil (go through a phase change).
Q4: The green line is ammonia liquid. The condenser cools the ammonia gas (Q3) to produce the working fluid which removes the heat from the refrigerated compartment.
Q5: The pink line is once again ammonia gas. The ammonia gas is produced by evaporation of the ammonia liquid (Q4) within the evaporator. The evaporation of the liquid ammonia is due to Dalton's law of partial pressures. The pressure vessel which is commonly referenced as a cooling unit in the RV industry has all of the air evacuated and a hydrogen charge of approximately 350psi (~2.4MPa).
Q2 + Q5 → Q1: In the absorber coil the circuit is completed. Q1 is reconstituted due to the fact that ammonia has an affinity for water.[3] The weak solution (Q2) absorbs the ammonia gas thereby returning the ammonia to a liquid state to reenter the boiler and start the process over.
Simplified Fluids Schematic
The RV fridge simplified fluids schematic more closely resembles the actual cooling unit. The same color schemes have been use for the fluid flow as the block diagram. This section will have more detail including function, safety, and troubleshooting for each component.
Boiler
The RV Fridge boiler is the heart of the refrigeration process. It performs two main tasks, one is to separate the ammonia from the water, this process is called distillation. The second task that the boiler performs is to pump (create flow Qx) the fluids within the cooling unit.
Process Flow
Q1 Holding Tank to Percolator Tube
Starting at the holding tank the fluids Q1 are delivered to the boiler through the inner annular tube. Heat is applied and the ammonia changes phase from liquid to gas. The ammonia gas rises up the percolator tube (pump tube) forcing the remaining water that has been depleted of ammonia up the tube also.
Q2 Weak Solution
At the top of the percolator tube the water Q2 drops down by the force of gravity into the outer annular tube. This water passes by the heat input to further deplete the flow Q2 of any remaining ammonia. In addition, the flow Q2 also provides preheating for flow Q1 as Q1 moves from the holding tank to the boiler.
Q3 Generation of Refrigerant
From the top of the percolator tube the anhydrous ammonia rises due to its density. The entire boiler assembly and percolator tube are also called a generator because the refrigerant is generated here.
Boiler Ammonia Accumulator
The ammonia accumulator is a bell shaped chamber. The bottom of the chamber is open to the boiler and the top is closed with the percolator tube passing through. The accumulator fills with ammonia gas from the bottom, this in turn forces out any liquids from within the accumulator. The ammonia accumulator serves two important startup functions as follows:
One is to provide an aria where the ammonia bubbles can gather when the heat source is started. This allows large bubbles to rise up the percolator tube upon startup rather than allowing the ammonia in the boiler to rise up the percolator tube without pumping the resulting liquid water.
The second function is to provide an ammonia reserve. When the heat is turned off at the boiler, as the boiler cools the same effect can occur as described above. If the ammonia is depleted from the boiler upon cool down the process can not restart upon startup. Thus, when the boiler temperature lowers, the ammonia gas trapped in the accumulator will return into solution reconstituting Q1 for a strong ammonia solution in the boiler for an easy startup.
Controls
The controls for the RV Fridge center around controlling the boiler heat source. For the most part the control is binary. If refrigeration is called for, one of the boiler heat sources is turned on. When the refrigerated space is cool enough the boiler heat source is turned off.
For the most part the RV Fridge controls have not change much in function sense 1923 when Kelvinator introduces the first refrigerator control with automatic temperature control.
Safety
One issue that is faced by any fired pressure vessel is how to prevent catastrophic rupture. This is why the typical RV Fridge has a pressure relief device which is commonly called a PRV. The RV Fridge PRV is very limited because it is a fusible plug. It is reported that the fusible plug will only vent the pressure vessel in the even of a fire. Unfortunately the fusible plug does not prevent a majority of fridge safety issues. The RV fridge PRV is a retroactive device which responds to a major failure mode that results in a fire.
One of the first major RV Fridge control changes occurred when the Absorption Refrigeration Protective RV (ARPrv) Controller[4]. was invented. The ARPrv control detects when Q2 is not flowing by the rise in temperature due to ammonia not being returned to the boiler. This discovery has solved most of the operator error issues such as off level operation of the fridge. The ARPrv controller is a proactive control that limits thermal stress on the boiler which increases both safety and longevity of the RV Fridge.
Condenser
Further reading please see [5]
References