Recreational vehicles (RVs) are designed to meet their own needs, making refrigerators on top of highly desirable tools. However, the RV is subject to conditions that make their refrigerator design challenging enough. The main problem is the vibration, acceleration, the energy needed to drive the cooling process, and the attitude of the vehicle that is often off-level, especially on the go. Refrigerator type-absorption overcomes most of these design challenges, and has therefore been popular in RVs since the 1950s.
Video RV Fridge
Overview
The following is a simplified block and graphic diagram of a special absorption type RV Refrigerator type. The block diagram is a simplification of the refrigerator cycle showing the key components combined with the fluid flow in the circuit. The simplified liquid scheme resembles the actual cooling cycle so readers can draw connections between the components in their refrigerator and what happens inside the cooling unit pressure vessel.
Maps RV Fridge
Brief history of absorption refrigeration
The first absorption cooling system was patented by Ferdinand Carre in 1859. This cooling system has a mechanical pump and a throttling valve that changes the pressure in the cooling cycle.
Refrigerator RV is considered as a single pressure absorption (SPAR) cooling system because the whole system is at the same total pressure. Since there is no pump or compressor, it is easy to keep this refrigerator in RV as long as propane (LP gas) is available. SPAR patented by Baltzar von Platen and Carl Munters in 1923.
Block diagram
The block diagram shows the fluid flow (Q x ) along with the direction of the arrow stream. Also, the heat input (endothermic process) and heat rejection (exothermic process) for each component have been identified. The fluid flow is as follows:
Q 1 : The yellow line is a mixture of liquid water container tank, ammonia, and rust inhibitor.
Q 2 : The blue line is a weak solution. Weak solutions should consist only of water and rust inhibitors as a result of the distillation process.
Q 3 : The pink line is pure ammonia gas (anhydrous ammonia). The heat input in the burned boiler causes liquid ammonia in Q 1 to boil (through phase change).
Q 4 : The green line is the ammonia fluid. The condenser cools the ammonia gas (Q 3 ) to produce a working fluid that removes heat from the cooling compartment.
Q 5 : The pink line once again is an ammonia gas. The ammonia gas is produced by evaporating the ammonia fluid (Q 4 ) in the evaporator. Evaporation of liquid ammonia is due to Dalton's partial pressure law The pressure vessel normally referred to as the cooling unit in the RV industry has all air evacuated and a hydrogen charge of about 350psi (~ 2.4MPa).
Q 2 Q 5 -> Q 1 : In the absorber coil, the circuit is completed. Q 1 is dissolved due to the fact that ammonia has an affinity for water. The weak solution of (Q 2 ) absorbs the ammonia gas thus returning the ammonia to the liquid state to reenter the boiler and start the process over.
Simplified fluid schemes
The RV refrigerator simplifies the liquid scheme more like the actual cooling unit. (The same color scheme has been used for fluid flow as block diagram.)
Boiler
RV Refrigerator boiler is the heart of the cooling process. It performs two main tasks; one of which is to separate ammonia from water using distillation process. The second task the boiler does is to pump the liquid that maintains the flow of Qx in the cooling unit. RV Refrigerator boilers are considered watertube type boilers that have low weight advantages and high circulation rates desired for mobile applications.
Process flow
Q 1 Holding Tank ke Percolator Tube
Starting from the liquid container tank Q1 is sent to the boiler through the inner annular tube. The heat is applied and the phase of ammonia changes from liquid to gas. The ammonia gas rises above the percolator tube (pumping tube) forcing the remaining water that has exhausted the ammonia up the pumping tube at the same time.
Q 2 Solusi Lemah
At the top of the percolator tube, water Q2 falls downward by gravity into the outer annular tube. This water passes through the heat input to further drain the flow of Q2 from the rest of the ammonia. In addition, the flow of Q2 also provides a preheater (regenerator) for Q1 flow when Q1 moves from the container tank to the boiler.
Q 3 Generasi Refrigeran
From the top of the percolator tube, anhydrous ammonia increases due to its density. The whole boiler and the percolator tube are also called generators because the refrigerant is generated here.
Accumulator of boiler ammonia
The ammonia accumulator is a bell-shaped space. The bottom of the open space for the boiler and the top is covered with a percolator tube through it. The accumulator fills with ammonia gas from below, this in turn forces the liquid out of the accumulator. The ammonia accumulator serves two important startup functions as follows:
One is to provide an area where ammonia bubbles can congregate when the heat source starts. This allows large bubbles to rise above the percolator tube at startup rather than allowing ammonia in the boiler to raise the percolator tube without pumping the water produced.
The second function is to provide ammonia reserves. When heat is off in the boiler, as the boiler cools, the same effect can occur as described above. If ammonia runs out of the boiler after cooling process can not be restarted at startup. Thus, when the boiler temperature decreases, the ammonia gas trapped in the accumulator will return to a solution that rebuilds Q1 for a powerful ammonia solution inside the boiler for easy startup.
Control
Controls for RV Refrigerator centers around controlling the boiler heat source. For most control it is binary. If cooling is requested, one of the boiler heat sources is turned on. When the cooling chamber is cool enough, the heat source of the boiler is turned off.
The RV Refrigerator control has not changed much since 1923 when Kelvinator introduced the first refrigerator control with automatic temperature control. Please see the next section of Security Boiler for modern Refrigerator RV control.
Boiler Security
Pressure
One of the problems faced by each hot pressed vessel is how to prevent a catastrophic outbreak. This is why the typical Refrigerator RV has a pressure relief device commonly called PRV. RV PRV refrigerators are very limited because these are plugs that can be doubled. It was reported that a fusible plug would only vent a pressure vessel in case of a fire. Unfortunately fusible plugs do not prevent the majority of security issues of refrigerators. The fusible fridge RV plug is a retroactive device that responds to the main failure mode that generates fire.
Limit the heat voltage
One of the first major Refrigerator RV control changes occurs when Refrigeration Absorption Refrigeration Cooler (ARPrv) is created. The ARPrv control principle of operation is based on the same physics that produces a constant temperature to boil water. The ARP control measures only the latent heat of ammonia vaporization. ARP detects when Q2 does not flow by temperature rise because ammonia is not returned to the boiler. When the ammonia does not return to the boiler, sodium chromate is destroyed by excess heat. After the sodium chromate is destroyed, intergranular corrosion, which causes a tense crack corrosion, will eventually result in a split of the cooling unit. This discovery has solved most operators' error problems such as outside operation of the refrigerator. The ARPrv controller is a proactive automatic boiler control that limits both the thermal pressures on the boiler and the pressure on the working fluid. In addition, by limiting the temperature on the boiler, the pressure in the cooling unit is controlled which in turn improves the safety and longevity of the Refrigerator RV.
Condenser
Refrigerator RV Refrigerators cool and melt pure Q3 ammonia gas that rises from the boiler. Q4 Liquid ammonia flows from the condenser to the evaporator where the actual cooling takes place.
Failure mode
The condenser RV refrigerator requires two requirements in order to maintain the cooling cycle. The first is the temperature of the condenser environment must remain below the ammonia condensation temperature at the system pressure, otherwise the ammonia will remain in the gas state. Therefore, the condenser requires proper air circulation. The second factor for correct operation is that the condenser is a level like the ammonia fluid will flow into the evaporator. When the refrigerator is operated outside the level of ammonia will be pooled in the condenser. This collection results in the termination of Q4 flow and thus halts the cooling process because ammonia can not enter the evaporator for refrigeration refrigeration. In addition, the collection of ammonia in the condenser prevents the ammonia back to the boiler causing the cessation of boiler function. When the boiler function stops heating the heat from the following boiler.
Evaporator
Evaporator The RV refrigerator is a refrigeration refrigerator. Basically all cooling processes use evaporation for cooling. In this type of absorption RV Refrigerator liquid ammonia Q4 enters from the condenser and flows down the evaporator. In the evaporator, the liquid ammonia in contact with the gas assistant is generally hydrogen. Due to Dalton's partial pressure law, liquid ammonia Q4 evaporates so as to absorb heat from within the cooled chamber. Since the ammonia gas Q5 is heavier than the assistant gas, the ammonia gas flows out from the bottom of the evaporator.
Construction
The evaporator is located inside the foam cabinet in the RV Refrigerator. When one looks into a refrigerator or a refrigerator, the back wall will generally have an aluminum plate that may have a fin to increase the surface area for heat absorption from the cooled chamber. Depending on the type and construction of the Refrigerator RV, as a rule of thumb, the top of the evaporator is in thermal contact with the freezer portion and the bottom contact with the refrigerated part of the cabinet.
Failure mode
Because of evaporator construction there are two failure modes. The first is not the failure of the evaporator but the consequence of the failed boiler assembly. For example, if the refrigerator has been operated outside the level and the boiler is left too hot, the volume of ammonia Q4 fluid decreases. The reduced ammonia flow of Q4 produces the evaporating ammonia before it reaches the lower half of the evaporator. As a result, the freezer may be very cold, while the cooling chamber is too warm to prevent food spoilage.
The second mode of failure is because the evaporator is not sealed properly. Often the screws hold the evaporator into place from the inside of the refrigerator. If the screw head is not sealed, the water from inside the cabinet will flow through the capillary action into the closed evaporator chamber inside the foam. Also, the rear side of the evaporator must be sealed so that ambient air can not infiltrate the closed evaporator chamber. When the ambient air is left in contact with the evaporator, the condensation results. The result for both of these problems is that water is trapped in the evaporator steel pipe, forming a galvanic cell that will create a hole in the evaporator, leading to premature refrigerator failure.
Absorber
The absorber or absorber coils are where the liquid is combined to complete the coolant fluid circuit. The ammonia gas Q5 falls from the evaporator to the adsorbent where it is in contact with the weak solution Q2 (water) flowing into the top of the absorber coil. When the ammonia vapor Q5 is in contact with the weakened Q2 liquid solution, the weak solution absorbs the ammonia gas because of the affinity of ammonia for water. The absorption of ammonia gas into the aqueous solution is an exothermic process, so the heat needs to be rejected from the absorber coil. The absorber coil is where heat from the refrigerator is turned around.
Failure mode
The absorber itself does not experience failure due to corrosion or stress when constructed properly. The absorber can be reduced in capacity if the temperature is too high in the cooling unit compartment. High temperatures slow down the rate at which ammonia gas is reabsorbed (Solubility of Ammonia in Water), therefore this can produce poor cooling in the cooled chamber. A simple test to see if the ammonia completes the circuit is to sense the absorber coil. If the absorber coil is not hot within an hour turning on the refrigerator is probably the ammonia not produced by the boiler or ammonia does not complete the circuit. In addition to the refrigerator cabinet temperature, knowing the temperature of the Refrigerator RV boiler and the absorbent temperature gives one information to see if the RV Refrigerator is working properly.
References
Source of the article : Wikipedia
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