64 pilot. Rich posted a refrigeration 101 UTube on an earlier thread you should read.
There are major difference between the design of a system with a capillary tube refrigerant flow and a system with a Thermo Expansion Valve (TXV). The main reason these small Danfoss 12 volt compressor systems use capillary tube expansion devices is costs. The Cap tube system does not require a refrigerant receiver tank or a sight glass or an expensive TXV.
CAP TUBE refrigerant flow control systems
Capillary tube systems: are generally not used in mobile refrigeration because their performance is greatly affected by ambient air temperature and condenser cooling medium temperature changes. When one of these small cap tube systems is serviced correctly in Boston and moved south to Miami
the temperature may be degraded because there is too much refrigerant in system. Normally there will be no liquid refrigerant in these cap tube systems when they are not in use when at standard day temperatures only gas vapor is present. When a complete heat soaked small Danfoss system is at 70 to 80 degrees F the static gas vapor pressure in system will not have any refrigerant phase changed to a liquid. In this example when high side pressure exceeds 190 psi liquid mixed with gas vapor will begin to build up against the long cap tube. As refrigerant flows through the long cap tube sub cooling occurs within tube condensing more vapor to liquid as it interiors evaporator coil.
A technician must remember that performance of these Small cap tube BD compressor systems requires correct volume of liquid refrigerant to within 15 grams of correct amount and reasonable control over condenser cooling medium’s temperature range.
Other than lower hardware
costs why are capillary tube flow control devices used instead of the TXV flow control systems? A. The job shops that assemble icebox
refrigeration units rarely manufacture the thin plate evaporators and companies that do include cap tube properly sized with each evaporator.
TXV refrigerant flow control systems
• The objective of any refrigerant control device is to cause refrigerant process of absorbing heat to occur in the evaporator. Thermo Expansion Valves are used where a wide variation of temperature range is needed. Because of extra refrigerant storage
in a receiver required for TXV flow control it allows for variations in refrigerant charge without impacting performance. The receiver also helps separate vapor from liquid to insure there will always be a liquid head
pressure at TXV. Instead of allowing pressure by itself as in a cap tube system to control flow the TXV compares temperature across evaporator to control refrigerant flow.
Mother Nature has laws on refrigerant that when broken can provide disastrous results to performance and even a systems service
life. An accurate history
of a unit will generally pin point poor performance or need for complete system replacement. Below is a list of risky practices that have caused systems to self destruct:
• Use of unapproved refrigerants.
• Connecting gauges to a small system or tampering in any way without confirming first refrigerant or its flow is a problem by non destructive means.
• Mixing any compressor oil
with manufacturer’s original oil.
• Adding Stop leak, Dye, or any additive to refrigerant.
• Allowing compressor to operate at excessive temperatures in warm climates especially incorrectly installed air cooled units and some water
cooled units without separate compressor cooling.
• DIY systems sometimes use low pressure side tubing including evaporator coil inside holding plates that is larger than compressor’s return vapor line. Over time with low suction line velocity oil will be trapped causing loss lubrication at compressor.
• Adding refrigerant to achieve frost on evaporator when amperage, low pressure and high pressure are all in proper range will only compound trouble over time by overheating