Technical
Marius La Grange
System diagram-DX type .
condition and the ambient air conditions the smaller the condenser ’ s surface area could be .
With a rise in altitude the ambient air density decreases , so a similar volume of air would weigh less requiring a greater volume of air to achieve the same relative exchange of heat to take place . ( A greater volume of air needed for the same relative mass flow at a hight altitude ).
Most domestic appliance rely on convection with the condenser coil mounted in such a way that will allow warm air to rise and draw cooler ambient air in from the bottom .
• Evaporators – DX systems An evaporator ’ s function within a system is to absorb and remove heat energy from and enclosed area . Within the evaporator the condensed liquid from the HP side would be exposed to the heat source that causes the liquid to boil off changing phase to vapour . The heat source in this case being the air within the enclosed area that needs to be cooled .
This air is of course also forced across the coil surface area by means of fans . Should the air across the coiled surface area decrease , the rate at which the liquid refrigerant boils off would also decrease with the expansion device modulating the mass flow of the refrigerant fed into the evaporator . At exit of the evaporator the refrigerant would need to be fully evaporated with few K of superheat as controlled by the expansion device with no liquid refrigerant remaining a vapour at the coil exit .
The temperature for the refrigerant on the inside of the evaporator will obviously always need to be lower than the required air temperature of the enclosed area in order for the flow off . This difference in temperature ( ΔT ) means that most cold rooms or freezer rooms have a suction temperature well below 0 ° C . In such cases the surface area of the coil would ice up constantly requiring regular defrosting . This is done with electrical heating elements in many cases .
A ‘ hot gas ’ defrost makes the most sense from a thermodynamic perspective , but it is less common on smaller systems . It adds a bit of complexity to the system and the sizable change in temperatures from cooling / freezing to defrosting several times a day results in the welded copper
joints leaking over time . Synthetic refrigerants leaking into the atmosphere uncontrollably of course never being a good thing . A fin and tube HE is always going to result in a drop in air humidity when cooling the air . The rate of de-hydration can be limited by design . A ΔT between suction temperature and air of temperature of 6-8K offers a good balance between cost and system operating costs or energy consumption . A lower ΔT would mean a greater surface would be required to achieve the same relative heat absorption capacity .
• Flooded evaporators – ammonia systems Similar to DX evaporators flooded evaporators have a surface area based on the capacity required and the air volume is forced across the surface area . The main difference in the this case being that the internal volume of the coil tubes are filled with liquid refrigerant so the transfer of heat is from the air on the outside of the coil towards the colder liquid refrigerant on the inside .
A flooded evaporator would be piped onto a surge vessel with the liquid refrigerant circulated / pumped to maintain a specific liquid level in the evaporator in order the achieve designed heat absorption . One benefit of a flooded R717 system being the very low , if any , suction superheat that the evaporators operate at resulting in a greater system efficiency .
The tubing circuits in contact with the refrigerant needing to be steel ( as special version ) or stainless steel ( no copper since ammonia corrodes copper ). With R717 applications , stringent regulations ( SANS10147 ) apply when making use of carbon steel as part of the pressurized system .
• Air coolers Air coolers are commonly used in air conditioning applications . Water used as a secondary coolant being circulated through the coil . The heat energy being transferred from the air volume ( requiring cooling ) and the secondary coolant . The secondary coolant is circulated with a pump between the air cooler and a heat exchanger of some kind to absorb the heat gain that the secondary coolant had . In such systems additional energy is needed to pump the secondary coolant .
One advantage of such systems would be the lower ΔT possible between the required air off conditions and the secondary coolant supply temperature . This can potentially lower the rate of dehydration than a comparable DX evaporator coil making it more suitable for specific applications .
The secondary coolant being water or a water and glycol mixture ( pre-mixed to the applications specific ratio by volume ). In applications where the secondary coolant is required to be below 0 ° C a mixture is required to prevent the mixture from freezing at any stage . Note that the glycol has a lower viscosity than clean water , so a greater amount of pump energy is needed to circulate the mixture . RACA
• Continued in the next issue of the RACA Journal
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RACA Journal I April 2021 www . hvacronline . co . za