2017 Tech Report Nov-Dec

not guarantee there will not be an accident, but guarantees everything possible has been done to mitigate an accident and possible injuries. A variation on the simple cascade system is the hybrid cascade system . The hybrid system is a typical cascade systemwith a water or glycol heat exchanger added to handle cooler or high temperature loads. A brine solution, like a propylene glycol/water solution, is cooled in the heat exchanger and pumped to the cooler or high temperature load. Heat is transferred to the brine as sensible heat and the solution returned to the heat exchanger where the heat is removed by the high stage refrigerant of the cascade system. The use of the hybrid system allows a refrigerant like ammonia to be used and still be isolated to a location outside the conditioned or occupied space. Yet another variant of the cascade system is the volatile brine system . In this type system liquid CO2 is circulated using a pump in the low stage of the system. The difference between a volatile brine system and a typical secondary refrigerant system is that a portion of the CO2 liquid flashes off as it

absorbs heat. The advantage of using this system is the circulated CO2 absorbs latent heat to boil the liquid. By doing this it picks up more heat per pound circulated, fewer pounds have to be circulated, and this requires less pump horsepower. In addition, there is no low stage compressor. The liquid and vapor from the evaporator is returned to an accumulator/ pump receiver, the liquid and gas separated, the liquid drops to the bottom to be pumped back to the evaporator, and the gas flows by natural convection to the evaporator/condenser. TRANSCRITICAL SYSTEMS Transcritical systems are those that operate in the area above the critical point. Traditionally, they been limited to areas that have moderate ambient conditions. However, as the technology develops, the use of transcritical systems will continue to move toward the equator. The refrigeration cycle in the transcritical cycle takes place both above and below the critical point. Below the critical

“Even used in a cascade system, CO2 has normal operating pressures higher than those encountered with most common refrigerants.”

either a halocarbon or a natural refrigerant such as ammonia,

in the high stage system. Cascade systems are used in both commercial and industrial applications.

Even used in a cascade system, CO2 has normal operating pressures higher than those encountered with most common refrigerants. At a temperature of -30ºF (-34.4ºC) CO2 has a saturation pressure of 163.1 psig (11.2 bar). And at a condensing temperature of +20ºF (-6.7ºC) CO2 has saturated condensing pressure of 407.2 psig (28.1 bar). But even with the high pressures there should be no concern about safely handling CO2 or servicing a system. The same precautions taken with other refrigerants have to be taken. Use the standard operating procedures (SOP) and personal protection equipment (PPE) the same as is done with any other system. Doing so does

point, the system operates much like a conventional system. Liquid is metered into the evaporator, the pressure and temperature

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