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View Subscription Details: Letter ( 04/30/2015 ) - Plain Text Format Evaporator Brining

The change in energy content of a refrigerant as it moves through an evaporator takes place in these two forms:
- Sensible (i.e. the refrigerant temperature increases but does not change state)
- Latent (i.e. refrigerant temperature is constant but the added thermal energy is changing the state of the refrigerant from a liquid to a vapor)
It is highly desirable that the need for sensible heating of liquid inside an evaporator be as limited as possible. Conversely, the more heat transfer taking place in the latent form is much desired. In a sensible heat transfer process fluid is pumped through a heat exchanger absorbing energy by increasing its temperature. During a sensible energy change, the working fluid does not undergo a change in phase. This type of process is common to a brine or water.
In contrast, a latent heat transfer process relies on the change in phase between a liquid and a vapor. Remember that during a change in phase (i.e. an evaporation process), the temperature of the refrigerant remains constant for all pure fluids, like ammonia. The advantage to the latent energy change process is that each pound of refrigerant we evaporate in a heat exchanger has the ability to carry away considerably more heat than if it only relied on sensible energy change.
If circulated ammonia does not boil inside our evaporator and it is merely heated sensibly, capacity of this evaporator will be significantly reduced. What prevents ammonia from boiling? The answer is subcooling; excessive liquid feed to an evaporators is a major cause of subcooling and brining. Oftentimes when evaporators are initially set up and adjusted, a technician might feel that about halfway is the correct hand expansion valve setting. This is usually wrong - more often than not, you will find that oversized hand expansion valves setting the valve as little as 1/8 turn open is about right. If you have gauge downstream of the hand expansion valve, set this valve for a pressure about 5 - 7 psi above the evaporating pressure, because many evaporator manufacturers used a 5 - 7 psi loss across their button orifices. This setting will prevent subcooling of the liquid ammonia inside evaporator and this evaporator will operate at maximum capacity.
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