Help needed
Help needed
(OP)
Hi everyone !!
I have just started my professional career in a chemical plant. I am facing problems in understanding the chiller operation.
We have a chiller equipped with a screw compressor of HANBELL company. The compressor is a 4 step capacity control system with a modulation slide valve installed over the screws.
When the compressor load increases the slide valve slides and change the capacity. But I am unable to predict what actually is being done in the whole cycle. As far as I have understood, as the load increases the evaporator temperature increases (increase of super-heat). Now the expansion valve opens and allows more refrigerant to flow which increases the suction/evaporator pressure. The increased flow then enters the compressor and capacity control system allows this increased flow rate to be compressed by screws.
If all this explained above is true, then what will be the effect on discharge pressure ?
Kindly correct me if I have got anything wrong
Regards
I have just started my professional career in a chemical plant. I am facing problems in understanding the chiller operation.
We have a chiller equipped with a screw compressor of HANBELL company. The compressor is a 4 step capacity control system with a modulation slide valve installed over the screws.
When the compressor load increases the slide valve slides and change the capacity. But I am unable to predict what actually is being done in the whole cycle. As far as I have understood, as the load increases the evaporator temperature increases (increase of super-heat). Now the expansion valve opens and allows more refrigerant to flow which increases the suction/evaporator pressure. The increased flow then enters the compressor and capacity control system allows this increased flow rate to be compressed by screws.
If all this explained above is true, then what will be the effect on discharge pressure ?
Kindly correct me if I have got anything wrong
Regards





RE: Help needed
RE: Help needed
RE: Help needed
If the point is at the discharge of the compressor then you won't see much pressure rise. If it's at the inlet to the compressor then the pressure will rise to achieve the higher flow rate through all the components.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Help needed
http://www.eng-tips.com/viewthread.cfm?qid=302526
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Visit the following threads to get some more ideas:
thread403-279014: Sight Glass Use in Refrigeration Piping
thread403-155434: high compressor discharge
thread403-144104: Lower condensing pressure
RE: Help needed
dcasto is correct in asserting that the condenser of a pure fluid cannot be subcooled. By definition a subcooled fluid is one that is condensed and subsequently cooled to a temperature BELOW its saturation temperature. This cannot happen to a pure fluid that is condensed in a closed system where it is in equilibrium with it own vapaor - such as a refrigerant. I know that this is applying semantics, but the point here is understanding what we mean by the term "subcooling" as it pertains to a condenser.
Steam, when condensed in a closed steam condenser can only condense as a saturated liquid. However, when exposed to the atmosphere and cooled further, it results in subcooled water. The water is subcooled because it is not in equilibrium with a water vapor; it is in equilibrium with the atmosphere.
The principle still holds for the topic of this thread: the OP doesn't understand that the vapor pressure of the condensed refrigerant is SATURATED (not subcooled), and as such will exhibit a pressure that is equal to its vapor pressure at the condensing temperature. The colder that the cooling medium is (cooling water in this example), the lower that the saturated vapor pressure will be. This can be clearly appreciated in a Mollier (or Temperature-Entropy) diagram. Chemical engineers should easily understand this, considering all the Thermodynamics and Phase Equilibria they have to study. Mechanical Engineers don't take Phase Equilibria, so it's a little tougher for them.
RE: Help needed
Gibbs' phase rule applies.