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Suppose your load is60KW and you using one60KW Chiller.
1- How you will do maintenance to the chiller?
2- What will you do if the chiller is out of order?
3- Why use a60KW chiller in the wintertime when the load drops to20KW or less?
Second question why not use two chillers
Same example as above. We have60KW load and are using two30 KW chillers
Our problem is solved partially
1- we can shut down one chiller when load drops below30 KW
but
2- We cannot do maintenance when load demand is high
3- When one of the chillers is out of order during high demand we are doomed.
Partial solution is using two60KW chillers we stop one and do maintenance on the other.
but we will be still using60 KW chiller when we have20 KW load demand
Best solution is use three30KW chillers which will be cheaper than two60KW chillers
2 chillers will operate and one stand by
during low demand one chiller will be operational and two chillers when high load demand.
as one of the chillers is stand by we can do maintenance on any chiller we want as the other2 will be operational.
when one of the chillers is out of order the other2 chillers will be sufficiant untill it is repaired.
I hope this will shed some light over the question.
3 element control system is good to use when the boiler load is more or in full load because this consider drum level,steam flow & feed water flow . But use of single element system lead to fluctuation of drum level in high load. single element system is better to use when boiler run in low load or in start up.
we use it where the process is critical.
we use three element loop in boilers because we want pre-action automatically without any time delay before the abnormal conditon or result accurs. in simple words3 elment loop or feed forward loop depend upon the criticality of the process.
Why is3-element not advised for boilers that need to change firing rate quickly? The answer is that3-element assumes that if you keep your water mass constant in the boiler, your level will stay at setpoint. This is incorrect. In fact, as the boiler load increases, the water vapor phase in the boiler rizers increases. Hence, for a constant level, your water volume will DROP as your firing rate is increased. This is the reason why steam flow trips boiler drum levels. We prefer using firing rate (or GT MW for CC plants) as a feedforward signal to drum level control - much better for rapid firing rate changes as it manages your volume balance.
In Boilers, Three , Two or One Element Controls are used based upon requirement or criticality of Operation.
Three Element Control Gives more in Depth Control of Boiler in terms of In-Flow of Feed Water, Out-Flow of Steam & Hence Level of Water & Steam in Drum of Boiler
3 element control is used for precise controling of water level in steam drum for preventing dryout cause by swell and shrink effect caused by In-Flow of Feed Water, Out-Flow of Steam.
