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Mucking fuddle
13-08-2013, 01:48 PM
Hey guys, i have just been challenged with the task of drawing a control diagram for an existing chiller, this unit has been modified so many times by so many people that I dont think the chiller knows what it is any more :p.

My question is there have been 2 timers added to bypass the low pressure switch, is this normal and why would you need to?

Also why does there never seem to be overload protection on compressors?

Sorry for my ignorence but i am a Controls Engineer from Manchester, now living in Canada

cadwaladr
13-08-2013, 09:08 PM
single phase hermetic compressors have a built in overload ;klixon;,as for the delay timer on the l/p can only guess its a preset l/p sealed switch so there to let it settle down?

sedgy
13-08-2013, 09:16 PM
hi MF, in the old days ( before global warming) BGW, it was known for engineers to fit a cold winter pack ( CWP) this was a 2nd LP switch set 5 PSI higher than no,1 LP so on chilly mornings( which the UK + Canada ) do have. the new LP would trip and a timer would keep this ON for half an hour or so hope this helps =sedgy

install monkey
13-08-2013, 09:21 PM
have contactors been replaced and some one omitted the overloads??
as for the delay timers- they can be fitted to act as a delay on start up
however- wouldnt it be better to fit an adjustable pressure switch without a delay
whats the model of the chiller- check the original wiring diagram-see what should be in there, then maybe suggest additional safety devices.
- surely a controls engineer should be able to answer a controls question? :D


Hey guys, i have just been challenged with the task of drawing a control diagram for an existing chiller, this unit has been modified so many times by so many people that I dont think the chiller knows what it is any more :p.

My question is there have been 2 timers added to bypass the low pressure switch, is this normal and why would you need to?

Also why does there never seem to be overload protection on compressors?

Sorry for my ignorence but i am a Controls Engineer from Manchester, now living in Canada

Gibbo
14-08-2013, 12:11 AM
LP switch with a bypass timer were normally fitted for low ambient conditions on older chillers

The Viking
14-08-2013, 12:28 AM
As others has said above,
The LP timer will prevent LP trips just after compressor starts, before the system reach a stable running condition. In my experience this would be set between 1/2 and 5 minutes.

Either it is done to allow for starts at low ambient temperatures or sometimes it is to allow a higher than normal low pressure cut out. On some industrial chillers, the LP switch is also monitoring for a lack of flow or build up of ice in the evaporator, then the LP switch would be set just below the pressure at which the refrigerant's saturated pressure is at the freezing point of the cooled media. (0 C for water) but when the system first start to circulate the refrigerant it will take a minute or so before the pressure in the evaporator reach that cutout point, especially if it pumped down for shut off.

:cool:

The Viking
14-08-2013, 12:41 AM
Another thought, others has mention that this is old school, modern chillers often has the same function as the LP timer built in to their software. Sometimes it's an adjustable function in the menus of the controllers other times it is hidden in the background.
About 2 years ago I even had the manufacturer's engineer out to look at brand new chiller that just been commissioned but kept dropping out on LP. He arrived late one Friday afternoon and stated on his report that "the chiller started an run OK". Looking at the key card log, he had spent less than 15 minutes on the roof. On his return visit (after repeated failures of the unit) he said that we must have a problem on the wet side as the chiller started up OK. Only after I cut the lead to the LP switch did he accept that there was a delay before the controls actually looked for LP faults. (On this particular chiller it was in the software to disregard the LP switch for 9 minutes after compressor start up)

:cool:

Mucking fuddle
14-08-2013, 10:12 PM
Thanks for the help guys this is for a HVAC chiller on a telecoms room on a gas platform in the Atlantic, so cold mornings are a definate so much so we have had to heat trace the compressor :). Compressors have built in thermal cut outs but it does not stop our client cooking them, the chiller was made by MARC controls in the USA and has never had overloads, as for controls engineer asking about controls i am used to process controls in the oil and gas industry, and also as previously said i am from manchester so i got my degree on the back of an embassy packet.LOL

The Viking
15-08-2013, 12:23 AM
Embassy packet???

In MANCHESTER??? :eek:

Must have been on the posh side!

:cool:

Mucking fuddle
15-08-2013, 06:27 PM
:o:eek: Love it.

install monkey
15-08-2013, 09:43 PM
lived here for 35yrs,dont know of any posh parts of manchester!!!- neither does google,bing !:o
yahoo came up with this- i disagree with the answer as 1/2 the places are either on a sk postcode-cheshire or bl for bolton!!!
http://uk.answers.yahoo.com/question/index?qid=20100908180217AAcbPAz
:rolleyes:



Embassy packet???

In MANCHESTER??? :eek:

Must have been on the posh side!

:cool:

The Viking
15-08-2013, 10:57 PM
So during those 35 years, how often have you seen a native smoking Embassy??

:cool:

Mucking fuddle
18-08-2013, 10:01 PM
Sorry i am from Stalybridge, it used to be nice but then the bars took over, My lecturer smoked Embassy i could only ever afford Lambert and Butler :(

install monkey
18-08-2013, 10:05 PM
staly vegas!!
i remember a curry house opposite the bus station- threw up all over the place- was about 20yr ago!!
im in audenshaw- bout 3 mile away!

Mucking fuddle
19-08-2013, 02:22 AM
served my apprenticeship in Audey at Carey Electrical