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sparrow
21-11-2007, 06:06 PM
Hello I was asked a question on a mock exam paper,and I am unsure of the correct answer.

The question was Capacity control of a screw compressor is generally accomplished by:

(a) Hot gas bypass

(b) throttling the suction vapour

(c) Cylinder unloading

(d) sliding valve

I did not get this correct,and i am unsure still.

I think it might be (a) though.

sparrow

Lowrider
21-11-2007, 08:47 PM
The correct answer I think is (d) sliding valve.

Brian_UK
21-11-2007, 08:55 PM
Hi Sparrow, if you do a Google on the subject you will find a wealth of information on the different methods of capacity control available.

Here's a starter for you ;)
http://www.emersonclimate.com/images/pdf/7.2.3_SrewCompressorBrochure.pdf

Pooh
21-11-2007, 10:09 PM
Definately "D"

Ian

The Viking
22-11-2007, 12:19 AM
I always been know to favour "D" or "DD"s and I won't change my mind because of this question:D

Brian_UK
22-11-2007, 12:34 AM
They go up to Js and Ks now I believe ;)

US Iceman
22-11-2007, 04:17 AM
They go up to Js and Ks now I believe ;)


Isn't that sort of overkill Brian?:eek:

sparrow
22-11-2007, 10:18 AM
Hi Sparrow, if you do a Google on the subject you will find a wealth of information on the different methods of capacity control available.

Here's a starter for you ;)
http://www.emersonclimate.com/images/pdf/7.2.3_SrewCompressorBrochure.pdf

Brian, thanks that was just the ticket, nice one

sparrow
22-11-2007, 10:24 AM
Just as an after thought and because I keep coming accross this (Throttling the suction vapour)

(A) What is it?
(b) What is its role and how is it done,and why?

Sparrow

TRASH101
22-11-2007, 10:40 AM
Hello sparrow

I hope I`m correct when I say that throttling the suction vapour is a process by which the vapour in the suction line is altered by means of a variable orifice situated closest to the suction port of the compressor. The concept of the orifice (valve) is similar to that of an EEV, that is a metering device, which will alter the saturation temp within the evap. by causing a pressure drop across itself.

I hope that this will help you:)

sparrow
22-11-2007, 10:51 AM
Hello sparrow

I hope I`m correct when I say that throttling the suction vapour is a process by which the vapour in the suction line is altered by means of a variable orifice situated closest to the suction port of the compressor. The concept of the orifice (valve) is similar to that of an EEV, that is a metering device, which will alter the saturation temp within the evap. by causing a pressure drop across itself.

Hello Trash, EEV? Why could we not just use a standard TEV or similar? does it have to be very specific, could we not just use a solonoid valve to allow the flow limitations into the TEV.

Sorry to be a pain.

TRASH101
22-11-2007, 10:56 AM
Furthermore Hot Gas Injection can be used in 2 ways (maybe more).
1 injected after the expansion device so as too reduce the liquid content in the evap by a small amount of volume displacement and a lot by dumping loads of energy into the evap.

2 injected after the evap. so as to condition the suction vapour without such an impact on the evap.

sorry If this is a little vague but I would have to read up on this to get into the nitty gritty.

Whats the slide valve then and all this d ,dd ,j & k
sounds more like cup sizes to me:D?

PaulZ
22-11-2007, 11:09 AM
Hi Trash
If you throttle the suction to a screw it will unload by adjusting the position of the slide valve as in normal operation.

Sparrow
D is definitely the correct answer. There is also another way that was not listed and that is to use a VF drive on the motor. This way the slide valve stays in the fully loaded position and does not slide back and forward causing wear.
Hot gas bypass valve is for starting the compressor if the motor is a bit small. It opens and equalizes the suction and discharge so the compressor does not have to push against discharge pressure.
A hot gas bypass can be used on a screw but not for cpacity control, it would be used where the suction can drop very quickly and then increase just a quickly, the compressor cannot unload quick enough and could trip. Also when the load comes back the compressor cannot load up quick enough. I have see this system on Carbo Coolers in a softdrink canning operations.
Cylinder unloading is for a recip compressor.
Hope this helps
Paul

guapo
22-11-2007, 11:23 AM
Hi,:)
I think a,b,c,d are all incorporated in the process of capacity control.

During unloading the slide valve will pull by the hydraulic cylinder it means the effect is throtling the suction, and if the capacity is at the minimum then the hot gas will blow at the suction to mentain the compressor running.

regards;)

TRASH101
22-11-2007, 11:27 AM
Hi again sparrow

Didn`t mean to confuse you. The EEV is just a ropey example of how the throtteling valve works i.e. it works off "normally" a proportional signal which adjusts the valve position, rather like the valve/ actuator arrangement you would find on a low pressure hot water heater battery. The throtling valve must work in conjunction with a normal expansion device.

If you used a solenoid in the system, especially the liquid line, for control modulation you would cause massive problems :eek: ,probabley destructive, as solenoids work as on/off devices.

TRASH101
22-11-2007, 11:33 AM
cheers paulz

but can you tell me the significance of all the letters
i.e. dd,j etc.:confused:.

I`ve never come across a slide valve arrangement and would always opt. for variable speed drive for compressor operation when applicable.

Grizzly
23-11-2007, 02:50 AM
D is the answer to the original question as many have already stated.
Also consider the fact that on some Screw Comps you have a side load port. Increaseing volumetric gas flow further.
Grizzly

PaulZ
23-11-2007, 03:15 AM
Hi Trash
If you don't know the significance of DD and all the way to J you must have led a sheltered life!!!!

All screw compressors have slide valves, or at least all the ones I have worked on. You might want to Google Screw Compressors and all will become clear.
Paul

TRASH101
23-11-2007, 12:57 PM
ah curse you paul for so easily identifying my cotton wool wardrobe;).

Its a case of too much AC not enough fridge I`m afraid. Seldom, in 10 yrs, have I worked on anything that wasn`t hermetically sealed. Have browsed for info and can appreciate the design (man/auto ported by-pass), but the lettering is still eluding me. Are they the graduation markers on the slide for manual setting ?:confused: surely not.

frank
23-11-2007, 01:56 PM
but the lettering is still eluding me. Are they the graduation markers on the slide for manual setting ?:confused: surely not.

I've got to put you out of your misery Trash. Have a look at this (http://www.videojug.com/film/how-to-measure-your-bra-size-2) - but don't get overly excited ;)

TRASH101
23-11-2007, 06:13 PM
Thanks Brian :o:o

I am going home to advise my good lady wife that if she wants to stop her screw unloading before the correct process reqiurements are met she should reduce from e to c.:D

p.s. will you be able to put me out of my misery when she reciprocates (that will entail massive HEATed Rejection and the compression of my non condesables ?):eek:

sparrow
23-11-2007, 09:51 PM
Trash you need some serious help lol, as for the young lady surely just a vest would suffice here.

or was i expecting a little more?

Sparrow

Yasser Khaldi
23-11-2007, 11:22 PM
hi
slide valve

PaulZ
24-11-2007, 01:18 PM
Trash just remember a good fridgie always takes care of his noncondensables.
Paul

Josip
24-11-2007, 02:46 PM
Hi, Sparrow :)


Hello I was asked a question on a mock exam paper,and I am unsure of the correct answer.

The question was Capacity control of a screw compressor is generally accomplished by:

(a) Hot gas bypass

(b) throttling the suction vapour

(c) Cylinder unloading

(d) sliding valve

I did not get this correct,and i am unsure still.

I think it might be (a) though.

sparrow

During start we can utilize (a) if it's installed, to reduce starting torque - easier start....also sometimes (after long shut down) we have to start the screw compressor with throttled suction stop valve thus (b) too,

...as we do not have any pistons and cylinders on screw compressors we cannot use (c) at all,

....normal operation capacity control of screw compressors is obtained by sliding valve (d) controlling opening or closing suction port,

...on new compressors we have also Vi slide valve to control the opening of discharge port, mainly, but having some affect also to suction port...according to relation between suction and discharge pressure...some of them with automatic some with manual positioning...

Best regards, Josip :)

sparrow
26-11-2007, 08:45 PM
Josip, hi sorry still trying to work all of that out, What you are saying is that quite possibly three of the answers could of been selected but for differing reasons? I think the answer they were looking for was D.

I know you are indeed very bright and so have many others in this thread proven themselves so, could i get a better understanding on the coefficient performance on refrigeration and heat pumps.

I have this thus far
The coefficient of performance, or COP (sometimes CP), of a heat pump (http://www.answers.com/topic/heat-pump) is the ratio of the output heat (http://www.answers.com/topic/heat) to the supplied work or

where Q is the useful heat supplied by the condenser and W is the work (http://www.answers.com/topic/work-physics-in-encyclopedia) consumed by the compressor.
According to the first law of thermodynamics (http://www.answers.com/topic/first-law-of-thermodynamics-1), in a reversible system we can show that Qhot = Qcold + W and W = Qhot - Qcold, where Qhot is the heat taken in by the cold heat reservoir and Qcold is the heat given off by the hot heat reservoir.
Therefore, by substituting for W,
The coefficient of performance, or COP (sometimes CP), of a heat pump (http://www.answers.com/topic/heat-pump) is the ratio of the output heat (http://www.answers.com/topic/heat) to the supplied work or

where Q is the useful heat supplied by the condenser and W is the work (http://www.answers.com/topic/work-physics-in-encyclopedia) consumed by the compressor. (Note: COP has no units, therefore in this equation, heat and work must be expressed in the same units.)
According to the first law of thermodynamics (http://www.answers.com/topic/first-law-of-thermodynamics-1), in a reversible system we can show that Qhot = Qcold + W and W = Qhot - Qcold, where Qhot is the heat taken in by the cold heat reservoir and Qcold is the heat given off by the hot heat reservoir.
Therefore, by substituting for W,, where Thot and Tcold are the temperatures of the hot and cold heat reservoirs respectively.

:confused:

It can also be shown that COPcooling = COPheating - 1. Note that these equations must use the absolute temperature, such as the Kelvin scale.
A geothermal heat pump heating a building and operating at COPheating 3.5 provides 3.5 units of heat for every unit of energy it consumes (e.g. such a heat pump consuming 1 kW would provide 3.5 kW to heat a building). The "provided" heat comes from both the heat source and the input energy, so this does not mean that the heat source is cooled at a rate of 3.5 kW. The COP of heat pumps seems to compare very favorably with high-efficiency gas-burning furnaces (90-99% efficient), and electric heating (100%), but the full costs of the energy consumed must be considered, and energy from gas is typically much less expensive than that from electricity. Even so, a heat pump of COPheating 3.5, such as in the example above, could be less expensive to use than even the most efficient gas furnace.
A heat pump (air conditioner) cooling a building and operating at COPcooling 2.0 removes 2 units of heat for every unit of energy it consumes (e.g. such an air conditioner consuming 1 kW would remove heat from a building's air at a rate of 2 kW, partly through cooling and partly through dehumidification).
COPheating applies to heat pumps and COPcooling applies to air conditioners or refrigerators.

Is this the standard accepted on heat pumps?

Sparrow

US Iceman
26-11-2007, 09:12 PM
Josip, hi sorry still trying to work all of that out, What you are saying is that quite possibly three of the answers could of been selected but for differing reasons? I think the answer they were looking for was D.


D would have been the conventional answer. What Josip was trying to tell you is that you will find other types of unloading on screw compressors.

In fact you could go so far as to say there is a fifth answer: variable speed operation.

Some screw compressors use solenoid valves to open cylinder bores for by-passing suction gas also.

Josip
26-11-2007, 11:27 PM
Hi, Sparrow :)

...it was not my intention to make things harder to you;), but US Iceman (Thanks Mike) repair my mistake...


D would have been the conventional answer. What Josip was trying to tell you is that you will find other types of unloading on screw compressors.

In fact you could go so far as to say there is a fifth answer: variable speed operation.

Some screw compressors use solenoid valves to open cylinder bores for by-passing suction gas also.

...as English is not my mother tongue, sometimes I face problem to explain something clearly...., but here are some links where you will find very nice explanations...of COP, EER...

http://www.canren.gc.ca/prod_serv/index.asp?CaId=169&PgId=1023

http://www.heatpumpcentre.org/About_heat_pumps/HP_performance.asp

http://tva.apogee.net/res/rehpeff.asp

http://tva.apogee.net/res/rehcop.asp
http://tva.apogee.net/res/reheer.asp

Hope this is of some help..

Best regards, Josip :)

US Iceman
27-11-2007, 02:25 AM
Happy to help Josip. I don't think you made a mistake at all. I think the confusion was based on sparrow looking for one answer, and you correctly told him more than he was prepared for.;)

sparrow
27-11-2007, 05:22 PM
Happy to help Josip. I don't think you made a mistake at all. I think the confusion was based on sparrow looking for one answer, and you correctly told him more than he was prepared for.

Yes I concur the reason for specifically looking for one definitive answer is that when I take the exam, and the computer is linked directly to the city and guilds board, they generally do tend to laugh off any students, multi selected answers.

Donít get me wrong if I could put more than one in I would be happy to do so.
This action will invalidate my submitted attempt, and hence no cigar would be the outcome.
The cop info you supplied Josip was the business, I was able to discuss with confidence today the findings.
I have now moved into designing a wiring diagram for a pump down system.
Knowing why each component follows directly after the previous one is also very important, Question 1

Would I be able following the thermostat, be rite in thinking that I could connect to a solenoid valve? Or would one have to go directly to the compressor, in order to move the refrigerant?

Sparrow

frank
27-11-2007, 05:59 PM
Question 1

Would I be able following the thermostat, be rite in thinking that I could connect to a solenoid valve? Or would one have to go directly to the compressor, in order to move the refrigerant?

Sparrow
Providing that the compressor has an LP (low pressure) switch fitted, then you can control the SV with a thermostat to effect pump down. If you position the SV close to the evaporator inlet then you effectively have all the liquid line to use for storage of the refigerant as well as the condenser (where no receiver is fitted).

With long pipework runs you can get cycling on the LP switch at startup while the evaporator fills, but this can be overcome by delaying the compressor start for a couple of minutes after the SV is energised.

sparrow
27-11-2007, 07:17 PM
HI Frank is it me or is everyone a site moderator and more inportantly a general nice guy?

Anyway thanks for you prompt reply, I have considered the information you have put forward, to this end I feel I should point out that the system has to have a liquid receiver incorporated also there is Surge pot, now I was under the misconception that this little piece of necessary equipment was called an accumulator.

My Tutor advises me differently, anyhow I have to incorporate not only the low pressure switch, I also have the high pressure switch and as you point out the Sv just shy of the evaporator inlet. This little beauty must include my black rod defrost set up,again these do not trouble me too much as there is only a live feed to these just like a switch either on, on defrost cycle or not. (could they take being on for two hours?)

Sparrow

John Hunter
27-11-2007, 07:53 PM
The method I prefer for load control on screw compressors on new equipment is speed control. We find that range between 4 - 6 pole on the 50 Hz system gives quite an efficient and smooth operational profile. Below 4 pole it is considered that slippage could occur. The operational percentage between 6 pole being a 100 % , 4 Pole produces 48%. Many installations still use the slide as the means of control and is quite acceptable but once the capacity position reduces below 90% the overall efficiency begins to drop off quite rapidly. Suction throttle. This is used on plants where the compressors suction is at a common low pressure but the plant may have two or more operational evaporation requirements in the field , i.e. chillers and freezers . The plant suction will be maintained at a pressure in the freezer range but the evaporation on the chiller system held say 2 bar (30 psi) higher by being throttled via a BPV such as a Danfoss PM . Works OK by not all that efficient but initial costs sometimes makes it attractive.

tinpoint
24-12-2007, 09:43 PM
An Rmo Observes A Gauge That Reads The Saturation Pressure Of A
Refrigerant, If Chilled Water Is Being Circulated As The Cooling
Meduim This Reading Can Best Be Used To Estimate:

A. The Temperature Of Chilled Water Bieng Pumped To Spaces
B. Chilled Water Temperature Return
C. Cooling Tower Water Efficiency
D. Condenser Water Flow Rate

hendry
03-01-2008, 05:56 AM
Hello I was asked a question on a mock exam paper,and I am unsure of the correct answer.

The question was Capacity control of a screw compressor is generally accomplished by:

(a) Hot gas bypass

(b) throttling the suction vapour

(c) Cylinder unloading

(d) sliding valve

I did not get this correct,and i am unsure still.

I think it might be (a) though.

sparrow


since it was an exam Q, i feel the educator is looking for answer D.

however, in real life all the above is applicable in capacity ctrl.

it has to do with the engineer's design concept & his/her selection of equipment.

sometimes, the lowest capacity after unloading the compressor is still too high cap for the job, you have to introduce something else to stabilize the system.

i feel, for exam, we have to answer smartly to fits the needs and not so much to our expertise.

conrad
05-01-2008, 10:29 PM
Hello I was asked a question on a mock exam paper,and I am unsure of the correct answer.

The question was Capacity control of a screw compressor is generally accomplished by:

(a) Hot gas bypass

(b) throttling the suction vapour

(c) Cylinder unloading

(d) sliding valve

I did not get this correct,and i am unsure still.

I think it might be (a) though.

sparrow
tha answer is [d]

Josip
05-01-2008, 10:48 PM
Hi, Tinpoint :)


An Rmo Observes A Gauge That Reads The Saturation Pressure Of A
Refrigerant, If Chilled Water Is Being Circulated As The Cooling
Meduim This Reading Can Best Be Used To Estimate:

A. The Temperature Of Chilled Water Bieng Pumped To Spaces
B. Chilled Water Temperature Return
C. Cooling Tower Water Efficiency
D. Condenser Water Flow Rate

An Rmo is-
a-Radar Management Officer
b-Regional Medical Officer
c-Responsible Managing Officer
d-Risk Management Office/Officer
e-Resident Medical Officer
f-Records Management Official
g-someone else;)


Is this question from some exam to confirm/denied your answer or maybe a question for RE members;)

What do you think we can estimate with this reading?


Best regards, Josip :)

SteinarN
22-03-2009, 10:50 AM
I always been know to favour "D" or "DD"s and I won't change my mind because of this question:D

I agrees definitely on this! :D

rajaprabhu
08-12-2009, 09:18 AM
The answer is Sliding Valve

Steve Wright
08-12-2009, 05:22 PM
Have you guys seen screw capacity control be suction cut-off or hot gas bypass?

I've only seen slide valves, port, and variable speed as cap control devices.

I have see Vi control to minimize over compression.

imran ansari
09-01-2010, 01:17 PM
Hello I was asked a question on a mock exam paper,and I am unsure of the correct answer.

The question was Capacity control of a screw compressor is generally accomplished by:

(a) Hot gas bypass

(b) throttling the suction vapour

(c) Cylinder unloading

(d) sliding valve

I did not get this correct,and i am unsure still.

I think it might be (a) though.

sparrow
answe is d

imran ansari
09-01-2010, 01:19 PM
correct answer is d

sparrow
18-01-2010, 03:55 PM
The method of compressor capacity control known as "suction valve lifting" is operated by

(a) hydraulic oil pressure

(b) changes in compressor discharge pressure

(c) internal gearing connected to the compressor crankshaft

(d) a transducer in the suction line

I got this incorrect can someone shed some light please.

Sparrow

TRASH101
18-01-2010, 05:12 PM
http://www.dallashermetic.com/pdffiles/Worthington2V-3VCapacityControl.PDF

Hi sparrow try this link

sparrow
18-01-2010, 07:49 PM
whoa, that's more like it trash thanks.

Sparrow