capacity control basics

[B_roche]

I'm having a hard time keeping up with the commercial side of this forum. could I get some help reguarding capacity control? I want to keep my question general because basicly I'm a blank slate and any advise helps.

heres where I stand though. I was very recently introduced to staging of a compresser. I have a fairly good idea of how the unloader works but havn't seen a hot gas bypass yet and I don't know how it works. so what does the hot gas bypass do exactly?

when are the unloaders generally engaged and disengaged?

Thanks,
Brian

[Dad]

when y2 calls

brain... drop me an email

[B_roche]

see the one I worked on the other day had two semi herm compressers. It was a chiller with a solid state dc signal thermostat. I would have expected compresser no. 1, then loaded, then compresser no. 2, then loaded.

It didn't seem to be going in that order, though. seemed to me like the unloaders had a mind of their own. I got a little confused. trying to observe them. compresser no.1 unloaded while no.2 was still running and that pretty much defied my preconcieved notions.

the tech tried to explain it to me but he doesn't really seem to understand what I'm asking him.

[Milk man]

Originally posted by lusker
when y2 calls

brain... drop me an email

Share with everyone.

[h2045]

Typically on a 4 stage system: #1unloaded#2off,#1unloaded#2unloaded,#1loaded#2unl oaded,#1loaded#2loaded, as load decreases system stages down in reverse order.

[ozone drone]

Usually the unloaders are staged by the suction pressure.
as the suction rises more capcity is added. As suction drops , the unloaders unload.

Hot gas bypass also looks at suction pressure...as your suction drops ...hot gas is introduced to add an artificial load....this is usually done on larger systems when it is more economical to add a false load than to have to start and stop the compressor.

[Shophound]

Unloaders can be staged by suction pressure, as ozone says, or they can also be electrically activated using solenoids, and staged by demand.

My own building has two split systems, one 40 ton and one 25 ton, that use two unloaders each per Carlyle 06E semi-herm. The air handlers are VFD units supplying a constant 55 degree discharge air to various VAV boxes located in the building.
The unloaders are controlled by a set of Johnson Controls A330 and S330 controllers that load and unload the compressors according to the demand necessary to maintain discharge air temp. The unloaders themselves are suction cutoff solenoid operated. When the solenoids are de-energized, the compressor is unloaded to the degree of staging necessary to maintain discharge air temp. They are energized as demand and heat load increase.

Both systems use hot gas bypass to induce a "false load" during low outdoor ambient conditions (winter operation). This is more economical than cycling a 25 or 40 ton compressor all winter long (not to mention much easier on the compressor and contactors! Imagine 480 volts arcing across a contactor all winter long as the unit constantly cycles)

Units of this capacity and larger also often stage condenser fans as another measure of capacity control. The idea is to maintain adequate head pressure in low ambient conditions and adequate subcooling during high ambient/high demand conditions. My 40 ton condenser has three fans. The first is on a Motormaster controller, which is essentially a freq drive for a condenser fan. It uses a sensor that reacts to the temperature of a chosen return bend on the condenser coil. Under lower ambient temps, it will slow the fan down. On very cold days that fan will barely turn.
The other two fans are cycled either by head pressure or outdoor ambient (fan #3 is OAT and #2 is head pressure).

Once you get one of these systems dialed in right, it's cool to see all that stuff working to self regulate system capacity. When I took our two systems over from outside contractors they both were way out of whack and often tripped offline in low ambient conditions. I took some time and research to get these things in order, and after some additional repairs I made last summer (leaks and oil return issues), we went through a whole winter without a single trip-out.

[airworx]

depends on design, how many circuits, if electric unloaders,
pressure and hydrolic unloaders will load and unload depending on suction pressure, unloaded at 58 lbs dx and 55 lbs chillwater.

i genrally like to see compressors loading up before other compressors are started to keep compressors from running unloaded for long period.
compressors could have problems if ran for long periods unloaded.

[gasnowman]

Hey Greenears, You may better clarify things with your make, model and your application to find what you are seeking.

[B_roche]

Originally posted by gasnowman
Hey Greenears, You may better clarify things with your make, model and your application to find what you are seeking.
nothin specific, I was wondering in general.

Danm I just learned alot...thanks for all the help. that turned into a good thread.

[websy]

Just to throw something new into the mix we used to run split condensors when I was still involved with supermarket refrigeration systems, they may still use this method. This of course is only necessary during very cold weather. Once the ambient outside drops below a certain temp, the one side of the condensor coil is actually taken out of the loop by closing one side of the twinned discharge line via an oil pressure controlled valve. This requires a large reciever and a lot of tweaking but its also very interesting to watch it actually work.

Not trying to confuse anyone here just thought I'd throw in my own experiences.

[DaleP]

Brian,

Get a compressor or unit model number (at least brand) and start by learning one type. There are a lot of differnet types of capacity control. Start by fully understanding one and go from there. It can get very confusing trying to look at all of them at once especially if you haven't worked with them. It ain't rocket science but it can be a little challenging at times. My .02 Dale

[fixitman]

Some of what you are seeing can depend on how the cooling coils are circuited. Some units have their circuits thoroughly entangled, so the whole coil gets sort of cold even with one stage of cooling operating. Other units of similar tonnage may stage by cooling the bottom half of the coil, then the top half.

In the former case, they might want to stage so that the compressors get equal run time. That may have been what you were seeing. First on / first off. In the latter case, they will always want to start by cooling the bottom half of the coil, and end by cooling the bottom half of the coil. First on / last off. In the latter case, the bottom goes on first and off last because doing it in any other order reduces the dehumidification.