Hot Gas Bypass (HGBP)

[CoolKids]

Hi All

Has anyone got any info on a hot gas bypass installation.
We a pricing a job at the moment and they require HGBP in a DX AHU
We are to install it on the first stage of cooling to regulate the cooling load.

Any diagrams/pipe selection tools would be great.

Regards

Coolkids

[Brian_UK]

Is this for capacity control ??

[CoolKids]

Yes

Coolkids

[US Iceman]

Hot gas bypass does not regulate the cooling load. The hot gas is injected into the evaporator as the cooling load decreases. A pressure regulator controls the flow of hot gas into the evaporator, which is then cooled off by the TXV injecting small amounts of liquid to cool down the hot gas.

The total volume of gas flows back to the compressor to keep it operating at it's minimum capacity. The hot gas valve is typically sized for the minimum step of compressor capacity available.

If the compressor has unloaders, then you size the hot gas valve for that minimum step of compressor capacity.

If the compressor does not have internal unloaders, then you have to size the valve for the total compressor capacity.

That way when the cooling load drops off, the compressor suction pressure remains constant. With this type of an arrangement the compressor is doing no cooling (because the load is decreasing or zero).

[chemi-cool]

What you need is evaporator pressure regulator.

HGBP is as explained by my wise friend

[rd_shetty2003]

For installation instructions search in google hot gas bypass 30ga900161

[Tycho]

Hot gas bypass does not regulate the cooling load. The hot gas is injected into the evaporator as the cooling load decreases. A pressure regulator controls the flow of hot gas into the evaporator, which is then cooled off by the TXV injecting small amounts of liquid to cool down the hot gas.

The total volume of gas flows back to the compressor to keep it operating at it's minimum capacity. The hot gas valve is typically sized for the minimum step of compressor capacity available.

If the compressor has unloaders, then you size the hot gas valve for that minimum step of compressor capacity.

If the compressor does not have internal unloaders, then you have to size the valve for the total compressor capacity.

That way when the cooling load drops off, the compressor suction pressure remains constant. With this type of an arrangement the compressor is doing no cooling (because the load is decreasing or zero).

wouldnt it be easier to just "short circuit" the hotgas line and suction line?

And I call this method the "BTSITH" or "Bite thy self in the hind" method.

Run a frequency converter on the compressor and at minimum load and maybe 1200 rpm it would have so little to work with that if it stops on LP, there really isnt any need for the compressor to start in a while, in comparison to stopping at 3000 rpm where the system might start calling for the compressor to start again after a few minutes and it only gets a 15 minute downtime each stop.

[US Iceman]

wouldnt it be easier to just "short circuit" the hotgas line and suction line?

And I call this method the "BTSITH" or "Bite thy self in the hind" method.

Yes, it would be easier to just dump the hot gas in the suciotn line, but then you also need a liquid injection valve for desuperheating the hot gas. And, with this method a suction accumulator is not a bad idea to install.

If you dump the hot gas into a side-port distributor the main TXV takes care of the desuperheating.

I think your name for the "short circuit" method is aptly named!

[Peter_1]

Avery un-economical way of regulating

[Tycho]

Yes, it would be easier to just dump the hot gas in the suciotn line, but then you also need a liquid injection valve for desuperheating the hot gas. And, with this method a suction accumulator is not a bad idea to install.

If you dump the hot gas into a side-port distributor the main TXV takes care of the desuperheating.

I think your name for the "short circuit" method is aptly named!

I see your point, the ones I have come across have been a simple solenoid/PM valve from HP to LP on the compressor.

for continued operation I see how bleeding it to the evap would be better.


the ones I have seen has a OM valve with a CVP attached, and as the suction pressure reaches the set opening pressure of the CVP you hear *WHOOOOOOOOAMH* as the PM valve opens and short circuits the compressor and also dumps alot more hotgas into the LP side than is really needed.


I am sorry to say I have been part of installing a system with this as part of the "capacity control" it was two horizontal plate freezers injected with HP liquid and a LP reciever (when the liquid boiled of it injected more hp liquid) fair enough an ok system.

I was laid of due to bad economy from my regular company at the time and to make ends meet I helped a competitor of my regular company to supervising the build of this plant on my spare time and got fairly paid, I felt like they worked by the "Let's use what we have availavle" instead of "We'll have the proper valve in three days, let's wait till we have it before we proceed"

anyways... the system was to have the LP hp Comnnection, and there was this huuuuge PM valve short circuiting the compressor...

I was up in the galley getting food one day after the plant had been started, and I could hear *FWOOOOMPH FWOOOOMPH FWOOOOMPH* as the valve was working, and it felt bad every time I heard it..

[US Iceman]

I like your sound effects in the post. Very descriptive.

[Billy Ray]

Gentlemen,

would there be any benifit dumping saturated gas from the top of the receiver into the suction, oposed to hot discharge gas? would this limit compressor discharge superheat?

Alternatively, could this be dumped into the evaporator instead?

Billy Ray

[US Iceman]

Hi Billy Ray,

That is one possibility. Although, what I think would happen is the receiver pressure could decrease and limit how much liquid flows to the TXV.

If I had to use a gas bypass for part load control in today's environment I think I would use an inverter to control the compressor speed down to the minimum allowable RPM (suggested by the compressor manufacturer) as Tycho suggested. Then at that minimum capacity use the gas bypass. This helps to minimize the power use by the compressor and you end up with a fairly simple system that would work well.

[Pooh]

The only disadvantage of using an inverter as against hot gas injection is that with the inverter the swept volume of the compressor effectively reduces as the speed reduces which reduces the velocity in the pipework leading to problems with oil return. With hot gas injection the minimum velocity required by the system can be maintained to aid oil return and still meet the rduced load on the evaporator.

Ian

[wambat]

Hot gas bypass (HGBP) has some short comings such as introducing problems stemming from insufficient oil return and refrigerant logging in the HGBP line. And in all cases, hot gas bypass inflates the life-cycle cost of the system:
HGBP increases the initial cost.such as requiring an additional refrigerant line, which also increases the likelihood of refrigerant leaks and oil/refrigerant logging.
HGBP greatly reduces operating efficiency because the bypassed vapor does no
useful cooling.
HGBP costs more to operate because as the load fluctuates, the compressor
consumes more energy because it’s forced to operate at a compression
step that is likely one stage higher than necessary.

You should only use hot gas bypass if all other design
options fail to meet the demands of the application.
Most comfort cooling applications can operate between steps of loading
without loss of temperature or humidity control. Not only does this statement
hold true between the third and fourth steps of cooling, when the outdoor
latent load is high, but it also holds true for the step between off and first-stage
cooling when the outdoor latent load is comparatively low.

If you still feel the need for HGBP then be aware that diverting
hot gas from the discharge line and around the condenser reduces the
head pressure of the air conditioning
system. As head pressure decreases, operation may become unstable — and
unreliable. To avoid this risk, iyou need to include a method for head pressure control in
every HGBP application.

[S.M.Gokhale]

Hi Coolkids,
I notice that you have mentioned you wish to regulate cooling in first stage.
Perhaps this isn't a comfort cooling application.

Hot Gas Bypass will artificially load the system and is very useful when you have to cool ambient air in a once through system. Such systems are online systems - i.e. you cannot switch-off the compressor and you need to balance the fluctuating load which varies with the ambient temperature and ambient R.H.

'Sporlan' has very good literature on this.
You can visit www.sporlan.com and download bulletin 90-40 which gives all the information you need.
If the system is piped as shown in this bulletin you will not encounter any problem at all.

I have installed this type of system - without any crankcase or evaporator pressure regulating valve where I needed to cool ambient air down to 2 Deg. C Dew Point (entering air was between 5 Deg. C DB to 32 Deg.C DB and also varying WB).
This was a once through system and the air was consumed in pneumatic conveying. The system is operating for more than 2 years without any problem.

Needed a monster coil for this one though - 18 rows deep to cater to the dehumidification load during monsoon.

In case you need any further help pm me.