Hi Mike
the "vertical" may be a statement of location rather than of function.
The "low loss" bit may indicate the function. Some engineers who have
suffered NPSH problems in LTWH (and low pressure) systems arbitrarily
design headers with very low velocities; around 0.5 m/s. Check your
peak flow rates and this may help uncover if this is the intention.
The only other reason I can think of is thermal shock avoidance as
you have suggested. Some novel solutions have been tried to overcome
the perennial "cold or hot standby boiler" question. A cold boiler
can slug the system with a gutful of cold water when called up but
doesnt waste heat. A hot standby boiler wastes energy even with a
good stack damper. I used to go halfway with a warm boiler,adjusted
by predictive software (built-in demand model).
In your case the header may have been designed so the sink velocity
of the cold slug counterbalances the upward discharge velocity, such
as is considered when designing stratified thermal storage vessels.
This would give good mixing and minimal thermal shock. However it is
difficult to make a judgement without a drawing.
Have never designed an absorption chiller into an MTHW circuit so
this may have something to do with it.
Ask the designer, and respond with his explanation. That may save
some time.
Cheers and good luck
Steve McKenzie
- In PipingDesign@yahoogroups.com, "mikekdixon" <mikekdixon@y...>
wrote:
> The system is MTHW. On the generation side of the header, there is
6
> boilers and associated pumps. On the distribution side there are a
> number of pumps and exchangers to provide space heating for
buildings
> and hot water for absorption chilling. I take on board what you
are
> saying but as there are pumps I don't think it is for natural
> convection. We are looking at physical sizes of 1.5m diameter and
7m
> high.
> My initial thoughts were that it was some sort of thermal buffer
with
> a thermal gradient up its length. Smoothing out any steps when
> tunring boilers on and off.
>
>
> --- In PipingDesign@yahoogroups.com, "Steve McKenzie" <Mechproj@x>
> wrote:
> > Hi Mike
> > I assume you have either LTHW or MTHW boilers feeding some
> radiators or fin
> > coil units. With this type of boiler, best performance is
normally
> achieved
> > by maintaining a fairly constant flow rate through the boilers
and
> varying
> > the firing rate according to the boiler discharge temperature,
> modified by
> > boiler differential temperature. The heating distribution mains
is
> normally
> > variable volume with constant differential pressure across the
> distribution
> > circuits. This means that the distribution mains flow rate varies
> with the
> > heat demand, but a constant flow rate through the boiler is
> required.
> > There are normally two approaches to this. The first is to use an
> injection
> > system.
> > This has two duty pumps; one to circulate through the boiler at a
> fairly
> > constant rate, and a second to circulate through the (secondary)
> > distribution system. Heated boiler water is bled off the primary
> circuit
> > into the secondary via a control valve. The valve is controlled
by
> the
> > secondary circuit flow temperature. This is the only way to go
when
> > distribution circuit pressure loss is high (long pipe runs).
> > The second is to use the primary boiler circuit and pump with an
> orifice
> > plate or control valve to generate a pressure difference for the
> > distribution mains and circuits. Good for short runs and low
> pressure drops.
> > As hot water rises, a bit more head is available by buoyancy but
> the effect
> > is normally not much.This is probably the intent of your low loss
> header -
> > very little if you are using pumps.
> > If you have a natural convection system you will have no pumps,
> very large
> > pipes, temperature control will be difficult and you will
> definitely need
> > the low loss header.
> > Natural convection can be and is engineered (e.g. nuke subs to
> eliminate
> > pump noise) but it seldom has a place nowadays in general
> engineering single
> > phase distribution services.
> > A couple of diagrams sure would help.
> >
> > Cheers
> >
> > Steve Mckenzie
> >
> >
> > -----Original Message-----
> > From: mikekdixon [mailto:mikekdixon@y...]
> > Sent: Wednesday, March 26, 2003 10:45 PM
> > To: <a href="/group/PipingDesign/post?postID=PeezPgbZaErWRLhS7UzqaXyRA_jjWcgSA8gSFJJV0j4JaeG11RQzOY_85J_w8KFGXjA5JMUH65k0MGWgl2aekWjA">PipingDesign@yahoogroups.com</a>
> > Subject: [PipingDesign] Low Loss Vertical Mixing Header
> >
> >
> > I have received a P&ID from one of our clients for a hot water
> > production and distribution system and there is an item called
> a 'Low
> > Loss Vertical Mixing Header' on it. It has hot water going in and
> > coming off at the top and the cold water returns coming in and
going
> > back to the boilers at the bottom. I think it is intended to be
> some
> > sort of thermal buffer. Is there any experience of these or
> > explanations behind them?
> >
> >
> >
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Received on Fri Mar 28 08:34:00 2003