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@yahoo.co.uk]
Sent: Wednesday, March 26, 2003 10:45 PM
To: <a href="/group/PipingDesign/post?postID=qNrNyiOfBSWNXHrytn311Qfi5rGJmqPw9gGl6Q1JQ5M5okZnlJex50RTF_VJQEoYt87_aIpaP6t9HJ-cBjaeWc2GRQs">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?
Your use of Yahoo! Groups is subject to <a href="http://docs.yahoo.com/info/terms/">http://docs.yahoo.com/info/terms/</a> Received on Wed Mar 26 06:51:00 2003
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