Date: Wed, 04 Jan 2006 23:17:48 GMT From: "WOOFER" <Woofernospamo.com> Subject: Re: Curious Heater Problem
Fellow Travelers, This has been a most enjoyable thread but it's time to dispel all the myths with the introduction of some real wisdom.....mine, of course. ; ) Simple water flow models are easily analyzed with Joule's Law...Power = Current x Volts. This axiom, together with Ohm's law, shows us that if you have enough Voltage (Pressure), small changes in the Resistance (Passageway options) will make no meaningful difference in the Power (Heat) delivered to the Loads (Radiators) because the Current (Available Flow Volume) part of the equation becomes insignificant to the Voltage (Pressure) part. As pointed out in a parallel thread, it's only a problem at low RPMs. Low pressure. Low Voltage. We have a classic Voltage sag resulting in inadequate Power delivery to the Load with the higher Resistance. The bathroom light goes dim when your wife turns on the hair dryer. With triple the Voltage there would be no problem. So...Is the water pump developing enough pressure (Voltage) to avoid these problems? A worn/defective impeller could do all these things. Does noone remember changing the thermostats in a SAAB 93 from a "Summer" thermostat to a "Winter" thermostat? It's the same problem. An inadequate (or malfunctioning) heat exchange system being asked to operate beyond it's abilities to perform at the extremes. Let's also remember that the divergence of coolant flow through the heater core on a hot summer day is unlikely to be a problem in a vehicle that controls its heat with a water valve!! Thanks to everyone for their input on this one! I've enjoyed the ride. Samhot "MH" <nonospamo> wrote in message news:dp9tuk$k3t$1nospam3.zwoll1.ov.home.nl... > > .. if you reduce the parrallel paths from 2 to one, and >> maintain the same pump pressure, the amount of flow through the remaining >> path will be increased. > > I do not know how you define 'amount of flow', but flow rate Q is defined > as a volume passing throug a surface area in a certain time; e.g. > litres/sec or m3/hr. > With the same pump at same rpm and the same flow rate, if you close one > leg of the parallel path, the same flow Q will go through a smaller area, > at higher speed; if you half the area, the _speed_ will double. > > If hot water flows through a radiator at higher speed, its retention time > in the radiator is less, and it will release less heat to the passing air. > For the flow speed to be as slow as possible, you would want the radiator > to have an as large as possible flow through area (for both water and > air). > > The thermostat changes nothing to the pump's flow rate, it just opens to > divert part of the flow out of the block to the radiator, instead of > directly back to the pump. > > -- > MH > '72 97 '77 96 '78 95 '79 96 > '91 900i
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