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Thermo 101

To: tigers@autox.team.net
Subject: Thermo 101
From: STUART_BRENNAN@HP-Andover-om3.om.hp.com
Date: Mon, 17 Jun 96 13:24:10 -0600
Item Subject: cc:Mail Text
     What's all this "Coolant Doesn't have time to cool off" stuff anyway?
     
     It's been a bunch of years since college, but I think I remember 
     enough Thermodynamics to cause a little trouble.  So here goes:
     
     Is there such a thing as coolant flowing too fast?  Let's not confuse 
     temperature and heat.  Think of a baseboard heater, a long pipe with 
     fins down it's entire length.  Hot water enters at one end,  and 
     leaves the other somewhat cooler.  The radiator is the interface 
     between the hot liquid and the cool air.  From Physics, we know that 
     heat flow is proportional to the temperature difference across the 
     interface.  The smaller the temperature difference, the less heat 
     flows. 
     
     If the colantflow is fast, the fluid doesn't have much of a chance to 
     cool, and the whole pipe stays pretty warm, heating lots of air.  For 
     the same inlet temperature, a slower flow will make the exit end of 
     the pipe cooler, so less heat is being transferred to the air at that 
     end.   Which type of flow do you want heating your house this winter?  
     Or cooling your car this summer?
     
     The answer is the same.  Don't slow down the coolant.  
     
     The engine is producing lots of waste heat as it works, which must be 
     carried away before things melt.  The longer the coolant spends within 
     the block, the more heat it absorbs, and the higher it's temperature 
     gets.  But as the coolant gets hotter, the temperature difference 
     between the block and the coolant gets smaller, reducing the flow of 
     heat into the coolant. This drives the block temperature higher, so 
     that the same amount of heat can flow into the hotter coolant.  If the 
     coolant flow is slower, the  temperature will drop more across the 
     radiator , but the coolant is moving more slowly through the engine 
     too, so the engine outlet temperature is going to be higher. This is 
     just what your thermostat does.  It slows the flow to force the engine 
     to warm up quicker, then opens to maximize the flow at high 
     temperatures.
     
     You wanted a new thread. Larry?  Let's see what this kicks up.
     
     Stu

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