Hot Water Physics Related To Solar Water Heaters - Rheem 52S160 Service Instructions Manual

Hot Water Physics Related to Solar Water Heaters

There are physical properties of hot water that are common to all types of heating
mediums. However, with solar heating an understanding of these properties will be of
assistance to servicing a solar water heater.
Stratification - The term used to describe thermal stratification within a water heater
where hot water will lie above cooler water without mixing. Stratification allows the storage
water heater to deliver hot water from the outlet, while refilling with cold water at the inlet.
Stagnation temperature - This is the temperature at which HEAT LOSS is equal to HEAT
INPUT. In this case when water stops circulating through the solar collector the
temperature will rise to the STAGNATION TEMPERATURE.
Density of water - Water is at its maximum density at 4ºC. When heated above that point
up to 100ºC it expands, unequally, an average of 1/23 of its volume. However between
10ºC and 65ºC the expansion is approximately 1/50 of its volume. This is known as
THERMAL EXPANSION, or expansion, and is relieved through the temperature and
pressure relief valve (T&PR valve).
Note: Water will expand relative to its rise in temperature. The discharge from the T&PR
valve is usually the result of thermal expansion due to heating, the quantity of the
discharge will be affected by:
The amount of water being heated.
The temperature rise from cold to hot.
The pressure rating of the T&PR valve.
The number of times a hot tap is opened during a heating cycle.
The amount of water lost through dripping taps.
Faulty Non-return valve fitted to cold water inlet.
It should be noted that a T&PR valve would not discharge water due to thermal expansion
when the heating cycle is not on.
Boiling point of water - The temperature at which water boils is directly related to the
pressure to which the water is subject to.
Water will boil at below 100ºC if the pressure is below 101kPa (atmospheric pressure at
sea level).
At sea level the boiling point of water is 100ºC.
Water will boil at above 100ºC if the pressure is above 101kPa (water at 1000kPa will
boil at approximately 183ºC).
Specific heat - The amount of energy required to raise 1kg of a substance by 1ºC.
Measured in units of kilo-joules (kJ).
i.e. 4.2kJ will raise 1 litre of water 1ºC.
Latent heat (Hidden or invisible heat) - The energy required to change the state of a
substance (water) into another state without a change in temperature.
i.e. Water to steam and steam to water.
Water to ice and ice to water.
The latent heat of steam is approximately 6 times the specific heat of water, i.e. to convert
water at 100ºC to steam at 100ºC will require approximately 252 kJ/kg.
TM005 Rheem Solar Hiline Service Instructions REV D
D.O.I 20/02/2009
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