Variable Volume Tank
by M. Arny 1982

	This subroutine models a vertical cylindrical tank, which contains a
variable quantity of liquid, using analytically solved differential equations.
The level of the liquid in the tank is allowed to vary between user specified
high and low level limits.  When the high level limit is exceeded additional
flow into the tank is dumped and the level indicator is set equal to +1.  When
the liquid level falls below the low level limit, further flow out of the 
tank ceases and the tank level indicator is set equal to -1.
	Since the flow rates into and out of the tank are not necessarily the
same, both flowrates are specified as tank model inputs.

INPUTS 4

1	Inlet Temperature
2	Inlet Mass Flowrate
3	Outlet Mass Flowrate
4	Ambient Temperature

PARAMETERS 11

1	Tank Volume
2	Tank Diameter
3	Wetted Tank Wall - U Valve
4	Dry Tank Wall - U Valve
5	Minimum Liquid Volume in Tank
6	Maximum Liquid Volume in Tank
7	Specific Heat of Liquid
8	Density of Liquid
9	Reference Temperature (usually mains water temperature)
10	Initial Temperature of Liquid in Tank
11	Initial Mass of Liquid in Tank

OUTPUTS 17
1	Outlet Temperature
2	Outlet Mass Flowrate
3	Mass Flowrate of Liquid Added to Tank (dM inlet - dM dump)
4	Q Dumped (tank overflow)
5	Q Loss by Conduction
6	Change in Internal Energy in Tank from Initial Conditions
7	Tank Level:	+1 for liquid volume >= max. volume
			 0 for min. volume <= liquid volume <= max. volume
			-1 for liquid volume <= min. volume
8	Tank Temperature at beginning of Timestep
9	Mass in tank at beginning of timestep
10	Tank temperature at end of timestep
11	Mass in tank at end of timestep
12	------
13	------
14	Q in - Energy added to Tank by Flow In
15	Q out - Energy removed from Tank by Flow Out
16	Energy Stored in Tank
17	Change in Mass in Tank from Initial Conditions

REF:  Arny, M.D. "Thermal Analysis for the Design of an Industrial Process
      Water Heating System Utilizing Waste Heat Recovery and Solar Energy"
      Masters' Thesis, University of Wisconsin-Madison  1982