Consider the following questions:
1. Which is bigger. a nickel or a dime?
2. Which weighs more?
3.Which is worth more?
The answers are obvious: a nickel is bigger and it weighs more. but a dime is worth more. So size and weight are the wrong measurements of a coin's value, The real value of a coin is how much it will buy.*
In much the same way, enthalpy (Btu/lb) is the wrong measurement for the value of steam. It tells what the heat content of the steam is, but heat content is not the same as value. The real value of steam is how much work can be obtained from it.
KENNETH E. NELSON
*$2 and 45c in nickel tockens weights as much as $10 in dimes .
Somehow exergy analysis is rarely used today to compare heating systems performance. The quality of heat is missed from the piture.
Exergy analysis is employed here because no experimental data is available in the literature of relative efficiency of hydronic and vacuum systems. The concept of exergy offers a way to define system efficiency based on the Second Law of Thermodynamics (SLT). Exergy deals with the energy quality and the ability to produce useful work in the form of electricity, heat, or mechanical work.
For an exergy analysis of hydronic and vacuum heat distribution systems the same 100,000 BTU/hr. heat source is assumed. An ideal hydronic system (A), a vacuum heating system with old steel piping (B) and a vacuum heating system employing modern piping similar to hydronic (C) are compared. Heat loss for hydronic systems is assumed to be 0%, for vacuum heating system with old steel piping and higher flue gas temperature is conservatively assumed to be 25%, and 5% for vacuum heating with upgraded piping. So the distribution systems efficiency factor DE is assumed to be 1.0, 0.75 and 0.95 (A, B and C, correspondingly).
Operating pressure and temperature of the ESB vacuum heating system are used for calculations. The first law of thermodynamics is used to calculate water/steam flow:
m = Q* DE*/(Hin - Hout) ( 1)
Where m – water/steam, lb
Q – boiler capacity, BTU/hr
Hin, Hout – heating media enthalpy at Tin and Tout, correspondingly, BTU/lb
Usable heat w delivered into the radiators depends on the reference (room) temperature and circulating water/steam temperatures]:
w = (Hin - Hout) – Tref * (Sin – Sout) (2)
Where w - exergy, BTU/lb
Sin, Sout – entropy at Tin and Tout, correspondingly, BTU/lb oF
Tref = (68 + 460) oR = 528oR - reference temeperature
Thermodynamic data for enthalpy H and entropy S are readily available [4]. Calculation results are presented in Table 1.
The amount of usable heat actually delivered into radiators is:
Wdel = m*w (3)
The calculated value of usable heat delivered to the radiators for an ideal hydronic system A is similar to vacuum heating system B with 25% heat loss in distribution. 30.3% higher energy efficiency can be expected from vacuum heating with upgraded piping.
Please note that the electricity used by hot water circulators is not considered at all..
