ENERGY BALANCE
Nur Istianah,ST.,MT.,M.Eng THP UB 2017
HEAT
• Heat is a kind of energy that is related to the temperature or phase change. It involved in thermodynamic transformations
• Sensible heat is defined as the energy transferred between two bodies at different temperatures
• Latent heat is the energy associated withphase transitions
EXOTERM
System
Energy
Environment
Q<0
ENDOTERM
System
Energy
Environment
Q>0
Sensibel heat
SENSIBEL HEAT
-40 -20 0 20 40 60 80 100 120 140
Temperatur
Entalpi
Sensibel heat
∆ H = H 2 – H 1 = Q = m
𝐶
𝑝𝑑𝑇 = 𝑚𝐶𝑝
(
𝑇
2− 𝑇
1)
𝑇2 𝑇1
PANAS LATEN (PERUBAHAN/TRANSISI FASA)
Q= mh f atau Q = mh v
Laten heat
Laten heat
-40 -20 0 20 40 60 80 100 120 140
Temperatur
Entalpi Heat of
Fusion (+)
Heat of Vaporization (+)
Heat of Solidification (-)
Heat of Condensation (-)
Relationship between sensible Heat and latent Heat
ICE at -50 C ICE at 0 C
water at 0 C water at 100 C
vapor at 100 C
vapor at 150 C
Q1 = sensible heat
Q3 = sensible heat
Q5 = sensible heat
Q2 = Latent heat Of Fusion
Q 4 = Latent heat Of vaporization
Heat term
• Units of heat: joule (J), Calorie (Cal), or British thermal unit (Btu)
• 1 Calorie: amount of heat needed to raise the temperature of 1 gram of water by 1 C0 (from 14.50C to 15.50C)
• 1 Btu: amount of heat needed to raise the
temperature of 1 lb of water by 1 F0 (from 630F to 640F)
• 1 cal = 10-3 kcal = 3.969 x 10-3 Btu = 4.186 J
ENTHALPY
• Enthalpy is the absolute value of which cannot be measured directly.
• Heat can be defined as enthalpy changes of materials
• Specific heat capacity (cp) : amount of energy needed to raise the temperature of 1 kg of a substance by 1K ( kJ/kgC or kJ/kg.K)
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Enthalpy dan specific heat Enthalpy:
sensible
ΔH = CpΔT = Cp (T-Tref)
Laten:
Presented in experimental data
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Specific heat
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Enthalpy dan specific heat Enthalpy:
ΔH = CpΔT = Cp (T-Tref)
Siebel`s Eq.:
Cavg = 0,4 F + 0,2 SNF + M BTU/(lb0F) or
Cavg = 1674,72 F + 837,36 SNF + 4186,8 M J/(KgK)
Where: fat(F), solids non-fat (SNF), dan moisture (M)
1
– Heldman and Singh (1981) proposed the following expression based on the components of a food
product [Introduction to Food Engineering, 4th ed., page 258, equation 4.4]:
– where X is the mass fraction; the subscripts on the right-hand side are h (carbohydrate); p (protein); f (fat); a (ash); and w (moisture)
Cp (J.kg-1.K-1)
=1424Xh +1549Xp + 1675Xf + 837Xa + 4187Xw
Enthalpy dan specific heat
2
Example
– A apple with 0,15 kg of weight was left at 0oC until the temperature raise up to 20oC. Calculate the heat transfered if the apple has composistion; water 84,4%, Protein 0,2%, carbohydrate 14,5%, fat 0,6%, dan Ash 0,3% ?
– Defined: m = 0,15 kg T1 = 0oC
T2 = 20oC
Xw = 0,844, Xp = 0,002, Xh = 0,145, Xf = 0,006, Xa = 0,003
– Q = ?
– Solusi: ∆H = Q = m 𝑇𝑇2 CpdT
1
• For unknown Cp, Cp can be calculated from Siebel Eq.
•
• ∆H = Q = m 𝑇𝑇2 CpdT
= 0,15 kg x 3,76 kJ/kg.1 oC x (20-0) oC = 11,28 kJ
Cp = 1,424Xh +1,549Xp + 1,675Xf + 0,837Xa + 4,187Xw = 1,424 (0,145) +1,549 (0,002) + 1,675 (0,006) + 0,837 (0,003) + 4,187 (0,844) = 3,76 kJ/kg.K
– A formulated food has the following composition:
water, 80%; protein, 2%; carbohydrate,17%; fat, 0.1%;
and ash, 0.9%. Estimate the heat capacity of the food!
ENTALPHY of FOOD
– For processes where temperature changes, we must use predictive models of specific heat that include temperature dependence. Choi and Okos (1986)
presented a comprehensive model to predict specific heat based on composition and temperature.
[
Introduction to Food Engineering, 4th ed., page 258, equation 4.5].
Their model is as followsCp = Σ(XiCpi) = X1Cp1 + X2Cp2 + X3Cp3 + X4Cp4 + XnCpn
3
Choi Okos Eq.
Exercise
– Calculate the specific of milk at 60oC 2% lemak (Cpf=1,675 kJ/ kgoC), 89,2% air (Cpw=4,187 kJ/kgoC),
3,3% protein (Cpp=1,549 kJ/kgoC),
4,8% karbohidrat (Cph=1,424 kJ/kgoC) dan 0,7% abu (Cpa=0,837 kJ/kgoC).
Solution
– By using Eq. of Choi dan Okos (1986), this is the specific heat of milk:
Cp milk = Σ(XiCpi) = XwCpw + XfCpf + XpCpp + XhCph + XaCpa
Cp milk = (0,892)(4,187) + (0,02)(1,675) +
(0,033)(1,549) + (0,048)(1,424) + (0,007)(0,837) Cp milk = 3,894 kJ/kgK
Problem
– Calculate the specific of milk at 60oC:
2% lemak (Cpf= ?), 89,2% air (Cpw=?),
3,3% protein (Cpp=?),
4,8% karbohidrat (Cph=?) dan 0,7% abu (Cpa=?).
There is information of specific heat of components
Choi Okos Eq.
Solution
THANKS FOR YOUR ATTENTION
The best person is one give something useful always
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