§ 17

Steam consumption during heating

How much steam do we use to heat the batch?

Answer: When steam condenses in a jacket or heat exchanger, it releases its latent heat (r). The calculator finds the heat requirement for the batch, adds the heat loss and divides by the latent heat at the steam pressure you run with. This gives steam consumption in kg and the steam flow needed to reach the temperature in the time you have.

Formula

Q = m × c × ΔT ÷ udnyttelse (kJ)

Dampforbrug = Q ÷ r, hvor r er fordampningsvarmen ved driftstrykket (damptabel)

Dampflow = dampforbrug ÷ opvarmningstid i timer

1 kWh = 3.600 kJ

Symbols and units in the formula
Symbol	Meaning	Unit
masse	Mass of the product	kg
c	Product's specific heat capacity	kJ/(kg·K)
delta_t	Temperature increase	K
r	Steam pressure (evaporation heat from steam table)	kJ/kg
eta	Efficiency (heat loss from cladding and pipe deducted)	%
tid	Heating-up time	min
=	Steam consumption for the batch	kg damp
=	Steam flow during heating	kg/time
=	Heat quantity incl. losses	kWh

Validity and source

Source: Damptabel (fordampningsvarme r) · Q = m · c · ΔT

Does not apply when

Calculates only with the latent heat i.e. that condensate leaves the jacket at steam temperature. If condensate is cooled more a bit less steam is used. c for water is 4.19 kJ/(kg·K) for other products it's in the product datasheet.

Worked example

The calculator's own default values inserted. The figures are calculated with exactly the formula the tool uses.

Substituted

Mass of the product2,000 kg
Product's specific heat capacity3.90 kJ/(kg·K)
Temperature increase60 K
Steam pressure (evaporation heat from steam table)2,133 kJ/kg
Efficiency (heat loss from cladding and pipe deducted)90 %
Heating-up time30 min

Giver

Steam consumption for the batch244 kg damp
Steam flow during heating488 kg/time
Heat quantity incl. losses144.4 kWh

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Sådan bruger du den

Når en kappetank eller en varmeveksler varmes med damp, skal du vide, hvor meget damp opvarmningen bruger. Det bruges til at planlægge, så kedlen kan følge med, når flere tanke varmes op samtidig, og til at regne energiforbruget ud.

Step by step

Indtast massen og produktets varmekapacitet (vand 4,19, mælk ca. 3,9 kJ/(kg·K)).
Indtast temperaturstigningen.
Vælg det damptryk, kappen kører med. Fordampningsvarmen kommer fra damptabellen.
Indtast udnyttelsen og opvarmningstiden.
Aflæs dampforbruget og dampflowet.

Typical errors

At bruge damptabellens totale entalpi i stedet for fordampningsvarmen.
At blande manometertryk og absolut tryk i damptabellen.
At glemme varmetabet fra kappe og rør.
At regne c i J i stedet for kJ. Her er enheden kJ/(kg·K).

Examples from everyday life

Mælk i en kappetank

2.000 kg mælk skal varmes 60 grader op i en kappetank med damp på 3 bar(o). Udnyttelsen er 90 %, og opvarmningen skal tage 30 minutter.

Mass of the product: 2,000 kg
Product's specific heat capacity: 3.9 kJ/(kg·K)
Temperature increase: 60 K
Steam pressure (evaporation heat from steam table): 3 bar(o) / 4 bar abs. (144 °C): r ≈ 2.133 kJ/kg
Efficiency (heat loss from cladding and pipe deducted): 90 %
Heating-up time: 30 min

It means: Opvarmningen bruger 244 kg damp, og kedlen skal kunne levere 488 kg/time i den halve time. Varmemængden er 144.4 kWh.

Stor vandtank på højere tryk

5.000 kg vand skal varmes 55 grader med damp på 6 bar(o) på 45 minutter. Udnyttelsen er 85 %.

Mass of the product: 5,000 kg
Product's specific heat capacity: 4.19 kJ/(kg·K)
Temperature increase: 55 K
Steam pressure (evaporation heat from steam table): 6 bar(o) / 7 bar abs. (165 °C): r ≈ 2.065 kJ/kg
Efficiency (heat loss from cladding and pipe deducted): 85 %
Heating-up time: 45 min

It means: Det kræver 656 kg damp og et dampflow på 875 kg/time. Kører en anden tank samtidig, skal kedlens kapacitet tjekkes.

Rule of thumb

1 kg damp afgiver ca. 2.100 kJ ved kondensering. Det varmer ca. 500 kg vand 1 grad.

How to check the answer

Dampforbrug × fordampningsvarme skal give m × c × ΔT ÷ udnyttelse.