§ 17

Cooling surface: power on air side

How many kW does the cooling surface deliver, and how much goes to dehumidification?

Answer: An air cooler cools not just the air, it also removes moisture. The total output is the air's mass flow times the enthalpy drop, which you read from the h-x diagram before and after the surface. The sensible part is what lowers the temperature. The rest is latent heat from the moisture condensing. The calculator divides the output and gives the ratio between sensible and total output (SHR), which you use when choosing a cooling surface.

Formula

Samlet ydelse Q = V̇ × ρ × (h_ind − h_ud) ÷ 3600 (V̇ i m³/h, h i kJ/kg, Q i kW)

Følbar ydelse = V̇ × ρ × 1,006 × (t_ind − t_ud) ÷ 3600

Latent ydelse = samlet − følbar

SHR = følbar ÷ samlet

Symbols and units in the formula
Symbol	Meaning	Unit
luftmaengde	Air flow through surface	m³/h
rho	Air density	kg/m³
h_ind	Enthalpy before the surface	kJ/kg
h_ud	Enthalpy after the surface	kJ/kg
t_ind	Air temperature before surface	°C
t_ud	Air temperature after surface	°C
=	Total cooling capacity	kW
=	Heraf folbar koling	kW
=	Heraf latent koling (affugtning)	kW
=	Palpable fraction (SHR)	%

Validity and source

Source: Energibalance for fugtig luft (Mollier h-x-diagram), cp,luft = 1,006 kJ/(kg·K)

Does not apply when

The standard values correspond to approx. 25 °C/50% RH in and 13 °C/95% RH out. The sensible part is calculated with dry air's heat capacity, a small estimate. The amount of condensation water is calculated with the calculator «Condensation from air cooler».

Worked example

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

Substituted

Air flow through surface2,000 m³/h
Air density1.20 kg/m³
Enthalpy before the surface50.30 kJ/kg
Enthalpy after the surface35.50 kJ/kg
Air temperature before surface25 °C
Air temperature after surface13 °C

Giver

Total cooling capacity9.87 kW
Heraf folbar koling8.05 kW
Heraf latent koling (affugtning)1.82 kW
Palpable fraction (SHR)82 %

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Sådan bruger du den

Når du dimensionerer eller kontrollerer en køleflade i et ventilationsanlæg, skal du kende både den samlede ydelse og delingen i følbar og latent køling.

Step by step

Mål luftmængden.
Mål temperatur og RF før og efter fladen, og aflæs entalpierne i h-x-diagrammet.
Indtast temperaturerne.
Aflæs ydelserne og SHR.

Typical errors

At regne kun med temperaturer og glemme den latente del.
At bruge entalpier fra et forkert diagram.
At glemme at luftmængden skal måles ved fladen.

Examples from everyday life

Køleflade i et kontoranlæg

Luften kommer ind med 25 °C og 50 % RF (h = 50,3 kJ/kg) og ud med 13 °C og 95 % RF (h = 35,5 kJ/kg). Luftmængden er 2.000 m³/h.

Air flow through surface: 2,000 m³/h
Air density: 1.2 kg/m³
Enthalpy before the surface: 50.3 kJ/kg
Enthalpy after the surface: 35.5 kJ/kg
Air temperature before surface: 25 °C
Air temperature after surface: 13 °C

It means: Fladen yder 9.87 kW i alt, heraf 8.05 kW følbart og 1.82 kW latent. SHR er 82 %.

Tørt vejr

En tør forårsdag kommer luften ind med 25 °C og h = 45 kJ/kg og forlader fladen med 15 °C og h = 34,8 kJ/kg, uden at der dannes kondensvand.

Air flow through surface: 2,000 m³/h
Air density: 1.2 kg/m³
Enthalpy before the surface: 45 kJ/kg
Enthalpy after the surface: 34.8 kJ/kg
Air temperature before surface: 25 °C
Air temperature after surface: 15 °C

It means: Ydelsen er 6.8 kW, og næsten det hele er følbart (6.71 kW). SHR er 99 %, fordi luften ikke affugtes.

Rule of thumb

1.000 m³/h og 1 kJ/kg giver ca. 0,33 kW.

How to check the answer

Følbar + latent = samlet.