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Environmental Engineering

6. Air-conditioning; CAV system

Bachelor degree course

Vladimír Zmrhal Winter semester 10/2021

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Heat Transfer

- convective - radiant

Air-conditioning systems

Medium

- air- water- air + water- refrigerant

Purpose

- comfort- technology- biology- safety

Number of zones

- one - zone- multizone

single-duct, dual-duct cooled ceilings, fan coil units induction units (chilled beams) split, multisplit, VRV

Air-conditioning

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Air-conditioning

Single-duct air system with constant air flow rateCAV system

Exh

O

FAN

REC

DFANHCCHCFM S

O

M v

Ext

S

t ,xrec rec

t ,xo o

t ,xi i

Q cl,i

I

Q hl

Q cl,e

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Air-conditioning

Single-duct air system with variable volume air flow rateVAV system

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Air-conditioning

Dual-duct air system

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Air-conditioning

Fan-coil unit system

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Air-conditioning

Induction unit, Chilled beams system

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Air-conditioning

Cooled ceiling system

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Air-conditioning

Refrigerant systems

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Split Multi-split VRV

Air-conditioning

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Single duct air system with constant air flow rate – CAV system

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Input data

Heat loss and Cooling load

sensible cooling load Qcl

sensible heating load Qhl

Outdoor air parameters – extreme weather conditions

Prague:

to,win = – 15 °C o = 100 %

to,sum = 32 °C ho = 58 kJ/kg

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Input data

Indoor air parameters

ti,win = 20 22 °C i = 30 70 %

ti,sum = 25 27 °C i = 30 70 %

Ventilation requirements

for the people – from 25 to 90 m3/h per person

for technology – according to mass balance of contaminants

Heating coil and cooling coil parameters

water heating 80/60 °C, …

water cooling 6/12 °C, … tc = tm,w = (6 + 12)/2 = 9 °C

evaporator tc = 4 5 °C

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Supply air/Room air temp. difference

It depends od the space air diffusion (distribution of the supply air)

WINTER t 15 (25) K

SUMMER 3 to 5 K - displacement vent.

6 to 10 K - mixing systems (grilles)

up to 12 K - swirl diffusers

i st t t

Input data

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Space air diffusion

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Space air diffusion

Displacement ventilation

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SUMMER process1) Volume air flow rate hygienic requirements Vo

Vo = n.R = number of person x vent. requirements [m3/h]

sensible cooling load V

if V > Vo than the recirculation air is used Vrec

Dimensioning of Ventilation

rec oV V V

cl

i s

QV

c t t

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2) h-x diagram

E: to,sum = 32 °C; ho,sum = 58 kJ/kg

I: ti = 25 °C

i = 30 až 50 % (approx.)

M: mixing Vcirc/Vo

CC: tc (according to cooling coil type)

S: MC x ts

Dimensioning of Ventilation

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Dimensioning of Ventilation

3) Humidification in the room

… production of people water vapor

… if not move the position of state I on isotherm ti

1.w wM n M

w i sM V x x

!

i s i scalc h xx x x x

wi s calc

Mx x

V

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Dimensioning of Ventilation

4) Cooling coil capacity

Množství vodní páry, která zkondenzuje na povrchu chladiče

sen m s

lat m s

cc sen lat m s

Q V c t t

Q V l x x

Q Q Q V h h

w m sM V x x

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WINTER process

1) Temperature of supply air

[K]

… if the heating system cover the heating load

… if not t = 0 K

Dimensioning of Ventilation

hls s i

Qt t t

V c

15(25)s it t

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Dimensioning of Ventilation

2) h-x diagram

O: to,win = to,calc – 3 °C

o = 100 %

I: ti = 20 °C

i = 30 až 50 % (approx.)

HR: tHR efficiency of heat recovery

M: mixing Vcirc/Vo

S: air heating to ts

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Dimensioning of Ventilation

3) Heat recovery

reuse of the sensible heat from the air leaving the room (zone,building) and trasfer heat into the supply (outdoor) air

heat recovery efficiency

...HR o

HRi o

t tt

t t

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Dimensioning of Ventilation

4) Humidification in the room

see summer process

5) Heating coil capacity

hc s m s mQ V h h V c t t

h hl ventQ Q Q

h s i i mQ V c t t V c t t

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Dimensioning of Ventilation

WINTER process with steam humidification

h-x diagram

O: input

I: input

HR: heat recovery

M: mixing

S´: heating to tsS: humidification to xs

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Dimensioning of Ventilation

6) Humidifier capacity

[kg/s]

[W]

´hum s sM V x x

hum humQ M l

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Example: Single duct air system

Example 1: Dimensioning of single duct air systemSummer Winter

Cooling/Heating load 33,6 kW 25 kWOccupancy 200 personSupply and room air temp. difference 10 K -Ventilation requirements 25 m3/h personTemp. of indoor air 26 °C 20 °CRH of indoor air 30 to 70 % 30 to 70 %Outdoor air conditions 32 °C; 58 kJ/kg -15 °C, 100 %Wate vapor person production 116 g/h 70 g/h Cooling coil temperature 9 °C -Heat recovery efficiency - 67 %

Vo = ?, V = ?, Vc = ?; Qhc = ?; Qcc = ?, Qh = ? Draw h-x diagram

Thank you for your attention

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