05 Component Sizing

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International MSc Programme Sustainable Energy EngineeringInternational MSc Programme Sustainable Energy Engineering

THERMAL COMFORT AND INDOOR CLIMATE

Lecture:

- HVAC SYSTEMS COMPONENT SIZING- AUTOMATIC CONTROL

Assist. Prof. Igor BALEN



Heating coil

- factors that should be considered in coil selection:

• Required duty or capacity considering other components

• Temperature of air entering the coil and air temperature rise

• Available heating media, its operating and maximum pressure(s) and

temperature(s)

• Air volume, velocity, distribution, and limitations

• Heating media volume, flow velocity, distribution, and limitations

• Permissible flow resistances for both the air and heating media

• Individual installation requirements, such as the type of control and

material compatibility

• Specified and applicable codes and standards regulating the design and

installation



Heating coil

- total design heat load for a building according to EN12831:



Heating coil

1

2

- humidity ratio stays unchanged while

temperature increases (heating) ordecreases (cooling) and relative

humidity changes

)t t (.hh

)hh(mQ a

1212

1221

011 −=−

−=−

&& [kW]

h2-h

1

h 2 =

c o n s t .

h 1 =

c o n s t .

[kJ/kgdry air ]

- sensible heating:



Heating coil

- coil sizing when heating load is known:

OA HL HC

QQQ &&& += [W]

- heating up of outdoor air to the room temperature is calculated from:

)( oi pOAOA t t cV Q −=

ρ &&

[W]

- coil is selected from the producer’s catalogue.

- coil sizing when designing the exchanger:

m HC t UAQ Δ=& [W]

Mean logarithmic temperature difference



Heating coil

- water coil:

- usually use the standard construction materials of copper tube and

aluminum fins.

- generally have horizontal tubes to avoid air pockets. Where water coils

may be exposed to a freezing condition, drainability must be considered.This also applies when glycol fluid is circulated in the coil.



Heating coil

- steam coil:

- for horizontal airflow, the tubes can be either vertical or horizontal.

- in basic coils, steam supply connection is at one end and the tubes are

pitched toward the condensate return, which is usually at the opposite end.

- when the entering air temperature is at or below 0°C, the steam supply to

the coil should not be modulated, but controlled as full on or full off. During

part-load conditions, air can be bypassed around the steam coil with full

steam flow to the coil.



Heating coil

Coil ratings (ARI Standard 410):

Air face velocity. 1 to 8 m/s, based on air density of 1.2 kg/m3

Entering air temperature. Steam coils: −29 to 38°CWater coils: −18 to 38°C

Steam pressure. 15 to 1700 kPa (gage) at the coil steam supply

connection (pressure drop through the steam control valve must be

considered)

Fluid temperatures. Water: 50 to 120°C

Ethylene glycol: Up to 93°C

Fluid velocities. Water: 0.2 to 2.4 m/s

Ethylene glycol: 0.2 to 1.8 m/s



Heating coil

Overall requirements:

- air face velocity is usually between 2.5 and 5 m/s.

- supply air temperature varies from about 22°C for ventilation only to

about 65°C for complete heating.

- steam pressure typically varies from 15 to 100 kPa (gage), with 35 kPa

(gage) being the most common. A minimum steam pressure of 35 kPa

(gage) is recommended for systems with entering air temperatures below

freezing.

- water velocities vary between 1.2 and 1.8 m/s. For high-temperature

water, water temperatures can range upwards of 200°C with operating

pressures of 100 to 170 kPa over saturated water temperature.



Heating coil

Overall requirements (continued):

- water quantity is usually based on about an 10 - 20 K temperature drop

through the coil.

- high-temperature water systems commonly have a water temperature

between 130 and 200°C, with up to 60 K drop through the coil.

-air resistance is usually limited to 100 - 150 Pa for commercial buildings

and to about 250 Pa for industrial buildings.

- steam coils are selected with dry steam velocities not exceeding 30 m/s

and with acceptable condensate loading per coil core tube depending on

the type of steam coil.



Cooling and dehumidifying coil

- design cooling load for a building according to CLTD method:

Total space sensible cooling load at time θ :

θ θ θ θ ,inf ,,, ssinsexs qqqq −−− ++= [W]

Total space latent cooling load at time θ :

θ θ θ ,inf ,, llinl qqq −− += [W]

TOTAL SPACE COOLING LOAD AT TIME θ

θ θ θ ,,, lsT qqq +=

[W]



- with calculated cooling load it is possible to size the system

Volume flow rate of the supply air:

AC p

s

AC t c

q

V Δ= ρ

max,&[m

3

/s]

Humidity ratio of the supply air:

AC

lr S

V r

q x x

&0

max,

ρ −= [kgw/kgda]

qs,max – sensible space cooling load for maximum total cooling load [W]

ql,max – latent space cooling load for maximum total cooling load [W]

Δt AC – temperature difference between the supply and room air for summer

period – selected 3 – 8(10)ºC for comfort air system




Cooling and dehumidifying coil- cooling and dehumidification:

- temperature and humidity ratio decrease

1

2

h1-h

2

)hh(mQ a 2121 −=− &&

2’

[kW]

- the state point 2 determined graphically

- the theoretical point 2’ read from the

table for saturation line




- coil sizing when sensible and latent cooling load is known:

F OACLT C C QQQQ &&&& ++= , [W]

- cooling down of outdoor air to the room temperature is calculated from:

)( ioOAOA hhV Q −= ρ &&

[W]

- including heating up of air by fan (sensible cooling):

F

AC T F

V pQ η

&

&

Δ

= [W]

- coil is selected from the producer’s catalogue.



- water coil:Cooling and dehumidifying coil

- usually have aluminum fins on copper tubes, although copper fins on

copper tubes are also used.

- core tube outside diameters are up to 25 mm, with fins spaced 1.4 to 6.4mm apart. Tube spacing ranges from 15 to 75 mm on equilateral

(staggered) or rectangular (in-line) centers, depending on the width of

individual fins and on other performance considerations.



- direct expansion (DX) coil:


- the TXV used to automatically regulate the rate of refrigerant liquid flow to

the coil in direct proportion to the evaporation rate of refrigerant liquid in

the coil, thereby maintaining optimum performance over a wide range ofconditions.

- to ensure reasonably uniform refrigerant distribution in multicircuit coils, a

distributor is placed between the TXV and coil inlets to divide the

refrigerant equally among the coil circuits.



- DX coil:

- the refrigerants usually HFC-134a, HFC-404A, HFC-407A, HFC-407C or

HFC-410A

- evaporating temperatures > 0ºC

- the superheat at the coil suction outlet

is continually maintained within the usual

predetermined limits of 3 to 6 K.

- air temperature leaving the coil is

normally 7 to 10 K higher than evaporating

temperature.




Ratings and performance

- Entering air dry-bulb temperature. 18 to 38°C

Entering air wet-bulb temperature: 15 to 30°C

- Air face velocity: 1 to 4 m/s

- Evaporator refrigerant saturation temperature: −1 to 13°C at coil suction

outlet

- Entering chilled water temperature: 2 to 18°C

- Water velocity: 0.3 to 2.4 m/s

- For cold ethylene glycol solution: 0.3 to 1.8 m/s, −18 to 32°C entering dry-

bulb temperature, 15 to 27°C entering wet-bulb temperature, 10 to 60%aqueous glycol concentration by mass.




Ratings and performance (continued)

- air-side ratio of sensible to total heat removed by dehumidifying coils

varies in practice from about 0.6 to 1.0 (i.e., sensible heat is from 60 to

100% of the total, depending on the application).

- dehumidifying coils for comfort application are frequently selected in the

range of 2.0 to 2.5 m/s air face velocity (limited to a value that prevents

water carryover into the air ductwork).

- reftigerant duty would be 3 to 6 K superheat for an appropriate balance at

7°C saturated suction.

- for water coils, circuitry would operate at 1.2 m/s, 5.5°C inlet water, 6.7 K

rise.




Humidifier

- humidifying load is the amount of water vapor required to be added to the

air by a humidifier so as to maintain a predetermined space relative

humidity. It depends primarily on the rate of natural infiltration or the

amount of outside air introduced by mechanical means.- other sources of moisture gain should also be considered. Space

(internal) moisture gains include the latent load from the occupants,

appliances, equipment, and products.

iwoioOAh m x xV V m ,inf )()( &&&& −−+= ρ [kg/s]

Internal moisture gains

- when outside air with low moisture content xo is introduced to the HVACsystem (winter), the supply air moisture content should be 4 to 8 g/kg

above that amount.



1

2w

s

- liquid water:

- water vapor:

- humidification by injection of water (liquid or vapor):

2v

x2

) x x(mm aw 12 −= &&

12

12

x x

hht ch

dx

dh wwww

−

−===

) x x(mm av 12 −= &&

12

12

x x

hhh

dx

dh vv

−

−==

from the table

for saturated

vapor [kJ/kg]

Humidifier



Humidifier

- water humidifier:

1. Atomizing humidifier

- centrifugal (high-speed spinning disk)- spray nozzles (compressed air, water pressure)

- ultrasonic vibrations





Humidifier

- water humidifier:

- usually designed at an air velocity between 2 and 4 m/s with respect to its

cross-sectional area at the water sprayers.

- length of humidifier section is usually 1.5 to 3 m.

- size of water droplets is up to 100 μm.

- total pressure loss of the airstream flowing through a humidifier depends

mainly on the configuration of the eliminators and the air velocity flowingthrough them. Usually it varies from 65 to 250 Pa, and typically it is 125

Pa.

- for humidification and evaporative cooling, the water-air ratio is usually

mw/ma=0.3 to 0.6. For a 1.5-m long air washer with mw/ma=0.45, that is, 0.5L/s of water per 1000 L/s of air, εsat = 0.85 to 0.9.



Humidifier

- steam humidifier:

1. Direct steam injection

- with steam-jacketed manifolds



Humidifier

- steam humidifier:

2. Self-contained, electrically heated

- electrode- or resistance type



Humidifier

- steam humidifier:

- temperature of the ambient air or airstream in which moisture has been

added is approximately unchanged.

- for direct steam injection, the steam source is a central steam boiler at

low pressure.

- capacity is often controlled by a microprocessor-based controller by

modulating the valve pin of the control valve according to the signal of ahumidity sensor and, therefore, the steam flow rate.

- accuracy of humidity control of ± 5 to 7 % for on/off control and ± 3 to 5 %

for modulation control.

- has to be installed where the air can absorb the discharged vapor beforeit comes into contact with other components, such as coils or dampers.

Otherwise, condensation can occur in the duct.



Fan

- the increase of air static pressure is created by the conversion of velocity

pressure to static pressure.

- fan total pressure rise is a true indication of energy imparted to the air

stream by the fan.

Centrifugal fan Axial fan



Fan

Pressure drop in an air system

- total pressure drop:

LF T p p p Δ+Δ=Δ [Pa]

- friction losses are the result of the duct surface roughness:

2

2

wd L pF ρ λ =Δ [Pa]

- dynamic/local losses in system parts like AHU components, entries,

exits, dampers, elbows, T-junctions...:

2

2w

p L ρ

ζ =Δ [Pa]



- fan – air handling component for supply and exhaust of ventilation air

EXT INT DIN STAT VENT p p p p p Δ+Δ≥Δ+Δ=Δ

REG MOT TRANSM VENT

VENT KL EL

pV P

η η η η ⋅⋅⋅

Δ=

&

- fan pressure:

- electric energy used for fan operation:

TOT p

VENT

c

pt

η ρ

Δ≈Δ

- heating up of the airflow by fan:

TOT

0,3-0,8

[ºC]

[W]

Fan



Fan

- selection:

Pressure-volume flow operating characteristics. Selecting a fan to

provide the required volume flow rate and total pressure loss for an air

system or a ventilating system is of prime importance. An undersized fanresults in an uncontrolled indoor environment. An oversized fan wastes

energy and money.

Fan capacity modulation. A variable-air-volume system operates at a

reduced volume flow rate during part-load operation. Effective andeconomical fan capacity modulation is an important factor that affects the

operation of an air system.

Fan efficiency. Fan efficiency is closely related to the energy use of an air

system. Fans should be selected so that they can operate at highefficiency during as much of their operation time as possible.



Fan

- selection (continued):

Sound power level. Most commercial and public buildings and many

industrial applications need a quiet indoor environment. Fans are the major

source of noise in an air system. Usually, the higher the fan total efficiency,the lower the sound power level of the selected fan. High-frequency sound

is more easily attenuated than low-frequency sound.

Airflow direction. In many applications, a straight-through or in-line flow

occupies less space and simplifies layout.

Initial cost . The initial cost of the fan modulation device, sound attenuator,

and space occupied by a particular type of fan, in addition to the cost of the

fan itself, should be considered.



Fan

- fan is selected to meet

the requirements of the

system.

- selection is performedfrom the manufacturers’

tabular or graphic data.

- optimum selection

range is the zone of the peak efficiency.



Fan

- commercial chart for the

fan selection – example

- fan pressure characteristics

have to fit the system pressurecharacteristic

- the total system must be

evaluated and the flow

requirements, resistances, andsystem effect factors at the

fan inlet and outlet must be

known.

- ducts should be connected

to a fan with canvas or other flexible material.

- sound power level data should

be obtained by the manufacturer



Automatic control

- automatic control in the HVAC is used for:

- providing the thermal comfort or other conditions necessary for

the production process

- protection from unnecessary system failures or damages- operation control (on/off, modulating) and selection of parameters

by users

- HVAC system usually includes the control of:

- temperature- humidity

- pressure

- flow rate



Automatic control

- control action:

1. Two-position (on/off, low/high)

- typical example – thermostat that starts and stops a device

- controller differential in two-position control action, is thedifference between a setting at which the controller operates to

one position and a setting at which it operates to the other.

2. Floating action

- controller can perform only two operations- moving the controlleddevice toward either its open or closed position, usually at a

constant rate.

- neutral zone between the two positions allows the controlled

device to stop at any position when the controlled variable is withinthe differential of

the controller.



Automatic control

- control action (continued):

3. Modulating

- output of the controller can vary infinitely over the range of the

controller.- 3 typical modes:

a) Proportional (P)

- controlled device is positioned proportionally in response

to changes in the controlled variable.- output of the controller is proportional to the difference

between the sensed value, the controlled variable, and its

set point.



Automatic control

b) Proportional-Integral (PI)

- adding another component to the control action thateliminates the offset typical of proportional control.

- increases stability, and eliminates offset, giving greater

control accuracy. PI control can also improve energy

efficiency in HVAC applications.

c) Proportional-Integral-Derivative (PID)

- control with a derivative term added to the controller that

varies with the value of the derivative of the error.

- faster response and greater stability, but more sensitiveto noisy signals and harder to tune than a PI controller.



Automatic control

- control action (continued):

4. Fuzzy logic

- an alternative to traditional control algorithms.

- controller uses a series of “if-then” rules that emulates the way ahuman operator might control the process.

- “fuzzy” element is introduced when the functions overlap and the

room temperature is, for example, 70% high and 30% OK.

- example:

IF the room temperature is high AND the rate of change is

decreasing, THEN increase cooling a little.

or

IF the room temperature is high AND the rate of change is

increasing, THEN increase cooling a lot.



Automatic control

- sensors:

- devices that respond to a change in the controlled variable. This signal is

sent to the controller.

- Temperature sensors

- Humidity sensors

- Pressure transmitters and transducers

- Flow rate sensors

- IAQ sensors (CO, CO2,...)

- Power sensors and transmission



Automatic control - HVAC

Fan control:

- most efficient way to change the output of a fan is to change its speed.

- variable-frequency drives are widely used.

- axial fans can be controlled by varying the pitch of the blade.

- dampers and ducting can simply bypass some of the air from the supply

side of the fan to the return side. Bypassing does not change the output of

the fan, but it can allow the fan to accommodate flow variations in the

distribution system without fan instability.




Fan control:

- Differential static-pressure control is used to pressurize a building or

space relative to adjacent spaces or the outside. Typical applications

include clean rooms (positive pressure to prevent infiltration), laboratories

(positive or negative, depending on use), and various manufacturingprocesses, such as spray-painting rooms.

- pressure controller usually modulates dampers in the supply duct to

maintain the desired pressure as exhaust volumes change.

- method for control of the return fan requires measuring the space andoutside static pressures.

- location for measuring inside static

pressure must be selected away from

doors and openings to the outside,

away from elevator lobbies, and,when using a sensor, in a large

representative area shielded from

drafts.




Fan control:

- Duct static-pressure control for variable air volume (VAV) and other

terminal systems maintains a static pressure at a measurement point. The

most common application for static-pressure control is fan output control in

VAV systems.

- multiple static sensors are required when more than one branch duct

runs from the supply fan. The sensor with the highest static requirement

controls the fan.




Economizer cycle control:

- reduces cooling costs when outside conditions are suitable, that is, when

outside air is cool enough to be used as a cooling medium.

- if outside air is below a high-temperature limit, typically 18°C, the return,

exhaust, and outside air dampers modulate to maintain a ventilation

cooling set point, typically 13 to 16°C.

- when the outside air temperature exceeds the high-temperature limit set

point, the outside air damper is closed to a fixed minimum and the exhaust

and return air dampers close and open, respectively.- in enthalpy economizer control, the high-temperature limit interlock

system of the economizer cycle is replaced to further reduce energy costs

when latent loads are significant.

- possible warm-up with no

outside air and night cooldown

(if outside air conditions are

acceptable) with 100% outside air




Cooling coil control:

1. Chilled water coil

- controlled by two- or three-way valves, that are usually closed to prevent

cooling when the fan is off. The valve typically modulates in response to

coil air discharge temperature or space temperature.

COOLING COOLING

CHILLED-WATER CHILLED-WATER CHILLED-WATER CHILLED-WATER




Cooling coil control:

2. DX coil

- controlled by solenoid valves in the refrigerant liquid line. Face and

bypass dampers are not recommended because they permit ice to form on

the coil when airflow is reduced. Control can be improved by using two or

more stages.

- modulating control (uncommon) can be achieved using a variable-

suction-pressure controller.

Two-position Modulating

A t ti t l HVAC




Heating coil control:- coils that are not subject to freezing can be controlled by simple two- or

three-way modulating valves.

- valve is controlled by coil discharge air temperature or by space

temperature. Valves are set to open to allow heating if control power fails.

- heating coils in central air-handling units preheat, reheat, or heat,

depending on the climate and the amount of minimum outside air needed.

- hot-water coils must maintain a minimum water velocity in the tubes (on

the order of 0.9 m/s) to prevent freezing (coil pump is common).

- in the conventional primary/secondary arrangement, the coil

pump and the pumps feeding the

coil are hydraulically independent.

It results in constant flow through

the coil and with constant flow

through the primary loop if a three-

way valve is used.




Heating coil control:

- the coil pump can be piped in series (A) with the primary pumps when the

three-way valve is open to the coil. In this case, flow through the coil will be

relatively constant, but can rise and fall depending on the differential

pressure available from primary pumps.

- the coil pump can be also piped in

parallel (B) with the primary pumps.

This design has the advantage that

hot-water flow can be achieved

through the coil even if either the

primary pump or coil pump fails.

Primary pump must be sized for the

pressure drop of the coil at this highflow rate.

(A) (B)

2-way3-way

mixing

3-way

diverting




Heating/cooling coil control:

- coil pump arrangements

mixing diverting

n o s e c o n

d a r y

p u m p

w i t h

s e c o n d a r y p u

m p

w i t h m i x i n g

w i t h i

n j e c t i o n




Humidity control:

- based on the output of a humidity sensor located either in the space or in

the return air duct.

- Dehumidification. One way is to override the control of the cooling coil.

The temperature of the coil is lowered until sufficient moisture is removed

from the supply air to maintain the humidity set point. Reheat coil may be

required to maintain the space temperature if the moisture removal

process results in too low a supply air temperature.




Humidity control:

- Dehumidification. If water condensing out of the airstream freezes on

the coil surface, airflow is restricted and, in severe cases, may be shut off.

The practical limit is about 5°C dew point on the coil surface. This results in

a relative humidity of about 30% at a space temperature of 24°C, which is

adequate for most commercial applications. When lower humidity is

needed, a chemical dehumidifier (desiccant-based) is required. Other way

is to use sprayed-coil dehumidifiers, but the problem is the cost of

maintenance, reheat, and removalof solid deposits on the coil.




Humidity control:

- Dehumidification. Desiccant-based dehumidifier can lower space

humidity below that possible with cooling/dehumidifying coils. This device

adsorbs moisture using silica gel or a similar material. For continuous

operation, heat is added to regenerate the material.




Humidity control:

- Humidification. A space or return air humidity sensor provides the

necessary signal for the controller. A humidity sensor in the duct should be

used to minimize moisture carryover or condensation in the duct. Usually,

the aim is to maintain design minimum humidity during the heating season.

Steam jet humidifier




Outside air control:

- provides ventilation air, space pressurization (exfiltration), and makeup

air for exhaust fans.

Fixed minimum outside air control with return-exhaust fan




Terminal units control:

- Reheat terminals. Use a single constant-volume fan system that serves

multiple zones. All delivered air is cooled to satisfy the greatest zone

cooling load. Air delivered to other zones is then reheated with heating

coils (hot water, steam, electric) in individual zone ducts.

- Throttling VAV terminal. A damper in the inlet controls the flow of

supply air. As the temperature in the space drops below the set point, the

damper begins to close and reduce the flow of air to the space. When the

airflow reaches the minimum limit,

the valve on the reheat coil begins

to open.





- Series fan-powered VAV terminal unit. An integral fan supplies a

constant volume of air to the space. When the space is occupied, the fan

runs constantly to provide a constant volume of air to the space. The fan

can draw air from the return plenum to compensate for the reduced supply

air. As the temperature in the space decreases below the set point, the

supply air damper begins to close and the fan draws more air from the

return plenum. When the supply air reaches its minimum level, the valve to

the reheat coil begins to open.





- Mixing box terminal. Have inlet dampers on the heating and cooling

supply ducts. These dampers are interlinked to operate in opposite

directions, and space thermostat positions the dampers through an

actuator to mix warm and cool supply air. By the VAV system, the airflow

controller controls the volume damper.

Dual-duct CAV mixing box

Dual-duct VAV mixing box





- example of throttling VAV terminal:

dpM

+ -

AIRFLOW AIRFLOW

-- AIRFLOW CHANGE AIRFLOW CHANGE

-- DETECTION AT TRANSDUCERDETECTION AT TRANSDUCER

-- REACTION OF CONTROLLERREACTION OF CONTROLLER

-- SIGNAL TO ACTUATORSIGNAL TO ACTUATOR

-- CORRECTION OF DAMPERCORRECTION OF DAMPER

BLADE POSITIONBLADE POSITION

-- AIRFLOW CORRECTED AIRFLOW CORRECTED




TVMTVM -- Dual Duct UnitDual Duct UnitM

M

dpw

+ -

dpw

+ -

Vcold

V w a r m

Vtotal

Room Temp: 22°C20°C Terminal units control:

- example of dual-duct VAV mixing box



dp

d P = r e q u i r e d

v a l u e

Reference Pressure

Room Pressure



- example of laboratory room pressure control: (1)






dp

V to VV to Vminmin

//maxmax

d P = 0 P a

Actuator Actuator running timerunning time

minmin maxmax

~~150 s150 s

Reference Pressure

Room Pressure

A i l HVAC






dp

d P > > r e q u i r e d

v a l u e

V =V = undefinedundefined

Reference Pressure

Room Pressure

Actuator Actuator running timerunning time

minmin maxmax

~~150 s150 s

A t ti t l HVAC



Mas ter

S la ve

V + dp

c o n t r o l

s i g na l

C o n t r o l s i g

na l s la ve ≙ ac t ua l

va l ue s u p p l y a i r

S U P P L YS U P P L Y E X H A US

T E X H A US

T

Reference

Pressure

Room Pressure




Correct solution

A t ti t l HVAC




Control of systems:

- example of heating, cooling and humidifying HVAC system

Task. Control of temperature during the heating and cooling season and

control of humidity during the heating season. System operates with 100%outside air. AHU consists of the fan, the heating coil, the cooling coil, the

reheat coil and the water humidifier. System is single-duct, single-zone,

constant air volume (CAV). Water distribution system is with variable flow.

Control. Performed with three control loops.First control loop controls the (pre)heat coil operation by maintaining the

dew-point temperature constant. Temperature sensor (16), behind the

humidifier, sends signal to the PI controller (R1). Depending on the

temperature change, the controller R1 moves the valve drive, thus

changing the coil capacity according to the PI characteristic.


Control of systems:

l f h ti li d h idif i HVAC t



- example of heating, cooling and humidifying HVAC system

Automatic control HVAC




Control (continued).

Second control loop controls the cooling coil and the reheat coil operation

by maintaining the set point of the space temperature. Room temperature

sensor (12) measures the change of room temperature in dependance with

the outdoor air temperature. The outdoor air temperature is measured withthe duct sensor (1). The PI controller (R2) compares values of the two

temperatures. When the outside temperature increases, R2 closes the

valve (14), thus reducing the water flow through the reheat coil. Valves on

the reheat coil and the cooling coil water supply operate in a sequence –when the valve (14) closes completely, the valve (7) on the cooling coil

water supply starts to open.

Third control loop controls the humidifier operation. There is the two-

position hygrostat (21) in the return duct, that limits minimum andmaximum humidity. The hygrostat starts or stops the water pump in the

humidifier.

Automatic control HVAC




System protection.

Freeze protection is performed with low-limit temperature sensor (9). When

temperature behind the preheat coil is below 5ºC, the supply and the return

fan stop, the air dampers close, the valve on the preheat coil water supply

fully opens, the pump of the preheat coil starts and the humidifier pumpstops.

Control of pressure drop on the filter is performed by the differential

pressure sensor. Differential pressure sensors on the fans react in the

case of low-flow/no-flow and stop the fans, close the valves and thedampers and stop the circulating pumps.

Fire protection is performed with stopping the fans and closing all air

dampers.

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05 Component Sizing