intro to primary systems

8/8/2019 Intro to Primary Systems

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Lecture 21: Introduction toPrimary Systems (CentralPlants)

Material prepared by GARD Analytics, Inc. and University of Illinoisat Urbana-Champaign under contract to the National Renewable Energy

Laboratory. All material Copyright 2002-2003 U.S.D.O.E. - All rights

reserved



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Importance of this Lecture to

the Simulation of BuildingsPrimary systems provide hot and

chilled water for the secondary

systems as well as other energysources that are needed by thebuilding

Some knowledge of the primarysystems (central plants) is requiredto accurately simulate buildings andto understand what the model inputparameters are



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Purpose of this Lecture

Gain an understanding of: Basic information about primary

plants (central plants) Interconnection between primary

plants and the rest of the building



Cooling Equipment

Chillers: Compression-Based andAbsorption

Heat Pumps

Rooftop/DX Packaged Units

Thermal Energy Storage (Water andIce)



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Compression-Based Liquid

Chilling Systems Compression Chillers and Heat Pumps both

work on what is commonly referred to as a“vapor compression cycle”

Thermodynamic cycle through which refrigerant goes Refrigerant is enclosed within cycle components

Components Condenser Compressor Evaporator (aka Liquid Cooler) Expansion Valve Primary and secondary fluids (refrigerant, water,

etc.)



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Condenser

Evaporator

Compressor

Expansion

Valve

Compression Cycle

Typical compression cyclediagram:

QE

QC

WorkHigh

Pressure

LowPressure



Compression-Based Liquid

Chilling Systems (cont’d) Cycle Details

High pressure side: from compressor outlet throughcondenser to expansion valve inlet

Low pressure side: from expansion valve outlet

through evaporator to compressor inlet Utilize the fact that the boiling point of the refrigerant

changes as the fluid pressure changes: lower pressuremeans a lower boiling temperature

Refrigerant picks up heat in the evaporator (refrigerantevaporates) because the chilled fluid temperature is

higher than the refrigerant temperature Refrigerant rejects heat in the condenser (refrigerant

condenses) because condenser fluid temperature islower than refrigerant temperature

Compressor drives the cycle by compressing therefrigerant through the addition of work

First Law of Thermodynamics



Chillers/Heat Pumps for

Conditioning Cooling: Normal operation mode

Goal is to provide cooling at the evaporator where there ischilled water or air that is produced

Coefficient of performance (COP) equal to cooling

achieved at the evaporator over the work required at thecompressor

Heating: Reverse operation (heat pumps) Goal is to provide heating at the condenser where there is

hot water or air that is produced

Typically this requires a reversal of refrigerant flow

Coefficient of performance (COP) equal to heatingachieved at the condenser over the work required at thecompressor




Conditioning (cont’d) Efficiency and Energy Issues

Work is required because we are trying to get heat to flow in adirection that is counter the natural flow of heat (naturalwould be from higher temperature to lower temperature)

COP is generally greater than 1.0 so we get more kW-h of cooling or heating than electric kW-h that we put into thecompressor

Performance (and COP) of the system is highly dependent onthe fluid temperatures that the condenser and evaporator arein contact with Lower evaporator temperatures result in lower COP Higher condenser temperatures result in lower COP More extreme temperatures lower COP and can lower available

capacity Temperature relation to performance can be a hindrance to the

system or a potential advantage Heat pump may struggle and require more energy as outside

temperatures become more extreme Presence of a more moderate/constant temperature source can

keep system running efficiently (e.g., ground)




Conditioning (cont’d)Chiller vs. Heat Pumps—what’s the

difference? Difference in system components: none

Chillers are generally cooling only device andare used to produce chilled water for coolingcoils (size range can be quite large)

Heat pumps can provide both heating andcooling and are typically smaller in size

(often residential units) Heat pumps are typically compression cycle

only and almost all use electric energy asinput

Chillers can use various cycles and may

actually use other energy sources as thesystem energy input



Condensers

Purpose: to reject heat from refrigerant tosurrounding environment, condensing therefrigerant from a (superheated) vapor to a(subcooled) liquid

Condenser is really a “heat exchanger” whichtransfers energy from one fluid stream to anotherwithout mixing the two streams

Water-Cooled Condensers Heat exchanged with water which is circulated to another

“component” (ground, lake, pond—natural or constructed,river, cooling tower, etc.) as closed or open loop

Condenser temperature depends on water sourcetemperature



Condensers (cont’d)

Air-Cooled Condensers Heat exchanged with outdoor air Fans required to improve heat transfer Condenser temperature linked to outside air dry bulb

temperature Evaporative Condensers

Heat exchanged sensibly and latently with outdoor air Fan and pump required: fan to circulate air through

unit, pump to circulate water

Added evaporation process increases performance Condenser temperature linked to outside wet bulb

temperature (less than or equal to dry bulb) Condenser water and evaporative water kept separat



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Condensers (cont’d)

Cooling Towers Similar concept as evaporative

condensers Condenser water “open” in the tower

Some water evaporates, requiring

make-up water Some systems eliminate the fan

requirement



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Condenser Examples



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Condenser Examples

(cont.)



16Digital images on this slidecourtesy of: Lisa Fricker, Graduate



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Condenser Examples

(cont.)



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Evaporators (Liquid

Coolers)Purpose: to absorb heat in the

refrigerant from the surroundingenvironment, evaporating the

refrigerant from a liquid (or liquid/vapormixture) to a (superheated) vapor

Evaporator is also a heat exchanger

Evaporator can be a cooling coil itself ora refrigerant (DX or direct expansioncoil) to water heat exchanger to thechilled water loop



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Heat Exchangers

Heat Exchanger Types (largest tosmallest): Shell-and-Tube

Plate/Plate-and-Frame Tube-in-Tube Shell-and-Coil

Heat Exchanger Issues: Larger exposed air means largest UA (more

heat transfer) Fouling can affect performance over time

(maintenance issues) Interior and exterior fins on coils



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Compressors

Purpose: to compress the refrigerant vapor to ahigher pressure (also increases the temperature)

Mechanical device: power input converted tomechanical energy

Types of Compressors: Positive-displacement: “squeeze”—increase pressure

be decreasing vapor volume Reciprocating Rotary

Scroll Trochoidal Dynamic: “spin”—increase pressure by transferring

angular momentum, momentum converted to pressureincrease Centrifugal

Centrifugal tend to be used in larger systems



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Compressors (cont’d)

Motor Types Open: motor and compression

chamber separated via shaft link Hermetic: motor and compression

chamber same, motor shaft andcompressor crankshaft integral

Semi-hermetic: bolted constructionallows field service



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Compression Cycle: Big

Picture

Condenser

Evaporator

Compressor

ExpansionValve

Cooling Coil

Air System To Zones…

Cooling Tower

Direction

of

heattran

sf e

r



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Absorption-Based LiquidChilling Systems

Concept Compression-based chillers use electrical energy (work) to

produce heating or cooling (in the opposite direction of natural energy flow)

Absorption-based chillers use mixture/solution chemistryand a heat source to produce heating (reverse cycle—alsocalled heat transformer) or cooling (forward cycle—morecommon)\

Absorption-based systems are most effective when a “free”or very inexpensive source of heat is available Solar energy

“Waste” heat Heat source must be high enough quality (temperature)to drive system

No compressor or other large rotating mechanicalequipment needed

Two “refrigerants”—primary and secondary (absorbent) Primary—usually water

Secondary—usually ammonia or lithium bromide (LiBr)



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Absorption Chillers (cont’d)

Components Generator (desorber)—high pressure side

Condenser—high pressure side Evaporator—low pressure side

Absorber—low pressure side

Heat Exchanger

Pump

Expansion valve/flow restrictors

Refrigerants



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Cycle Details (LiBr system) Pure water (vapor/liquid) in the condenser and evaporator Primary refrigerant (water) and absorbent mixtures of

varying concentrations in generator and absorber Weak liquid solution is introduced into the generator

along with heat from some source Generator process: boils water out of solution

accomplishing two things Pure water vapor is sent over to condenser side of

chamber Strong(er) solution (liquid) is sent to absorber Water vapor in condenser is converted to liquid

(condensed) by the removal/rejection of heat



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Cycle Details (LiBr system, cont’d) Condensed water is pushed to the evaporator as a result of the

pressure difference/gravity Liquid water in the evaporator is boiled off with the addition of heat

at low temperature/pressure

Water vapor boiled off from evaporator is sent to absorber Absorber: Water vapor condenses (potential heat rejection) and gets

reabsorbed into the water-LiBr solution, weakening the solution Absorber sends weakened solution back to generator where cycle

starts over again Pumps used to send solution from absorber to generator and to

circulate liquid water over evaporator coil Heat exchanger used between lines connection generator and

absorber—reduces heat addition needed in generator (improvingefficiency)

Goal is cooling at the evaporator (forward cycle) or heating at thegenerator (reverse cycle)

Many slight variations on this basic cycle



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Performance Issues Capacities typically range from 180-almost 6000 kW

(big!) though smaller units on the range of 18-35 kWavailable internationally

Typical COP values are much lower than for compressioncycle chillers: 0.7-0.8 or lower is common Low COP not necessarily a problem if heat source is free:

COP = Usable cooling/energy input

Other Issues Is a heat source available that can be used?

Concerns about water in contact with metal insideabsorption system (rust formation)

Potential toxicity of absorbent Noise—far less than a compression cycle chiller



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Thermal Energy Storage

Concept Produce and store energy for use during another time

Initially, this was as simple as cutting ice blocks fromLake Michigan and storing those until summer

Now, energy storage is produced during off-peak hours

when energy costs are lower Overall dollar effect is a reduction in the conditioning costs

for the buildingprimary (or only) benefit is economic Reduction in cost per kW-hr and reduction in demand costs

Costs based on type of power plants running Cost of start-up and shutdown of power plants

Mainly an issue for industrial customers, usually used forcooling Utilities have in the past actually paid (in part) for systems

Reduced demand reduces need for new power plants Shift of electric load uses power that might not

otherwise be used (hydroelectric, nuclear, etc.)



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(cont’d) System Types

Tempered Water Storage Storage of hot or cold water in a large tank above

or below grade

Water is kept stratified, taking advantage of density differences of water at differenttemperatures

Inlet diffusers must be designed to avoid mixing

Some energy transfer does occur between hot andcold sides

Water in tank can serve as emergency water sourcein case of fire

Water temperatures for cooling same as forstandard chiller only system

Large tank needs large space, tank losses



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(cont’d) System Types (cont’d)

Ice Storage Storage of cooling energy in the form of ice

Latent heat of solidification allows largeamount of energy storage in a much smallerarea than a water system

System types: Ice-on-coil outside melt (obsolete)

Ice-on-coil inside melt Encapsulated ice (ice container) Ice harvester Ice slurry



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(cont’d)Efficiency Issues (Ice Systems)

Process for producing ice less efficient than chilledwater production (temperatures required for makingice are much lower, resulting in lower efficiency/COP

and capacity of chiller) This may be offset somewhat be reduced condenser

temperatures due to cooler outdoor conditions atnight

Systems can produce lower supply air temperatures,reducing the flow rates needed to provide same

cooling (which lowers fan energy) Do ice storage systems save dollars and energy?



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Thermal Energy StorageControls

Full Storage (discharging) Minimizes on-peak energy consumption,

maximizes energy consumption shift Largest storage requirements and perhaps

largest chiller (and initial costs) Probably largest potential savings on operating

costs

Partial Storage (discharging) Types:

Chiller priority: chiller runs during on-peak only up

to some set demand limit, ice meets all other needs Ice priority: storage meets demand up to some limit

and chiller is turned on if the demand is higher thanthe limit

Some shift of energy consumption to off-peak,also savings on demand costs

Smaller chiller requirements than full storage orno storage



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Thermal Energy Storage Controls(cont’d)

Charging Strategies Zero prediction—chiller charges system

at its capacity as soon as off-peak period starts

“Optimal” strategies Delay start of charging to take advantage of

presumably cooler outdoor air in early

morning hours And/or run chiller at less than full capacity

at whatever its optimal fraction of full loadis



Heating Equipment

Boiler

Furnace

Heat Pump



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Heating Equipment

Electric resistance heating

Heat pump in heating mode

Solar panels

Boiler Water

Steam

Furnace (air)

same basic principle, just a different fluid



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Boilers

Definition: equipment whose solepurpose is to provide hot water or steamfor various uses within a building

Size (capacity) range:

15 kW 30+ MW

Fuels: coal, wood, fuel oil, (natural) gas,electricity



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Boiler Uses

Steam: Heating coils (reheat, preheat) Hot water heat exchangers Absorption cooling Laundry Sterilizers

Water: Heating coils (reheat, preheat) Domestic hot water



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Boilers: Basic Layout

Goal:

Try to getmost efficient

transfer of heat from fluegas(combustion

products) towater

stack/flue/chimney

air/fuelmix

burner

wate r

water

water

water



39

Boiler Example

(continued)

Digital image on this slide courtesy of: Lisa Fricker, Graduate Student,



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Boilers: Types

Dry Base/Back

Wet Base/Back/Leg

Base (bottom), back (with respect to multi-pass boilers), leg (top and sides)

Condensing Flue gas condensing due to low return

temperature of water

More efficient, but potential for rustgreatly increased



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Boilers: Efficiency

Fuel Boiler (combustion efficiency) Efficiency = (input – stack loss) / input

Non-condensing 75-86% Condensing 88-95+%

Electric Boiler (overall efficiency) Efficiency = output / input

Range of efficiencies 92-96%



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Furnaces

Heats air indirectly Combustion products do not mix with

circulated air dangerousFuels:

Natural gas (most common)

LPG (liquefied petroleum gas) Oil

Electric



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Furnaces (continued)

Sizes: Residential units (smallest)

Commercial (44 600+ kW) Generally smaller than boilers

Various configurations: Combustion systems Air flow variations (single/multi-pass)

Furnace



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Furnace(AHU)

Example



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Boiler/Furnace Stack



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Furnace Efficiency

ANSI/ASHRAE Standard 103 Annual Fuel Utilization Efficiency (AFUE)

AFUE includes: latent and sensiblelosses, cyclic effects, infiltration, pilot

burner effects, and losses from astanding pilot when furnace not in use AFUE ≈ 78-80% for non-condensing,

90+% for condensing

InputFuelOutputHeatUsableAFUE ≈



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Big Picture Review

air

surroundings

Zone (Loads)mix box

supply fan

cooling coilheating coil

chillerboiler

coolingtower

pumppump

pump

SecondarySystem

Primary System

A Buildingand itsHVAC

System



Summary

Primary systems convert one form of energy (fuel, electricity, etc.) tothermal energy

Chillers/heat pumps are used toprovide cooling (direct expansion orchilled water)

Boilers are used to provide steam orhot water for heating coils

Furnaces are used to provide hot air

intro to primary systems

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