Chill Train

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chiller

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Glossary of Terms

Approach — Tem perature difference between th e leaving fluid an d th e evapo-

ratin g refrigeran t

ARI Standard Conditions — 54°F. water in let; 44°F. wat er out;

35°F. refrigeran t; 0.00025 add itive fou ling factor

Flow Rate or velocity — Speed at wh ich t h e fluid travels through th e evaporator.

Fouling — Dirt and scale build u p th at imp edes heat transfer.

One Ton — 12,000 Btu/ h r

Pressure Drop — Differen ce in pressure between th e incom ing an d leavin g fluid

pressures.

Range — Tem perature difference between t he en tering warm fluid an d th e

leaving coo led fluid.

Specific Gravity — A m easure of the d ensity of a fluid com pared to water.

Specific Heat — A m easure of a flu id’s ability to ab sorb and transport h eat.

Superheat — Extra h eat carried by th e refrigerant after it h as chan ged from a

liqu id int o a gas.

Thermal Conductivity — A m easure of th e po tent ial rate of heat transfer.

Viscosity — A m easure of th e ability of a fluid to flow.



Refrigeration Cycle

Refrigeration is defin ed as a process of removin g heat

from an en closed space or material, an d m ain tainin g

th at space or m aterial at a temperature lower than its

surround ings. Cold an d h ot are relative terms th at are

not generally used when sizing heat transfer equip-

ment. Objects and space being refrigerated become

relatively colder an d colder (or less and less h ot) as

h eat is removed.

Removal of heat lowers temperature and may be

accom plished by th e use of ice, sn ow, ch illed water, or

m echan ical refrigeration. Mechan ical refrigeration

can be defined as an arran gemen t of comp onen ts in a

system for th e purp ose of tran sferring h eat.

Refrigeran t is on e of the key comp on ent s that m akes

m ech an ical refrigeration work. A refrigerant is a

chem ical compou nd that is altern ately com pressed

and con densed int o a liquid, and t hen permitted to

expand into a vapor or gas as it is pumped through

th e mech an ical refrigeration cycle.

This cycle is based on th e ph ysical principle th at a liq-

uid extracts heat from the surrounding area as it

expands (boils) in to a gas.

Refrigeran ts like Am m on ia, R–134A, and R–22, are cir-

culated throu gh th e system by a com pressor, which

increases the pressure and temperature of the

vaporous refrigeran t and pu m ps it into th e conden ser.

In th e con den ser, refrigeran t vapor is cooled by air or

water un til it con den ses in to a liqu id.

The liquid refrigerant then flows to the flow control

device, or expan sion valve, where flow is m etered and

the pressure is reduced, resulting in a reduction in

temp erature. You can u n derstan d th is concept if you

th ink of carbon d ioxide as a nat ural refrigerant. Wh enCO 2 is released from a h igh pressure fire extin guisher

cylin der to at m osph ere, it cools form in g ice crystals,

just like a like a halocarbon refrigerant, but less

ef ficien t.

After the exp an sion valve, refrigeran t flo ws into th e

lower pressure evaporat or, where it bo ils by absorbin g

h eat from t he space bein g cooled, and ch an ges in to a

vapor.

The cycle is completed when the compressor draws

the refrigerant vapor from the evaporator and, once

again, com presses th e gas so tha t th e cycle can con -

tin ue.

Chiller Barr els

Most Stan dard catalog m odels are direct–expan sion .

In d irect expan sion , th e refrigerant evaporates inside

th e tubes as th e medium to be cooled flows through a

baffled course on th e outside of th e tube bun dles. Th e

baffles assure proper m ixin g an d in crease h eat trans-

fer. The other common heat exchange barrels are

flooded chillers and water–cooled con den sers.

refrigeration cycle

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There are distinct differences in operation between

direct–expan sion chiller barrels, flood ed chiller bar-

rels, an d w ater–cooled con den sers. All are specializedheat exchangers that operate by removing heat from

on e fluid an d tran sferring it to an oth er. The differen ce

is in th e location of the water, and wh ether th e refrig-

eran t is chan gin g state from a liqu id to a gas, or from

a gas to a liquid.

DX Chiller

In t h e pop ular DX (or direct–expan sion ) chiller, heat

is drawn from water wh ich is located outside the tube

as refrigerant boils and evaporates inside. The tube is

finn ed in ternally to increase h eat transfer efficiency.

The direct–expansion ch iller barrel is po pu lar due to

th e therm ostatic expansion valve an d its exception al

ability to cont rol loads.

Direct–expansion chiller barrels are described by the

num ber of refrigeran t circuits, refrigeran t pa sses, an d

th eir capacity.

A sin gle–circuit chiller has on e refrigerant in let and

outlet. A dual–circuit has two refrigerant inlets and

two out lets. Each circuit can be used for two separate

but similar loads. There’s also a qu ad–circuit, design ed

for four separate loads. Direct–expan sion ch iller bar-

rels are also distinguish ed by th e n um ber of tim es that

refrigeran t passes back and forth with in th e len gth of

th e vessel. In a sin gle–pass un it, liquid refrigeran t

ent ers at on e end, passes straigh t th rough an d leaves

as a vapor in a single pass. In a dual–pass m odel, the

liquid refrigerant must go down and back the length

of th e ch iller before it exits as vapor. Refrigerant ent ersand exits at the same end of the chiller when the

n um ber of passes is even, and at opp osite end s wh en

th e nu mb er of passes is odd.

Flooded Chiller

The other commonly found type of chiller barrel is

th e flooded style. In th e flooded chiller, water travels

through the tubes and transfers its heat to a boiling

refrigeran t bath th at covers 1 ⁄ 2 to 3 ⁄ 4 of the tube bun dle.

Flooded ch illers, com mo n ly used in low temp erature

ammonia applications, can be ordered on specialrequest from Stand ard.

chiller barrels

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Water–Cooled Condensers

Standard’s other major heat exchanger line, the

water–cooled con den ser, is arran ged in a m ann er sim-

ilar to th e flooded chiller, except th at cool water trav-

eling th rough an extern ally fin n ed tube removes heat

from con densing refrigerant vapo r outside of the tube,

producin g liquid refrigeran t at th e bottom of the con-

denser shell.

Perfor mance FactorsSelecting th e right ch iller barrel or water–cooled con -

denser for a particular application depends on the

same basic perform an ce factors.The factors are: flow rates or velocity, pressure drop,

fouling, t ypes of flu ids (refrigeran ts, as well as cooling

fluids), an d tem peratu re differen tial or TD.

There are also som e terms to kn ow wh ich refer to spe-

cial tem peratu re different ials. They are: ran ge,

approach , and superheat.

Ra nge

Range is th e difference between in comin g and outgo-

ing water or fluid tem perature. It is th e temp erature

difference across the ch iller.

The Standard chiller catalog contains performance

data for ran ges for water from six to twent y degrees.

Approa ch

Approach is the temperature difference between the

leaving water or flu id an d th e evaporating refrigerant

tem perature.

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performance factor

New Terms



Standard measures evaporating temperature as the

saturated equivalent temperature to the refrigerant

outlet pressure, a meth od th at takes in to account pres-sure drop in th e barrel. Th is is a con servative meth od

of ensuring th e capacity claim ed.

Performan ce is governed by both range and ap proach,

especially when sizing at other than nominal or

Am erican Refrigeration Institut e con dition s.

Here’s an example of the dramatic influence of

approach tem perature on sizin g. A Stan dard TX–10 at

a 10°F ran ge has 9.5 ton s cap acity at a 8.7°F app roach ,

but can handle 13.4 tons when the approach is

increased to 12°F. Th at is a 30% perform an ce increase.

As a gen eral rule, every on e–degree ch an ge in

App roach is approximat ely equ ivalent to a 13% d if-feren ce in ch iller barrel capacity.

In spite of th e dram atic in crease in capacity, th ere are

good reasons to limit approach. For one, high

approaches can risk costly freeze–up damage.

Evaporating temperatures below freezing can destroy

a chiller barrel, unless it is protected by a freezestat,

flow switch , low pressure cut–off, or special fluids like

glycols an d brin es.

Second ly, as th e suction temp erature and evaporatin g

temp erature are lowered, th e capacity of th e comp res-

sor is reduced, lowering th e am oun t o f refrigeration

available in the system.

Superhea t

Sup erh eat is the extra h eat th at is carried b y a gas after

it has chan ged from a liqu id in to a gas. Steam at a tem -

perature greater than 212° Fah renh eit is a good exam-ple of a super–heat ed gas. Super–h eating occurs in a

refrigeration system wh en refrigerant gas in th e evap-

orator tubes and suction line contin ues to absorb heat

after it h as und ergon e its ch an ge of state from liquid

to vapor. The p resence of som e sup erheat is ben eficial,

since it is proof that the refrigerant has completely

evaporated and th at n o liquid refrigerant is left in t he

line to slug back to th e com pressor.

Stan dard Refrigeration sizes chillers to m eet th e ARI

standard of a minimum 7°F superheat. Some other

manufacturers use virtually zero degrees of superheat

to boost the apparent rated capacity of their units.Higher superheat ratin gs reduce th e overall rated p er-

formance of a chiller, since tubing that could carry

efficient heat absorbin g liquid refrigeran t is n ow d ed-

icated to less efficient refrigeran t vap or.

Manufacturers that reduce superheat ratings below

the ARI standard of 7°F gain chiller barrel economy,

bu t increase the dan ger of liqu id slugging. Low super-

h eat ratin gs increase th e need for a suction lin e accu-

mu lator or a suction line h eat exchan ger to p rotect

th e comp ressor from severe dam age.

Let’s review th e oth er five facto rs that affect perfor-

m ance, flow rates or velocity, pressure drop, fouling,types of fluids, an d tem peratu re differen tial or TD.

performance factors

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Performance Factors



Flow Ra tes

Water flow rates (velocity) in a ch iller barrel must be

h eld to 41 ⁄ 2 feet per secon d to avoid impin gemen t cor-

rosion dam age. All Stan dard selection recom m enda -

tions in our literature are un der 41 ⁄ 2 feet p er second .

Pressu re Drop

Pressure drop is th e d ifference between th e ent ering

and leavin g water pressure. Th e pum p capacity mu st

be great enough to overcome the combined pressure

drop across the chiller barrel and piping. Choose

anoth er model un less you’re sure that your pum p h as

th e capacity.

Fou ling

Fouling is scale or foreign material build–up thatreduces heat transfer. Most packaged chillers for

air–conditioning have sealed loop systems. There is

little opp ortun ity for fouling with in th e chiller barrel,

because the cooling flu id is sealed from external envi-

ron m ent al factors. Process ch illers in p lastics m oldin g

and other industrial applications often have open

chilled wat er tanks or op en processes wh ere the wat er

can foul badly an d ad ded capacity m ust be available

to com pen sate for th e fouling.

Stan dard rat es its chiller barrels with an add itive fou l-

ing factor of 0.00025 (0.0005 tot al).

Types of Fluids

Fluid types and refrigerants can vary, depending on

th e application . Th e most comm on flu ids are water,

eth ylene glycol, prop ylene glycol, and brine solut ions.

Fluids are the gasses or liquids that exchange heat in

h eat transfer. Fluids oth er than water can be con sider-

ably less efficien t an d h ave a substantial effect on th e

sizing of a ch iller. Special ch iller designs m ay be

required if glycols are used to preven t freeze dam age

in low tem perature application s.

The refrigerant itself must be taken into considera-

tion. Standard’s cataloged capacity data is based on

R–22. However, information for other refrigerants,

including R–502 an d am m on ia, are readily available

from t h e factory or sales represen tative.

The major fluid characteristics that affect chiller per-

form an ce are Specific Heat , Specific Gravity , Viscosity,

and Thermal Conductivity.

Fluid Characteristics

Specific Hea t

Specific Heat is a measure of th e ability of a flu id to

absorb and transport h eat. Th e specific heat value of

water is 1.0. Glycols and brines used in m an y low tem-

perature process cooling applications have lower

specific h eat values than water an d are less efficient.

The greater th e percentage, the lower the specific h eat.

Specific Gra vity

Specific gravity is a m easure of th e den sity o f a fluid as

com pared t o wate r. As the specific gravity in creases,

the ability of a fluid to absorb and hold heat also

increases. The specific gravity of water is 1.0. Most gly-

cols and brines h ave specific gravities greater th an

on e.

Viscosity

Viscosity is a m easure of th e ability of a flu id to flo w.

It is m easured in a u n it called cen tipoise. Th e viscosi-

ty of water at 67° Fahrenheit is 1.0 centipoise, and

varies only slightly with temperature. Other fluids,

like glycols, may have much higher viscosities that

chan ge considerably with tem perature. Th is can result

in unacceptable pressure drops at low temperatures

and reduction in capacity.

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fluid characteristic

Fluid Characteristics



Therm a l Cond uctivity

Therm al Con ductivity is th e fourth fluid factor tha t

can affect performance. It is related to the potential

rate of heat transfer across a temperature differential

for a fluid. Th e th ermal condu ctivity of pure water at

44° Fahren heit is 0.338, while a 30% solution o f eth y-

lene glycol at th e sam e tem perature is 0.256.

It is always best to con sult with your Stan dard sales

representative for fluids other than water. Standard

h as complete data on eth ylene an d propylene glycols,

as well as sod ium an d calcium brin es. If you are deal-

ing with a more exotic fluid, try to have the specific

h eat, specific gravity, viscosity, an d th erm al con du c-

tivity ready when you call Stand ard for help with your

selection .

Ma teria ls

Remem ber that m ost stock ch iller barrels contain cop-

per tu bes; th erefore, am m on ia refrigeran t or ch loride

brines cannot be used. Cupronickel construction

should be specified for chloride brines. Carbon or

stain less steel shou ld be specified for am m on ia appli-

cations. Be sure to consult the factory for a special

selection if you h ave any q uestion s.

Sizing

Sizing by Nominal Ton sThere are th ree basic selection m etho ds you can use to

size a chiller. The first an d easiest is to size by nom inal

system s ton s. Th e second m etho d is to use com pressor

capacity. The th ird and recomm end ed m etho d is siz-

ing by ran ge, flow and approach.

Sizing by no m inal ton s is don e according to ARI stan -

dards. Chillers can be selected on a n om inal system

ton s basis, as sh own in th e catalog, or reflected in th e

m od el n ame. For example, an TX–50–2 is a nom inal

50 ton dual–circuit barrel.

Nom inal ton ratin gs are based on th e con ditions of

ARI Stan dard 480 u tilizing R–22, w hich are:54° F water in

44° F water out

35° F refrigerant tem peratu re

7° superh eat

0.00025 additive fouling factor (0.0005 total)

100° F liqu id refrigeran t ent ering th e flow cont rol

This m eth od is reasonab ly accurate for sizin g air–con-

ditioning systems. However, it is not recommended

for evapo ratin g temp erature below 34° F, or wh en th e

fluids used are other th an w ater and R–22.

sizing

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ARI standard

Tube Selection



Sizing by Compressor Ca pa cit y

You ma y also size by com pressor cap acity.

Com pressor m anu facturer perform an ce curves of

com pressor Btu per ho ur data can be used to select

ch iller barrels for a given system. By reading th e per-

forman ce curve for the com pressor at the cond ition s

th at you require, you can determine th e maximum

capacity of the chiller barrel you will n eed.

Sizing by Ra nge an d Flow

The m ost precise way to size a chiller barrel wh en

water is used is by ran ge and flow, as seen in t h is for-

m ula.Btu/ hr capacity = Range x gpm x 500

To obtain th e Btu p er hou r capacity, just m ultiply the

Ran ge or Temperature Drop by gpm flow, and convert

to pounds of water per hour by multiplying by 500.The Btus can then be divided by 12,000 to yield the

ton s of load.

For example—

With a incom ing water temp erature of 55° F.,

outgoin g water temp erature o f 45° F., and a 479 gpm

water flow. Btu capacity can be calculated like this:

(55°– 45°) x 479 gpm x 500 = 2,395,000 Btu/ hr

2,395,000 Btu/ hr ÷ 12,000 Btu/ ton = 199.5 tons

Stan dard ’s sales cata log is designed to m ake your selec-

tion job easy after you have determined the cooling

load. Once fluid range has been determined, locate

th e table for th at range. If no table exists, use the n ext

lowest range.

Next, locate an acceptable approach at the to p of th e

table and read down th e tons colum n to th e capacity

th at is adequ ate for th e job.

The water pressure drop in psi is listed to t h e righ t. To

determin e gpm m ultiply tons by th e gpm factor at th e

bott om of each ran ge chart.

In th is case, an FSX–150 will provide th e du ty at a 10°

range and 11° approach with a 34° suction .

In cases where the fluid being cooled is other thanwater, capacity can be determined by adding the

specific h eat and specific gravity into th e equation :

Btu/ hr = Range x gpm x 500 x Sp Heat x Sp Gravity

This equation will give you the capacity required, but

chiller selection should be made by your factory represen-

tat ive since the fluid is not water.

All of Stan dard’s ratin g dat a is based on ARI stand ards,

a suction temperature of thirty–five degrees, using

water as the fluid . Con tact your local Stan dard repre-

sentative for special fluid con ditions.

All h eat exch an gers h ave capacity limits. Careless siz-

in g of chiller barrels leads to n eedless perform anceproblem s.

Undersizing can lead to insufficient cooling and

inefficien t com pressor operation.

Oversizin g can lead to control valve hun ting, poor

perform an ce, oil logging, and refrigerant slugging.

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sizing

Chill Train

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