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Steam systems use the vapor phase of water to

upply heat or kinetic energy through a piping sys-

em. As a source of heat, steam can heat a condi-

oned space with suitable terminal heat transfer

equipment such as fan-coil units, unit heaters, radia-

ors, and convectors (finned tube or cast iron), or steam

an heat through a heat exchanger that supplies hot

water or some other heat transfer medium to the ter-

minal units. In addition, steam is commonly used in

heat exchangers (shell-and-tube, plate, or coil types)o heat domestic hot water and supply heat for indus-

rial and commercial processes such as in laundries

and kitchens. Steam is also used as a heat source for

ertain cooling processes such as single-stage and

wo-stage absorption refrigeration machines.

Advantages

Steam offers the following advantages:

Steam flows through the system unaided by ex-ternal energy sources such as pumps.

Because of its low density, steam can be used intall buildings where water systems create exces-sive pressure.

Terminal units can be added or removed withoutmaking basic changes to the design.

Steam components can be repaired or replacedby closing the steam supply, without the difficul-

ties associated with draining and refilling a watersystem.

Steam is pressure-temperature dependent; there-fore, the system temperature can be controlled by

varying either steam pressure or temperature.

Steam can be distributed throughout a heating sys-tem with little change in temperature.

In view of these advantages, steam is applicableo the following facilities:

Where heat is required for process and comfortheating, such as in industrial plants, hospitals, res-taurants, dry-cleaning plants, laundries, and com-mercial buildings.

Where the heating medium must travel great dis-tances, such as in facilities with scattered buildinglocations, or where the building height would re-

sult in excessive pressure in a water system. Where intermittent changes in heat load occur.

Fundamentals

Steam is the vapor phase of water and is gener-

ated by adding more heat than required to maintains liquid phase at a given pressure, causing the liquido change to vapor without any further increase in tem-

perature. Table 1 illustrates the pressure-temperatureelationship and various other properties of steam.

Temperature is the thermal state of both liquidand vapor at any given pressure. The values shown in

Table 1 are for dry saturated steam. The vapor tem-perature can be raised by adding more heat, resultingin superheated steam, which is used (1) where highertemperatures are required, (2) in large distributionsystems to compensate for heat losses and to ensurethat steam is delivered at the desired saturated pres-sure and temperature, and (3) to ensure that the steamis dry and contains no entrained liquid that could dam-age some turbine-driven equipment.

Enthalpy of the liquid hf (sensible heat) is theamount of heat in Btu required to raise the tempera-ture of a pound of water from 32F to the boiling pointat the pressure indicated.

Enthalpy of evaporation hfg (latent heat of va-porization) is the amount of heat required to change apound of boiling water at a given pressure to a poundof steam at the same pressure. This same amount ofheat is released when the vapor is condensed back toa liquid.

Enthalpy of the steam hg (total heat) is the com-bined enthalpy of liquid and vapor and represents the

total heat above 32F in the steam.Specific volume, the reciprocal of density, is the

volume of unit mass and indicates the volumetric spacethat 1 lb of steam or water occupies.

An understanding of the above helps explainsome of the following unique properties and advan-tages of steam: Most of the heat content of steam is stored as la-

tent heat, which permits large quantities of heat tobe transmitted efficiently with little change in tem-perature. Because the temperature of saturatedsteam is pressure-dependent, a negligible tem-

perature reduction occurs from the reduction inpressure caused by pipe friction losses as steamflows through the steam. This occurs regardlessof the insulation efficiency, as long as the boilermaintains the initial pressure and the steam trapsremove the condensate. Conversely, in a hydronicsystem, inadequate insulation can significantlyreduce fluid temperature.

Steam, as all fluids, flows from areas of high pres-sure to areas of low pressure and is able to movethroughout a system without an external energysource. Heat dissipation causes the vapor to con-dense, which creates a reduction in pressurecaused by the dramatic change in specific volume(1600:1 at atmospheric pressure).

As steam gives up its latent heat at the terminalequipment, the condensate that forms is initially atthe same pressure and temperature as the steam.When this condensate is discharged to a lower pres-sure (as when a steam trap passes condensate tothe return system), the condensate contains moreheat than necessary to maintain the liquid phaseat the lower pressure; this excess heat causes

some of the liquid to vaporize or "flash" to steam

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PERCENTAGEOFFLASH

STEAM

20

15

10

5

0

-20 0 50 100 150 200 250

CURVEBACK PRE

PSI

A

B

C

D

E

F

G

-10

-5

0

10

20

30

40

A

B

C

D

E

F

G

PSI FROM WHICH CONDENSATE IS DISCHARGED

25

30

FLASH STEAM lb/h

0 1000 2000 3000 4000 5000

30

25

20

15

10

5

0

FLASH

TANKINTERNALDIAMETER,in.

at the lower pressure. The amount of liquid thatflashes to steam can be calculated as follows:

100(hf1-h

f2)

% Flash Steam = ______________ (1)h

fg2

wherehf1 = enthalpy of liquid at pressure p1

hf2 = enthalpy of liquid at pressure p2hfg2 = latent heat of vaporization at pressure p2

Flash steam contains significant and useful heatenergy that can be recovered and used (see the sec-tion on Heat Recovery). This reevaporation of conden-sate can be controlled (minimized) by subcooling the

condensate within the terminal equipment before itdischarges into the return piping. The volume of con-densate heat that is subcooling should not be so large

as to cause a significant loss of heat transfer (con-densing) surface.

Heat Recovery

Two methods are generally employed to recoverheat from condensate: (1) the enthalpy of the liquidcondensate (sensible heat) can be used to vaporizeor flash some of the liquid to steam at a lower pres-

sure, or (2) it can be used directly in a heat exchangerto heat air, fluid, or a process.

The particular methods used vary with the typeof system. Facilities that purchase steam from a utility

generally do not have to return condensate and, there-fore, can recover heat to the maximum extent pos-sible. On the other hand, facilities with their own boiler

generally want the condensate to return to the boiler

as hot as possible, limiting heat recovery because anyheat removed from condensate has to be returned tothe boiler to generate steam again.

Flash Steam

Flash steam is an effective use for the enthalpy

of the liquid condensate. It can be used in any facility

that has a requirement for steam at different pressures,

regardless of whether steam is purchased or gener-ated by a facilitys own boiler. Flash steam can be used

in any heat exchange device to heat air, water, or other

liquids or directly in processes with lower pressure

steam requirements. Equation (1) may be used to cal-culate the amount of flash steam generated, and Fig-

ure 1 provides a graph for calculating the amount offlash steam as a function of system pressures.

Although flash steam can be generated directly

by discharging high-pressure condensate to a lower

pressure system, most designers prefer a flash tank

to control flashing. Flash tanks can be mounted either

vertically or horizontally, but the vertical arrangementshown in Figure 3 is preferred because it provides bet-

ter separation of steam and water, resulting in the high-

est possible steam quality.

The most important dimension in the desigvertical flash tanks is the internal diameter, which mbe large enough to ensure a low upward velocit

flash to minimize water carryover. If this velocity isenough, the height of the tank is not important, bis good practice to use a height of at least 2 to 3 The graph in Figure 2 can be used to determineinternal diameter and is based on a steam velo

of 10 ft/s, which is the maximum velocity in msystems.

Installation is important for proper flash tank

eration. Condensate lines should pitch towardsflash tank. If more than one condensate line dischato the tank, each line should be equipped with a sw

check valve to prevent backflow. Condensate lines

Fig. 1

Fig. 2

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RELIEF DEVICEMAKEUP CONTROL VALVE

HIGH PRESSURE STEAM

DISHCHARGE TO HIGH PRESSURE

CONDENSATE RETURN

PRESSURE GAUGE

SAFETY RELEIF

VALVE

LOW PRESSURE STEAM

THERMOSTATIC AIR VENT

PIPE TO DRAIN

FLASH TANK

SWING CHECK VALVE

HIGH PRESSURE

CONDENSATE RETURN

DRIP

POCKET

F&T OR IB TRAP

LOW PRESSURE RETURN

he flash tank should be well insulated to prevent any

unnecessary heat loss. A thermostatic air vent shouldbe installed at the top of the tank to vent any air thataccumulates. The tank should be trapped at the bot-om with an inverted bucket or float and thermostatic

rap sized to triple the condensate load.The demand load must always be greater than

he amount of flash steam available to prevent the low-pressure system from becoming overpressurized. A

afety relief valve should always be installed at theop of the flash tank to preclude such a condition.

Because the flash steam available is generally

ess than the demand for low-pressure steam, amakeup valve ensures that the low-pressure systemmaintains design pressure.

Flash tanks are considered pressure vessels and

must be constructed in accordance with ASME andocal codes.

Rule of thumb: Horizontal flash tanks should be2 times tank diameter or 24" minimum length; Vertical

ash tanks should be 3 times tank diameter or 36"

minimum length.

Fig. 3

Copyright 2008, American Society of Heating, Refrigerat-ng and Air-Conditioning Engineers, Inc. www.ashrae.org.

Reprinted by permission from ASHRAE 2008 Handbook-HVAC Systems and Equipment.

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Type VFT Flash Tank (Vertical)

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Type HFT Flash Tank (Horizontal)

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Type FRV Units (Flash Recovery Vessel)

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ORM BULL 161 Revised 12/11 SHIPCO IS A REGISTERED TRADEMARK OF SHIPPENSBURG PUMP CO., INC.

HIPPENSBURG PUMP COMPANY, INC., P.O. BOX 279, SHIPPENSBURG, PA 17257 PHONE 717-532-7321 FAX 717-532-7704 WWW.SHIPCOPUMPS.COMRINTED IN THE U.S.A. BEIDEL PRINTING HOUSE, INC., 717-532-5063 COPYRIGHT 2011 SHIPPENSBURG PUMP COMPANY, INC.

Other Specialty Products

Specialty models include Vertical or Horizontal Flash Tanks, Flash Tanks, Discharge Chemical Feeders,Blow Down Separators, Blow Down Tanks, Fill Tanks and Turbine Models.

Benefits

* Reduce high-pressure steam to low-pressure steam.* Economically add chemicals into system.* Pipe blow down chemicals and debris to drain.

* Add glycol into system without contaminating the water source.* Lubricate and flush mechanical seals or stuffing boxes on large municipal or industrial pumps.

BDT

BDS

DCF

FT