Carrier 16JB User Manual

Carrier	Application Data
			16JB
	Application Detail

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Hermetic Absorption Liquid Chillers

GENERAL

Application details in this publication cover various methods of applying the 16JB absorption machine to meet liquid chilling needs. Throughout this publication, the chilled liquid will be water.

Subjects covered are chilled water temperature control, condenser water temperature control, system design for steam and hot water machines, and general information.

CHILLED WATER TEMPERATURE CONTROL

The absorption machine is basically a water chiller that can be connected to any conventional open or closed system. However, circulation of chilled water must be continuous during operation of the machine and during the shutdown dilution cycle. Chilled water flow may be restricted at partial load.

For fine chilled water temperature control within narrow limits, such as required in precision control of industrial processes or maintenance of laboratory conditions, the chilled water system may require additional storage volume to allow the machine to adjust slowly to changes in load. Normal air-conditioning applications are not subject to such requirements.

Systems having large storage volumes of chilled water transmit load changes to the machine slowly, allowing accurate chilled water temperature con­ trol. Small storage systems transmit load changes rapidly, making temperature control more diffi­ cult. For fine temperature control, the chilled water system volume should be at least ten times the gpm flow through the cooler. If a tank is added to the system for extra storage volume, it should be located in the line from the load to the cooler.»

Two-Pipe Cooling-Heating Systems — When machines are used in conjunction with a two-pipe cooling-heating system, certain precautionary steps should be taken during changeover from heating to cooling.

Maximum water temperature permitted thru the evaporator is 130 F because of the possibility of tube stress. If system water temperature is above 80 F but less than 130F at changeover time, evaporator flow should be throttled to prevent machine overload.

It is recommended that hot water temperatures be reset, based on outside air temperature. If a reset-type control is used, the entering hot water temperature at changeover will normally be lower than 130 F.

STEAM MACHINES

Boilers — Generally, any boiler capable of modu­ lating its input to maintain design operating steam pressure within 1 psi under varying loads, is suit­ able for application with the absorption machine. This generally includes all gasand oil-fired boilers.

Some oil-fired boilers are conversions from coal-fired to oil-fired and may have control systems which are too sluggish to give proper response to machine load changes. Direct control of oil feed rate normally ensures proper response.

Coal-fired boilers, due to slow buildup and shutdown characteristics, should be used only when the absorption machine represents less than 15% of boiler operating load. This generally limits coal-fired boiler applications to large industrial jobs where process steam is generated in large quantities year-round.

BOILER CAPACITY — Minimum boiler capacity for use with the absorption machine is equal to full load steam consumption, plus sufficient capacity to offset piping radiation losses. In the absence of a detailed study of radiation and vent losses, a minimum 10% safety factor should be used.

Pressure Reducing Valves — Maximum unit ratings are based on 14 psig steam pressure at the generator inlet. Operation at higher inlet pressures or with more than 100 F superheat is not per­ missible. Higher inlet pressures may lead to over­ concentration.

Where steam supply pressures are above 15 psig (14 psig + 1 psig for control) and below 20 psig, the steam control valve can be used to reduce the pressure. If steam supply is above 20 psig, a pressure reducing valve must be provided between the steam supply and the control valve inlet. A safety relief valve should be provided between the steam control valve and the generator inlet. This valve must be set in accordance with paragraph UG-133 (f) of the ASME code to relieve at a pressure not exceeding 17 to 18 psig or the setting determined by apphcable local codes.

Further specific details relative to pressure reducing stations should follow accepted standards, such as the ASHRAE Guide and manufacturer’s recommendation. For applications on high-pressure district heating, the steam utility should be con­ sulted for local codes or standards.

Steam Piping should be sized to avoid excessive pressure drop or excessive velocities. Recommenda­ tions and pipe sizing tables are given in the Carrier System Design Manual. It is recommended that lines be sized on the basis of design system flow for the machine plus a 10 to 20% safety factor to allow for normal radiation losses.

© Carrier Corporation 1971

Form 16JB-2XA

Start-Up Demand — Steam demand by the absorp­ tion machine is greatest at start-up (see Table 1 for values).

Table 1 — Maximum Condensate Flow (Ib/hr)

VALVE	VALVE INLET STEAM PRESSURE
SIZE (in.)	20 psig	14 psig	12 psig		10 psig
2	2450	2025	1880		1750
2Y2	4825	4000	3710		3430
3	8175	6760	6285		5810
4	14540	12025	11190		10350
5	21650	17900	16655		15400

When boiler capacity is unable to keep up with start-up demand, the steam pressure will fall off. On boilers serving only the absorption machine, this reduction in steam pressure will have no adverse effect on the absorption machine other than to lengthen start-up time. However, the increased steam demand may have an adverse effect on the boiler, causing it to run dry and fail. As steam pressure is reduced, the steam control valve pressure drop will eventually limit the de­ mand on the boiler provided the steam control valve is properly sized.

On boilers serving other loads simultaneously, the start-up demand can reduce boiler pressure sufficiently to cause adverse effects on other steam-driven equipment. When a reduction in boiler pressure cannot be tolerated without up­ setting other equipment, the boiler capacity avail­ able for absorption machine operation (with other loads deducted) must equal or exceed the start-up demands. If it does not, the start-up demand can be reduced by using demand Hmit controls, or installing a back-pressure regulator in the steam line(s) between the boiler and the control valve(s).

VALVE LOCATION AND PIPING - The steam control valve should be located a minimum of 3 ft away from the generator inlet. This is dictated by good piping practice, to allow equal distribution of steam in the generator tube bundle. Unequal distribution of steam in the tube bundle may cause a loss of capacity. Recommended steam supply piping for low-pressure steam applications is illustrated in Fig. 1.

STEAM CONTROt

NOTE; Separate supoiy piping "for each end of machine sizes

16JB077 thro '24,

Fig. 1 — Low-Pressure Steam (2 to 15 psig) Supply Piping

Machine sizes 16JB077 thru 16JB124 have steam supply inlets on each end. These are to be considered as two generators and should be piped from a common steam header as in multiple machine installations (see Fig. 2). Each inlet should then be piped in accordance with Fig. 1.

HEADER FOR N0.2 END

NOTES:

1.Piping appiies to moitipie macbiries connected in paraiief (3 shown).

2.Each end most be consdered as a separate generator.

3.The feed to each end of each generator should be piped as shown in rig. 1.

Fig. 2 — Steam Piping For 16JB077 thru 124

Steam piping to the absorption machine should be designed and supported to allow for thermal expansion without imposing undue stresses on the generator inlet. The machine is not designed for, nor expected to act as, a piping support or anchor for withstanding thermal stresses.

Condensate Systems — Satisfactory operation of the absorption machine requires a condensate handling system designed with the specific characteristics of the absorption machine in mind. The following is intended to supplement available reference data on condensate systems such as Carrier System Design Manual, ASHRAE Guide and individual manufacturer’s recommendations.

ATMOSPHERIC CONDENSATE RETURN SYSTEMS (VENTED) - These systems usually consist of steam traps, vented receiver, condensate pump, and condensate cooler. Fig. 3 illustrates typical atmospheric condensate return systems. On larger machines, with dual steam generators, the condensate outlet from each generator must be piped thru separate steam traps.

Trap Selection — Steam traps should be located as far below the generator outlet as possible. Actual pressure drop available for trap selection will depend on exact trap location below the generator

FROM SEMERATOR (LEFT END)

MACHINE Sizes 16J8077 THRU IZA

Fig. 3 — Typical Atmospheric (Vented) Condensate Return System

outlet, and trap outlet pressure. A vacuum breaker is factory installed to ensure that operating steam pressure in the generator does not fall below atmospheric pressure. Use the following formulas to determine available trap pressure drop:

Trap pressure drop = trap inlet ~ trap outlet psig.

Trap inlet pressure = 0 psig + hydrostatic head to trap inlet -- condensate leg pressure drop.

Trap design outlet pressure receiver pres­ sure + line pressure drop from trap outlet to receiver.

In determining trap outlet pressure, discount any liquid head drop to the receiver. This line may

not run full. If there is liquid lift from trap outlet to the receiver, it must be added to trap outlet pressure.

Either float-and-thermostat or inverted-bucket traps may be used, provided the trap is recom­ mended by the manufacturer for rapid handhng of noncondensables. For fast start-up with invertedbucket traps, install an external thermostatic air vent around the trap.

Traps should be sized for capacity to handle more condensate than twice the design full-load steam rate. See steam trap manufacturer’s recom­ mendations. Maximum load on the trap will occur during start-up when generator pressure falls to atmospheric (0 psig) and steam condenses rapidly.

At this time, pressure drop			across	the steam
control valve	is	maximum. If	boiler	capacity is
large enough,	the	control valve	inlet pressure will

stay at design, then flow rate will be limited by control valve capacity.

Table 1 gives maximum condensate flow for different inlet steam pressures. Interpolate for intermediate pressures.

If steam demand on start-up can be held within a controlled limit, the trap(s) may be sized accordingly.

When traps are undersized, condensate wiU back up in the generator with loss in machine capacity and may cause dangerous water hammer. Depending on boiler size, the boiler water makeup system could operate and add water to the boiler. Sooner or later, excess water would return to the boiler room and either overflow the hot well to drain, or if it flows directly into the boiler, it may shut the boiler down on high boiler water level control.

Condensate Cooler is used on some atmospheric condensate systems to reduce or eliminate loss of flash steam from the open receiver vent. The condensate cooler must be sized for handling and condensing flash steam as well as cooHng the condensate. Condensate is normally cooled to about 180 F. Pressure drop thru the condensate cooler should be very low, as it must be added to trap outlet pressure. If there is a liquid leg down to the condensate receiver, the condensate cooler and trap should be located at the bottom of this leg.

When a condensate cooler is used, it is desirable to use either cold boiler feed water or other cold water source which can benefit by heat rejected from hot condensate. Cooling tower bleed water can be used, but it may be heavy with dissolved solids and may rapidly foul the cooler. Tower makeup water can be used when large cooling towers are part of the system. Extra load to the tower would be insignificant.

Receiver and Condensate Pumps — When open receivers are used, the vent should be directed outside the equipment space to eliminate fogging.

Be careful in using small receivers and closeconnected condensate pumps. Some commercially available systems may work well on standard heating systems but can present problems in handling condensate from absorption machines. The basic difference in absorption machine oper­ ation lies in higher condensate temperatures and greater amounts of flash vapor. Commercial heating systems normally deliver condensate to the receiver thru long return runs. This lowers con­ densate temperature to 200 F or lower with relatively little flash steam.

Absorption machines commonly located close to the condensate receiver have little or no condensate cooling. During full load, condensate may be delivered to the trap at close to 12 psig and 240 F. This creates large amounts of flash steam at the trap outlet and in the condensate receiver. Very hot condensate drawn into the condensate pump may cause cavitation.

To minimize these effects, the following guides are offered:

1. If equipped with a vented receiver, the inlet line to the receiver should enter above the receiver water level. Flash steam can go directly out the vent without creating turbulence or frothing.

2.Locate the condensate pump as far below receiver water level as possible to give maxi­ mum Net Positive Suction Head (NPSH) to the pump.

3.If pump suction pipe is located at bottom of the receiver, use a vortex breaker at the receiver outlet.

4.Locate pump suction at opposite end of re­ ceiver from the condensate inlet. This will minimize agitation and frothing at pump inlet.

5.A properly selected condensate cooler, as previously described, will eliminate problems with flashing.

VACUUM PUMP CONDENSATE RETURN SYSTEMS are sometimes used to return con­ densate from space heating installations. The vacuum pump maintains the condensate return system at a subatmospheric pressure and permits the heating system to operate with subatmospheric pressure when the heating load is small.

It is generally impractical to use an existing vacuum pump condensate return system. Con­ densate from the absorption machine is far higher in temperature than condensate from the original heating system for which the vacuum return pump

■; was selected. Hot condensate forms excessive quantities of flash vapor when released into the vacuum return system and will usually cause vapor lock in either the return piping or the vacuum return pump, or both. When the existing condensate return system is a vacuum pump type, the recommended method of returning condensate from the machine is a separate wet-return system, if possible.

As an alternate choice, condensate can be discharged thru a steam trap to an atmospheric vent receiver. The receiver discharges flash-cooled condensate thru a second trap into the vacuumreturn system.

If a condensate cooler is used, condensate may be cooled to an acceptable level and discharged