Chiller Systems Dr. A. Hammoud BAUBAU -2007 007
Terminal Unit Performance & Control
Chilled Water System Type of Chilled water system y
Air cooled chilled water system
W t cooled Water l d chilled hill d water t system t
Mini--Chilled Water System Mini
Air Cooled Water Chiller
Outdoor unit is Noisy system, system, (open (open large area )
Chilled water system ¾Heavy Duty & Long life cycle ¾Air Cooled Large g Capacity p y Chiller Range g ¾Economical for large systems (high Efficiency). ¾Low L Noise N i ( fan f coil il indoor) indoor i d ) ¾Part Load or full load (25 (25-50 50-75 75-100%) 100%) using multi compressors for capacity control. ¾Plumbing works is required ¾Extra control & Accessories ¾Double cost with respect to DX system ¾Application: ¾Universities , Hospitals , ¾Commercial Centre & Hotel,…etc
Water cooled chiller
Closed area ( basement or very Hot place)
Water cooled chiller
Water cooled chiller system
Hot Air out Fan stack Fan with gearbox, gearbox shaft and motor
Drift eliminator
Spray Nozzle
Spray area Air In
cold water basin
Cooling Tower Components
COMPACT DESIGN SHELL AND TUBE WATER COOLERS
Air cooled-Condensing unit chiller with multi-compressors p for capacity control.
Fan coil units
The Fan coil air flow rate in CFM ranges from 200 200,,300 300,, 400 400,, 600 600,, 800 and and1200 1200. 1200. 3 speeds (L ,M ,H)
D Ducted d
Chiller Equations q
Summer 2004
Heat Transfer equations used in Chiller system The size of the chiller is rated in tonnage or tons of refrigeration, where the historic definition of a ton comes from making one ton of ice in 24 hours:
q W = 500 × Q(gpm) × ∆T (Bth/hr) 1 ton = 12 12,,000 000BtuIh; BtuIh; tons x 12 12,,000 000= = 500 gpm × ∆ T
Tons =
gpm × ∆T gpm × ∆T = 12 000 24 500
This is a good formula for estimating flows versus ∆T. ∆T= T=10 10..8F
In the condenser, condenser, the heat transferred to the condenser water t iincludes l d th the h heatt f from th the E Evaporator, t plus l th the heat h t of compression. For most practical comfort airair-conditioning applications applications, a value of 14 14,,400 Btu/h may be used as the total heat transferred to the condenser water. water.
q W = 500 × Q(gpm) × ∆T (Bth/hr) 14 400 = 500 × Q(gpm) × ∆T 28.8 × Tons gpm = ∆T
Example 1: What chilled water flow will a 100 -ton chiller handle for a 12°F rise in water temperature and a 8°F drop in tower water ? First First, determine the flow rate in the evaporator (chiller flow):
gpm × ∆T ; Tons = 24 Tons × 24 100 Tons × 24 gpm = = = 200 12 F ∆T The flow in the condenser:
28.8 × Tons gpm = ∆T
28.8 × 100 gpm = = 360 8F
Chiller Pump p discharge g & Pipe p sizing We could also using the basic equation:
C . L. = 510 × gpm × (Tin i − Toutt ) C . L. is the cooling load in BTU / hr . gpm is the flowrate or pump disch arg e Tin − Tout is i the th temperatur t t e drop d off water across Fan coil in F , (about 5.5 − 6 C ) o
o
Example
Suppose you want to calculate the required flow rate in gpm for a chiller Fan coil , assuming that, that the required C.L. is 1 Tons Tons--ref and ∆T =10 =10..8 º F, (6 (6 ºC ) Solution: gpm × ∆T Tons =
; 24 Tons × 24 1 Tons × 24 gpm = = = 2.22 ∆T 10.8 F
Or
1 × 12000 = 500 × gpm × 10.8 F The flow Rate = 2.22 gpm The correspond ing pipe size is = 1 / 2" o
F From Ch Chart Black steel pipe is recommended
Closed system
Chill Pump Chiller P mp selection s l ti n Care should be taken when selecting chiller pump :
1- The required q flow rate & head 2- Upp feed or down feed , If Up p feed system y is used ,the shut off head of the pump must be greater than the required head to push water up to highest level, However if down feed system is used remember to include gravity assist flow effect. effect
Example p Suppose pp yyou want to calculate the required q flow rate for a chiller centrifugal pump ,assuming that, the C.L. is 180 Tons and ∆T = 10 10..8 º F. F The D.F=75 D.F= D F=75 F=7575-80 80% % is included in the calculation of the total Cooling Load. S l ti Solution: 180 × 12000 = 500 × gpm × 10.8 F Th Pump The P di h discharge = 400 gpm The corespondi p ngg p pipe p size = o
Typical T pi l Fan F n coil il arrangement n m nt
Fan Coil The Fan coil inlet cold water temperature is about 7 ºC C the outlet 13 ºC C. Whereas the hot water temperature inlet is about 40 ºC. Practically, the temperature change across the Fan coil varies from 5 -7 º C in the summer. Whereas ,in Winter the hot water temperature difference is about 11 11ºC ºC.. Maximum water operating pressure is about 3 Bars.
C’ont C ont Fan Coil Th F The Fan coil il air i flow fl rate in i CFM ranges from f 300,, 400 400,, 600 600,, 800 and and1200 1200.. 200,,300 200 The Fan coil gp gpm is about = 2.4 gp gpm/ton.. Based gpm/ton on temperature difference between in & out 10 ºF, which is about 5.5 ºC.
Chiller Piping Systems Chiller Pipe Systems
Two pipe Systems
Three pipe Systems
Four pipe Systems
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Two--pipe systems Two
Boiler - Off Consists of one supply and one return pipe for either chilled ch lled or hot water supply . The he twotwo-p pipe pe system iss limited l m ted during changeover season. 24
Two--pipe systems Two Chiller - Off
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Three-Pipe p System y
It has three pipes to each terminal unit. These pipes are a cold water supply supply, a hot water supply and a common return. return. The return pipe has a mixture of chilled & hot water during changeover operation,. These systems are rarely used today because they consume excess energy. 26
Four-Pipe System
Four-pipe systems have a cold water supply, cold water Fourreturn, hot h water supply, l and d hot h water return. The Th terminal unit usually has two independent secondary water coils: one served by hot water, water, the other by cold water. water. 27
Comparison The fourfour-pipe system has the following advantages advantages:: 1- Responding quickly to load changes changes.. 2- Operates O t with ith the th summersummer-winter i t changeover changeover. h . 3- Efficiency is greater and operating cost is lower, lower though initial cost is generally higher higher.. 4-The system can be designed with no interconnection of the hot and cold water secondary circuits. circuits.
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Two & Three way -valve
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Types of valves.
Control of water flow through the coil is typically accomplished li h d using i either i h two-way valves l or three-way h valves. l Two-way valves are available in single-seat or double-seat bodies (see Figure 1 a). ) Single seat bodies are most common but require adequate actuator size to overcome water system differential p pressures. Figure 1-a
TwoTwo -way valve
Figure 1-b
ThreeThree -way valve
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Three-way valves, Threevalves, available as a mixing or a diverting pattern, are sometimes i considered id d where h continuous i system fl flow iis desired (see Figure 1-b ). However, this will have increased energy impacts on the system in pumping power and chiller load. Another application is where flow diversion is required, such as condenser flow to or bypassing yp g a cooling g tower. ThreeThreeway control valves used in this manner throttle the flow through the coil from 100% 100% down to minimum and increase flow in i th the bypass b from f minimum i i up to t 100 100% 100%. % %. A balancing valve is provided in the bypass to set a pressure drop equal to the terminal drop when on full bypass bypass. The port controlling the flow through the terminal should be chosen with an equal q percentage p g characteristic and the bypass yp port p selected with a complimentary linear characteristic to maintain nearly constant total flow.. 31
Controlling Water Flow
Figure 1-a
Two way-Valve 32
Figure g 1-b
Three way-Valve w y V
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Figure 1-b
Three way-Valve w y V
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Figure 1-b
Three way-Valve
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Flow coefficient Cv The flow coefficient Cv is defined as the volume flow rate of water at 60 °F, in gallons per minute (gpm), that will flow through the valve at a decrease in pressure of 1 Psi across the valve.
∆P Q = Cv Sg
Where: Q= flow rate gpm ∆P= Pressure drop Psi
Sg = Specific gravity of water =1 Now
Cv =
4300 × d 2 Kv
, where d in ffeet 36
The Flow rate through the control valve
Q = 0.67 × Cv ∆H Where: Q fl Q= flow rate gpm ∆H= Pressure drop ft Note that ;
the control valve size is not necessarily y the same as the pipe pp size, but is based on the control valve Cv and may be one or two sizes smaller than pipe size. The Cv is based on the control manufacturer's valve test data. This Cv rating g may y vary when comparing different manufacturers' valve sizes.
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Primary y –Secondary y pumping p p g system
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Figure (3-a)
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PrimaryPrimary -secondary pumping Controlling water temperature in a primaryprimary-secondary pumping arrangement with a two way valve is another approach to improving valve coil control, as shown figure (3 (3-a )in the previous Figure .This permits constant flow at all times at a variable temperature in the coil circuit, at the design velocity, to maximize coil heat transfer. transfer A common pipe (a(a-b )is connected to both the primary and secondary circuits with no pressure drop. This commoncommon-pipe is usually located in a bridge between the supply and return mains of the primary. The common pipe is selected with "no"nopressure drop" to either the secondary or primary circuits. circuits.
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As the temperature controller calls for an increase in cooling, the two two--way valve opens, opens permitting primary water to mix into the secondary and an equal amount of secondary return water is displaced into the return main. When VV-I is full open, chilled water flows from supply main (a) into the bridge and into the secondary at (b), through the load and returns to the bridge at (c), (c) where it flows through V V--I to the return main. main As valve V-I is throttled by the zone thermostat, less chilled water is supplied to the bridge at (b) and the secondary pump draws the balance of flow from the common from (c) to (b), thereby causing a mixing action. When the zone thermostat is completely satisfied, valve VV-I is closed and all the secondary flow is rere-circulated from (c) to (b). Adding a check valve to the common to prevent the possibility of shortshort-circuit flow in the common as shown in the next Figure . When the check valve closes, this will cause the primary pump to go into series with the secondary. See figure 41 (3-b)
Fi Figure (3-b) (3 b)
42
The primary-secondary concept allows the distribution pumping of the source supply from a central pumping facility. facility (see Figure 4-a) or distributing the pumping to remote buildings or zones of a large facility (see Figure 4-b). The primaryprimary-secondary concept allows continual flow g the source and still permits p twotwo-way y valve control in through the loads. There is flexibility in dedicating a pump to a chiller or boiler or manifolding the pumps. This might simplify i lif the th need df for h having i backup b k pumps for f every system. t Pumps, manifolds, accessories and associated pumping control may be assembled to match installation constraints, constraints or can be factory prepackaged as an assembly for a designated g mounting g location. The designer g must weigh g the pros and cons of cost, flexibility and installation requirements of the various concepts to determine the best arrangementt . 43
Fi Figure (4-a) (4 )
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Figure (4 (4-b) b) 45
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Remarks Reverse return systems are closer to a natural balance of flows than direct return systems. If automatic control valves are employed, the design pressure drop selected should be as high as practical. practical A pressure drop at least equal to the drop in the terminal unit coil is a desirable goal. The valve should be sized for the design flow with the Cv flow formula, which may not be the same size i as the h coil il inlet i l piping. i i Centrifugal pumps with flat characteristics should be selected for systems y with control valves. TwoTwo -way valves should be considered over threethree-way valves because they vary the volume of water flow in direct relationship with the control signal. Three--w Three way y valves provide p a continuous n nu u f flow w regardless g of f the load and n are not suitable With variable volume pumping systems. Manual balancing valves should be chosen for a minimal pressure drop and provide the means to measure flows in various loops in the field as well as provide a shutoff valve for coil servicing. Performance is best assured by requiring proportional balancing after the system is operating. operating Variable volume pumping systems should be checked and adjusted for balance at 50%, 50%, 75 75% % and 100%design 100%design flows. 47
Chiller Pumps &
Pump’ss selection Pump
Centrifugal Pump Basics “Closed” System Balancing Valve
Isolating 3-Way Control Valve Valve LOAD
LOAD
LOAD
BUILDING HEIGHT
LOAD
EXPANSION
SOURCE
Centrifugal Pump Basics “Closed” System Resistance Curve
System Resistance Curve Friction Losses
CAPACITY
Centrifugal Pump Basics “Open” System Static Head
Static Suction Head
CONDENSER
Centrifugal Pump Basics “Open” System Resistance Curve
Friction Losses
HEA AD
System Head Curve
Total Static Head
CAPACITY
Centrifugal Pump Basics “Closed” System with Minimum Maintained Pressure BALANCING VALVE
2 WAY CONTROL ISOLATING VALVE VALVE
LOAD
LOAD
LOAD
BUILDING HEIGHT
LOAD
EXPANSION
SOURCE
Centrifugal Pump Basics “Closed” System Resistance Curve with Minimum Maintained Pressure
Friction Losses
HEAD D
System Head Curve
Minimum Maintained Pressure
CAPACITY
Pumps in parallel & series
Refer to the “Fundamental of pumps” by Dr. Hammoud
Each pump operates @ this hi point i Both pumpspumps-ON
System S t operating ti point Both pumpspumps-ON
Single pump Operating point
Parallel pump operation
System Operating Point Both pump -On
Single pump operating ti Point One pump is -ON
Each pump operates @ this point B th pump -On Both O
Pumps in parallel & series
What happens to Flow, Head and Power with Speed? So: Fl Flow changes h DIRECTLY (linear) (li ) with ith RPM RPM… Head changes as a SQUARE of RPM… Power is proportional to Flow times Head – it changes as CUBE of RPM…
Q ~ RPM H ~ RPM2 SP ~ RPM3
Affinity laws (For the same pump)
Affinity y laws Doubling D u g the pump rotational speed p leads to: 1- Double the discharge. discharge 2- Increase the total head value by a f t of factor f 4. 3- Increase the power by a factor of 8.
Two –Speed pumping Multiple-speed motors can be used to reduce system over over-pressure at reduced flow flow. In this example standard two-speed motors are available in models with speeds of 1750/1150 rpm, 1750/850 rpm, 1150/850rpm and 3500/1750 rpm. The corresponding figure shows h the h performance f of f a system with ha 1750/1150 rpm multiple speed pump. In the figure, curve A shows the system's response when h the th pump runs att 1750rpm. 1750 Wh the When th pump runs at 1150rpm, operation is at point 1 and not at point 2 . If the system was designed to operate as shown in curve B, the pump would operate at or above the shut-off , it will be damaged if it runs at 1150 rpm rpm. For that reason the designer must analyze the system carefully to determine the pump's limitations and the effect of lower speed on performance. performance
Two multimulti-speed pumps in parallel
It can Replace the use of Multi Multi--speed ((3 3 speeds) manual adjusted j circulated pump p p by y VFD operation in heating system. (heating (heating system circulated p pump) p))
He
1st duty d t point i t
H
2nd duty point H1
3nd duty point Q1
Radiators OnOn-off !!! Refer to the “VFD ” by Dr. Hammoud
Q
Speed reduction
Pump’s Shaft power
Pump & System curves
Application pp of chiller pumps p p
4 pipe systems
Down - F feed d pumping D Down Feed pumpsystem system
Operating point Hstatic t ti = Negative N ti
Fan Coil
Shut off head
Shut off head O ti point Operating it
H
Q
Hstatic i = zero
Up--feed pumping UUp Up F Feed d pumpsystem system
Standby Stan y pump It is always l s good d pr practice ctic tto c consider nsid r a b back ck up pump of f equal qu l capacity and proper valves to permit operation when the normal pump is inoperable. Usually this is an application for a parallel pump. Failure F l can occur in extremely l cold ld weather h for f heating h or in the h middle of a hot spell during the cooling season, and the original investment cost of a by y pass p will be trivial compared p to the inconvenience for the building occupants or the operator. Depending on the system curve and the pump curve and how many pumps are in n the full system, a standby pump can provide prov de up to 80 80% % of design flow.
Types of Pumps needed
Pump performance must be considered not only at the design point but across its entire characteristic curve. Centrifugal pumps are available with steep curves that drop from high head at low flow to low head at high flow versus those with flat curves that show a small change in head between shutoff to design flow
Some designers like to limit this to a 15% to 25% rise-to-shut off curve. These flat curve pumps are always recommended where two-way valves are applied to unit terminals. At part-loads, the valves will be operating at lower flows and this will move the system operating differential pressure up the pump curve.
Pump’ p s selection When selecting a pump for chiller application, the follow following ng quest questions ons & factors should be considered:
Pump’s selection
1-What is the nature of the liquid to be pumped? (Water or antifreeze liquids , cold or hot etc.. ). 2-What is the required volume flow rate (discharge)? What is the diversity factor?, what is the maximum and minimum amount of liquid q to be pumped? q from the p pump p and the 3- What is the head required pipe system curve ?. For circulating pump the head required from the pump is to overcome the total h d loss head l only. l 4-What are the conditions on the suction (inlet) side of f the th pump and d on the th discharge di h (outlet) ( tl t) side id of f the pump? check the NPSH. That is to say “Cavitation Cavitation must be avoided avoided”.
5- Check the specific speed. Read the corresponding catalogue Select the range at which your pump may exist catalogue. exist. 6-Flat curve pumps rather than steep curve pumps is selected 7-Plot ot the th pipe p p syst system m curve cur on each ach selected s ct pump characteristic curve and then compare the head discharge relationship, efficiency and power of the d ff different pumps (f (from catalogues). l ) The h pump, which h h is operating at or near the point of B.E.P, ( maximum efficiency)) should be selected. efficiency selected Afterwards, Afterwards you should be able to identify one or two pumps that are suitable. g cost of f the p pump p i.e. 8- Choose the lowest initial and running (power requirements). 9- What is the type of power source (electric motor, diesel engine, i etc.)? ) 10-- Check the space, weight, and location ( indoors or outdoors 10 ) of the pump. pump 11-- Refer to the governing codes and standards. 11
Remember this Single pump - Selected for a simple application. application Single pump with trimmed impeller – Optimizing pump capacity for a specific application. . Single pump with backup pumppump- In addition to a selected application application, provides I00 I00% % backup. backup TwoTwo -speed pump - Provides limited variable flow steps with an added investment. P Parallel ll l pumpspumps- Flexible Fl ibl capacity i controll without i h iincreasing i system head; good for two two--way valve control. Series pumps - Steep p head change g with limited flow change; g twotwo-way y valves l would ld require high h h differential d ff l pressure operation and capability. Primary-secondary Primary secondary pumping pumping- Flexible zoning approach approach, with minimum pumping energy. Distributed pumps - Special application of primaryprimary-secondary pumping. pumping Variable speed pumpspumps- Applied to pumping systems to reduce power by p y lowering g pump p p speed p to meet control differential pressure in selected l d locations; l usually ll applied l d to parallel ll l pumping distribution systems employing primaryprimary-secondary two-way control valves. or distributed pumping, with two-
End of the lecture
