Condenser Water System Savings

Rather than relegating condenser water system design to standards developed 50 years ago, consider taking a second look. Recent developments, such as increased chiller efficiency, the escalating cost of materials, and the increased use of variable-speed drives on pumps and cooling tower fans provide significant opportunities to the owner and designer of condenser water systems.

Taking advantage of these opportunities requires determining the optimum condenser water flow rate. Present industry guidance has shifted from the “standard” three gpm/ton flow rate (and oft-assumed 10F ΔT) to lower flow rates and larger ΔTs as evidenced below.

• From the ASHRAE GreenGuide:¹ “The CoolTools Chilled Water Plant Design and Performance Specification Guide recommends a design method that starts with a condenser water temperature difference of 12F to 18F (7C to 10C).” 

• Kelly and Chan² state, “In most cases, larger ΔTs and the associated lower flow rates will not only save installation cost but will usually save energy over the course of the year.” 

• Taylor³ states, “Calculate the condenser water flow rate for all pipe sections assuming a range of 15F (8C).”

From a design parameters perspective, reducing condenser water flow rates by increasing the design temperature difference is advantageous since it reduces both installed and operating costs. In addition, using variable-speed drives on cooling tower fan motors is beneficial during times when the heat rejection load is lower.

SYSTEM DESIGN

Why do ASHRAE, Kelly and Chan, and Taylor recommend flow rates that are lower than the AHRI standard rating conditions?

To answer that question, let’s examine the relationship between the condenser water flow rate and other system variables (see HPAC April 2013 for a brief overview). We note that as the condenser water flow rate decreases:

• The chiller leaving condenser water temperature and chiller power rise. 

• The condenser water pump power drops.

• If the same cooling tower is used, its approach temperature decreases.

• If the pipe size is reduced and some of the pipe cost savings is used to oversize the cooling tower, the tower fan power can also be reduced.

As an example, let’s illustrate these effects by looking at a 700-ton chilled-water system using different condenser design parameters (as shown in Table 1).

Due to the reduction in pump and tower fan energy, system power is reduced at design conditions. Sometimes if the condenser system pressure drop is very low (< 20 feet of head) the design “GreenGuide” system power may be similar to a system designed at the AHRI standard rating conditions. How do the energy and life cycle costs compare? Taylor’s analysis concentrates on life cycle costs and shows that reducing the condenser water flow rate (increasing the ΔT) reduces life cycle costs. In his summary he states: … life-cycle costs were minimized at the largest of the three ΔTs analyzed, about 15F (8.3C). This was true for both office buildings and data centers and for both single-stage centrifugal chillers and two-stage centrifugal chillers. It was also true for low, medium and high approach cooling towers. 

SUMMARY

The information from the ASHRAE GreenGuide, Kelly and Chan, and Taylor all show that project teams should design condenser water systems nearer a 15F ΔT (1.9 gpm/ton). Reducing condenser water flow rates allows efficient system design, can reduce system first cost due to reduced condenser water pipe, pump, and cooling tower size, and reduces life cycle costs.

SYSTEM OPERATION

Now we can examine the control of the condenser water system – specifically, cooling tower fan and condenser water pump speed operation.

For a better understanding of the options, let’s examine two control modes:

•  Mode 1 varies cooling tower fan speed only

•  Mode 2 varies both cooling tower fan and condenser water pump speeds 

Mode 1: Cooling tower fan speed control

Many projects today use a fixed setpoint to control the cooling tower leaving water temperature. As the heat rejection load and/or wet bulb temperature drop, the tower fan speed is reduced to maintain the setpoint. The result is a reduction in cooling tower fan power. A number of parties* have found that “near-optimal” control can reduce the sum of chiller-plus-tower energy consumption.

The premise of finding a control point that minimizes the sum of chiller-plus-tower energy consumption can most simply be demonstrated by examining a point in time. Figure 1 shows chiller and cooling tower fan performance at a point in time during the year when the chiller load is 40 per cent and the outdoor air wet bulb temperature is 65F. 

If the tower fan operates at full speed, it can produce 69.5F leaving water temperature. The chiller power is lowest, but the tower power is high.

The optimal system setpoint occurs at a tower leaving temperature of 75F and 60 per cent fan speed. The chiller power rises. At these operating conditions, the chiller-plus-tower fan power is reduced by 8.7 per cent. 

Some assert that a system with a variable-speed chiller may benefit from operating the tower fan at full speed all the time. While the optimal tower fan speed is higher than for the constant speed chiller, running the cooling tower fan at full speed is not optimal. Figure 2 shows that for a system with  a variable-speed-drive chiller, the same conditions (40 per cent load and 65F) result in optimal control at ~71F and a tower fan speed of 70 per cent, saving 7.5 per cent of chiller-plus-tower power. Note this speed is higher than for the constant-speed chiller but still not at full speed.

This is one point in time. How much can be saved over the course of a year? In their ASHRAE Journal article, Crowther and Furlong¹⁰ found that for the system analyzed, optimal tower fan speed control saved 6.2 per cent in Chicago, 4.7 per cent in Las Vegas, and 8.5 per cent in Miami. In a study performed for this article, a 720 000 ft² hotel was analyzed in a number of global locations (see Figure 3) using the following possible control setpoints:

• Fixed tower setpoint at design tower leaving temperature (85F in humid climates, 80F in dry climates)

•  Near optimal setpoint

•  Fixed tower setpoint at 55F

When compared to making the tower water as cold as possible, near optimal control savings ranged from just under two per cent in Paris to 14 per cent in Toronto. Although in a relatively dry climate with a short cooling season (e.g., Paris) the savings are small, it is clear that chiller-tower near optimal control saves energy and operating cost in all locations. This control is available from at least three control providers; therefore highly consider specifying it on new projects and implementing it on retrofit applications. 

Mode 2: Variable-speed condenser water pump and cooling tower fans

Now that we understand near-optimal cooling tower fan speed control, let’s add the variable of additionally changing condenser water pump speed.

Recently, a few parties have examined variable-speed drives on both cooling tower fans and condenser water pumps:

• Taylor provides a methodology that can be customized for each specific chilled-water system. It requires extensive modeling for each system.

• Hartman¹¹ reveals only concepts with few details that allow project teams to implement such control themselves.

• Baker, Roe and Schwedler¹² provide a simple method for controlling condenser water pump speed and cooling tower fan speed, but the method may not
be optimal for all chilled-water plants or at all conditions.

None of the methods presently available are simple, understandable, all-inclusive, and straightforward at this time. So what are the issues with varying both condenser water pump and cooling tower fan speed?

• There are limitations to minimum condenser water flow rate.

• Changing condenser water flow rate affects performance of the cooling tower, condenser water pump, and chiller.

• The control method is not easily understandable.

Let’s examine each of these issues.

FLOW RATE

First, there are limitations to how far condenser water flow can be reduced. The minimum condenser water flow for a specific application is the highest of:

• The minimum flow rate allowed by the tower provider to maintain proper distribution over the fill. Proper distribution keeps tower surfaces wetted, heat transfer at good rates and avoids scaling.

• The minimum condenser flow rate allowed by the chiller provider to keep heat transfer in an acceptable range.

• The minimum pump speed required to produce the tower static lift.

Component performance

Much changes when the condenser water flow is reduced. As previously mentioned:

• Pump power goes down.

• Chiller power rises (as flow rate goes down, leaving condenser water temperature rises).

• Initially, cooling tower heat exchange effectiveness gets better, since the cooling tower receives warmer water. However, as flow is reduced further, heat exchange effectiveness is reduced. This may occur even above the minimum flow rate allowed by the cooling tower manufacturer.

CONTROL METHOD

Finally, the optimal interaction of the cooling tower, condenser water pump, and chiller is not simple to determine since optimal control changes at all system loads, operating combinations, and outdoor air wet bulb temperatures. In addition, slowing condenser water pump and cooling tower fan speeds too much when the chiller is heavily loaded and the wet bulb temperature is high will cause a centrifugal chiller to surge.

An example:

To provide a high-level understanding of the trends, system performance is shown for a 700-ton system designed at:

• The AHRI standard rating conditions of three gpm/ton design condenser water flow rate. (column 1, Table 1)

• The ASHRAE GreenGuide recommended conditions; for this example, two gpm/ton condenser water flow rate was chosen. In addition, this system was designed using an oversized cooling tower to reduce design cooling tower fan power by 50 per cent. (column 3, Table 1)

Figures 4 through 15 depict various chiller loads and outdoor wet bulb temperatures. The black dot indicates the minimum chiller + condenser water pump + cooling tower fan power for each figure.

Trends are noted in Table 2.

General observations of these operating choices:

AHRI Standard conditions (three gpm/ton)

• Not surprisingly, when design condenser water pump power and cooling tower fan power are high, there are significant system energy savings (four to 27 per cent) available at all part load and reduced wet bulb conditions.

• Reducing condenser water flow rate is beneficial at all the conditions examined.

• Reducing cooling tower fan speed is beneficial at 70 per cent chiller load and lower – and necessary to reduce system energy use at chiller loads of 50 per cent or lower.

• At most operating conditions, optimal system control can reach that of a system designed at a lower condenser
water flow rate.

• With that said, below 50 per cent chiller load, the spread between the minimum and maximum system power (per cent max savings) is large. Proper control at these loads is imperative when the system design condenser water flow rate is high (three gpm/ton). Therefore energy-saving retrofit control opportunities are available on condenser water systems with design flow rates of three gpm/ton.

GreenGuide conditions (two gpm/ton was used)

• Since pump and tower fan power are lower at design
conditions, there is less advantage to optimizing the off-design control.

• Reducing condenser water pump speed is detrimental at high wet bulb conditions.

• Reducing condenser water pump speed is beneficial at reduced ambient wet bulb conditions.

• Reducing tower fan speed has little or no benefit until chiller load is less than 50 per cent.

Overall conclusions

• On existing systems designed at AHRI conditions, reducing condenser water pump speed and cooling tower fan speed offers significant savings.

• Designing new systems at the AHRI standard flow rate significantly increases the risk of control decisions resulting in inefficient system operation – at all load and wet bulb conditions.

• Control is project, load, and ambient condition dependent.

• On new projects, designing to the ASHRAE GreenGuide conditions (12-18F temperature differences resulting in 2.3 to 1.6 gpm/ton) and oversizing the cooling tower to reduce fan power offers savings at all conditions and savings are less dependent on coordinating system control-especially at lower load conditions.

Simply put, designing condenser water systems at flow rates of 1.6 to 2.3 gpm/ton results in reduced installed costs as well as a much higher probability that the system will operate efficiently – no matter how the condenser water pump and cooling tower fan are controlled. 

Summary

1. Use the ASHRAE GreenGuide guidance of 12-18F ΔT for condenser water systems (2.3 – 1.6 gpm/ton) to reduce plant installed and life cycle costs.

2. Consider varying cooling tower fan speeds on all installations.

3. Consider varying condenser water pump and cooling tower fan speeds on systems not designed using the ASHRAE GreenGuide guidance, and where the plant operators are on board, trained, and retrained when the operators change. When used, keep the control method understandable, transparent, and as simple as possible (but not any simpler). <> 

REFERENCES

1. American Society of Heating, Refrigerating and Air-Conditioning Engineers. 2010. ASHRAE GreenGuide: The Design, Construction, and Operation of Sustainable Buildings, 3rd ed. Atlanta, GA: ASHRAE.

2. Kelly, D. and T. Chan. 1999. “Optimizing Chilled Water Plants.” Heating/Piping/Air Conditioning (HPAC) Engineering. 71(1).

3. Taylor, S. 2011. “Optimizing Design & Control of Chilled Water Plants; Part 3: Pipe Sizing and Optimizing ΔT” ASHRAE Journal. 53(12):22-34.

4. Braun, J.E., and G.T. Diderrich. 1990. “Near-Optimal Control of Cooling Towers for Chilled Water Systems.” ASHRAE Transactions. (Volume 96, part 2, paper number SL-90-13-3): 806-813.

5. Cascia, M. 2000. “Implementation of a Near-Optimal Global Set Point Control Method in a DDC Controller.” ASHRAE Winter Meeting Transactions: 249-263.

6. Crowther, H., and J. Furlong. 2004. “Optimizing Chillers and Towers.” ASHRAE Journal. 46(7): 34-40.

7. Hydeman, M., K. Gillespie, and R. Kammerud. 1997. National Cool-Sense Forum. Pacific Gas & Electric (PG&E).

8. See note 2.

9. Schwedler, M. 1998. “Take It to the Limit…or Just Halfway?” ASHRAE Journal. 40(7): 32-39.

10. See note 6.

11. Hartman, T. “Direct Network Connection of Variable-Speed Drives.” HPAC Engin
eering (March 2003): 22-32. Available at <http://www.hpac.com/member/archive/pdf/2003/0303/hartman.pdf>

12. Baker, M., D. Roe, and M. Schwedler. 2006. “Prescription for Chiller Plants.” ASHRAE Journal 48(6): H4-H10.

See Condenser Water System Components in HPAC April 2013.

What about the operator?

All of the previous analysis is predicated on optimal control working properly and being allowed to continue to work without “manual override.” Often the chilled-water system operator wants to understand system operation so he or she can change that operation when necessary. For example, they may ask, “What is the cooling tower setpoint?” When condenser water pump and cooling tower fan speeds are varied, there is no cooling tower setpoint-since the leaving cooling tower temperature is a result of the system operation decisions. This can be unsettling to some system operators. It is imperative that the system operator be “on board” with the control methods, understand them, and have the ability to both monitor and change them if necessary.

Cooling tower cells

A cooling tower cell consists of the structure, media, and fan. It should be noted that it is more efficient to operate multiple tower cells at part speed than one tower cell at full speed. For example, one cell operating at full speed (40 hp) and the other off gives about 58 per cent of the tower’s capacity. Two cells with fans each operating at 60 per cent – a total of 20 hp-gives 60 per cent of the tower’s capacity.