de junio de 2017 Descripción General

Figure 4-33 shows a CHW plant with primary-only pumping. Conceptually the design is fine, but the designer of this plant unnecessarily increased first costs.

First, reverse-return piping was provided for both condenser water and CHW systems. Reverse return, where the first device supplied is the last returned, is a pip- ing scheme intended to self-balance hydronic systems but, more importantly, to keep the pressure drop across modulating two-way control valves relatively low and equal among all valves as they open and close with changing loads (see the Balancing Variable-Flow Systems section). However, in this case, the valves at each chiller are essentially two-position and the difference in pressure across each valve is very small even if the system is direct return (first supplied, first returned) and was not manually balanced. A balanced direct-return piping system will result in the same flow rates across each chiller as the reverse-return design, regardless of how many chillers are enabled. So, there is no value in performance to reverse return in this application, yet it substantially increases first costs.

Another often expensive design concept is to group pumps together and pipe all the pumps first to a larger common pipe before distributing the supply water to the chillers. In this example, the common pipe size is 12 in. on the CHW side and 14 in. on the condenser water side. It is not uncommon to see all the pumps squished into a corner of the chiller room. There is little synergy to grouping pumps next to each other, and doing so can increase first costs and reduce space around the pumps for maintenance.

Figure 4-33 Expensive CHW plant.

Figure 4-34 shows the same plant as Figure 4-33 but instead of reverse-return and grouped pumps, the pumps are aligned with chillers and are piped into a common header on the discharge side of the pump and also on the discharge side of the condensers. (The discharge side of the evaporators is the same as that in Figure 4-33.) This design still allows any pump to serve any chiller, but it short- ens piping runs and it reduces pipe sizes because there is no common pipe that serves the total system flow. On the condenser water side, all pipes are 10 in.; the 14 in. common pipe is eliminated. Similarly, the 12 in. CHW piping at the dis- charge of the pump to the evaporators is eliminated; all pipes are 8 in. First costs are substantially reduced with no impact on performance.

This layout also usually reduces the footprint of the plant, reducing the floor area required for the chiller room. In fact, locating the equipment as close together as possible is key to the first-cost savings. Figure 4-35 shows a

Figure 4-34 Less expensive CHW plant.

floor plan of the plant shown schematically in Figure 4-34. The CHW and condenser water pumps align with the chillers but are offset to provide tube pull space and piping connections between the pumps. This is a very compact layout, but it still provides adequate maintenance access for all equipment.

References

ASHRAE. 2016. ANSI/ASHRAE/IES Standard 90.1-2013, Energy standard

for buildings except low-rise residential buildings. Atlanta: ASHRAE.

CBSC. 2016. 2016 California building standards code. California Code of Regula- tions, Title 24. Sacramento, CA: California Building Standards Commission.

Figure 4-35

CHW plant floor plan.

Taylor, S. 2002. Degrading chilled water plant Delta-T: Causes and mitigation.

ASHRAE Transactions 108(1).

Taylor, S., and J. Stein. 2002. Balancing variable-flow hydronic systems.

ASHRAE Journal 10.

Taylor, S. 2011a. Optimizing design & control of chilled water plants: Part 1: Chilled water distribution system selection. ASHRAE Journal 6:14–25. Taylor, S. 2011b. Optimizing design & control of chilled water plants: Part 2:

Skill Development Exercises for Chapter 4

4-1 Aggressive CHW temperature reset

a. Causes issues with thermal comfort because higher supply water tem- perature yields significantly higher supply air moisture content for a given set of entering air conditions and thus higher space humidity. b. Typically increases pump energy usage significantly enough to offset

the benefit of the decrease in chiller energy use. c. Has little to no impact on space humidity control. d. Both (a) and (b).

4-2 A plant consists of two identical fixed-speed centrifugal chillers, each with a dedicated constant-speed primary CHW pump. The chillers supply chilled water to one large built-up air handler that primarily serves daytime commer- cial office space loads and another large air handler that serves an auditorium space most frequently occupied in the evening. Both air handlers have three- way CHW control valves and are of approximately equal size. Which of the following are true?

i. The design will require two chillers to operate when the auditorium air handler is at full load, even if the office air handler is off.

ii. The plant will operate least efficiently in the rare instances that both the office air handler and auditorium air handler are at near design load.

iii. The plant will operate most efficiently in the rare instances that both the office air handler and auditorium air handler are at near design load.

iv. Headering the primary pumps would increase the controls complex- ity with no benefit in system redundancy.

a. (i), (iii), (iv) b. (i), (iii) c. (ii), (iv) d. (i), (ii)

4-3 For three-way valve systems

a. The flow rate through the branch serving the coil is constant, irrespec- tive of valve position.

b. The flow rate through the branch serving the coil peaks when the valve is fully open to the coil.

c. The flow rate through the branch serving the coil peaks when the valve is 50% open.

4-4 The back-loaded position of the common leg shown below

a. Causes one chiller to be almost fully loaded with the remainder of the load handled by the other chiller.

b. Causes unbalanced flow through the two chillers.

c. Results in the same energy performance as a system with a common leg located in the normal position just upstream of the secondary pumps. d. Is a reasonable location for most plants if it is less expensive due to the

physical layout of the plant.

4-5 Construction of a variable primary CHW plant is just about complete when it is discovered that the design includes only two-way valves and no means to maintain minimum flow. Which of the following last-minute design change options will resolve the problem at minimum cost?

a. Install a CHW bypass locally at the CHW plant. Measure DP across the chillers to indirectly measure flow.

b. Install a CHW bypass locally at the CHW plant. Install a flowmeter in the main return line at the plant to measure flow.

c. Install a CHW bypass at the end of the line. Install a flowmeter in the main return line at the plant to measure flow.

d. Install enough three-way valves at end-of-line coils to maintain mini- mum flow.

4-6 True or false?

Both headered and dedicated pump per chiller configurations are equally appropriate for plants requiring a standby pump.

a. True b. False

4-7 Which balancing method is most appropriate for all but very large distribution systems?

a. Manual balancing using CBVs to measure and adjust flow. b. No balancing.

c. AFLVs at all coils. d. Reverse-return piping.

4-8 True or False: Air-side economizing systems can contribute to low CHW T

issues in systems with highT designs. a. True

b. False

4-9 What factors constrain the number of cooling towers that can be operated with a given number of constant-speed condenser water pumps enabled?

a. Minimum per-tower flow requirements. b. Maximum per-tower flow requirements. c. Neither (a) nor (b).

d. Both (a) and (b).

4-10 Isolating cooling towers by means of isolation valves on the tower supply pip- ing only

a. Requires that the equalizer be oversized. b. Does not require the equalizer to be oversized.

c. Requires that all towers be operated whenever the plant is enabled. The isolation valves are only installed to prevent tower overflow upon plant shutdown.

d. Is not a viable design option.

4-11 True or false?

Most modern centrifugal chillers can operate with an integrated WSE without any means of head pressure control.

a. True b. False

Instructions

Read the material in Chapter 5. Verify the examples presented in the chapter with your own calculations. At the end of the chapter, complete the Skill Development Exercises without referring to the text. Review those sections of the chapter as needed to complete the exercises.

Introduction

Previous chapters discussed the basic principles behind central CHW plant components and distribution system design. This chapter provides procedures and analysis techniques for optimizing the design to minimize first costs and operating costs (in particular, energy costs) over the plant’s life cycle. This chapter primarily applies to new CHW plants, but many of the techniques can be used for retrofit projects as well.

To rigorously optimize a central plant design would be a Herculean task due to the almost infinite number of design decisions that affect energy costs and first costs. For instance, energy costs are determined by the

• full-load and part-load/part-lift efficiency of each piece of plant equipment (e.g., chillers, towers, pumps),

• quantity and staging of each type of equipment,

• design of the distribution system (e.g., variable flow versus constant flow, primary only versus primary/secondary),

• control sequences,

• pipe and valve sizing, • flow rate sizing,

• and more!

First costs can be even more complex to account for during initial design. There are many reasons for this, including the fact that

• costs are not a continuous function of capacity,

• capacity for some equipment and materials is only available in discrete sizes, • costs vary by manufacturer and by market conditions, and

• costs vary widely depending on the physical layout of the plant and other design details.

Rather than trying to account for every design variable, this chapter sug- gests a chiller plant design approach that combines detailed analysis and rule- of-thumb recommendations. This approach provides much better results in terms of plant performance and cost compared to traditional design procedures with little or no more effort.