ASHRAE Journal - September 2011 - 27

A
Cooling Tower No. 1 Cooling Tower No. 2 Cooling Tower No. 3

B
Cooling Tower No. 1 Cooling Tower No. 2 Cooling Tower No. 3

C
Cooling Tower No. 1 Cooling Tower No. 2 Cooling Tower No. 3

Chiller No. 1 Chiller No. 1 CHW Pump No. 1 Chiller No. 2 CHW Pump No. 2 Chiller No. 3 CHW Pump No. 3 Optional Standby Pump CHW Pump No. 3 CHW Pump No. 2 Chiller No. 3 CHW Pump No. 1 Chiller No. 2 CHW Pump No. 1 CHW Pump No. 2 CHW Pump No. 3 Chiller No. 1 N.C. Chiller No. 2 N.C. Chiller No. 3

Figure 1: Condenser water pump piping options. Option A (left): Dedicated pumps. Option B (center): Headered pumps with condenser auto-isolation valves. Option C (right): Headered pumps with manual isolation valves. 1. The pump can be custom-selected for the condenser it serves. Pump selection can then account for variations in condenser pressure drop and flow rates when chillers are not identical. This can reduce pump energy compared to Option B where the head of each pump must be the same and sized for the condenser with the highest pressure drop; balance valves at the other condensers must be throttled to generate this same pressure drop. 2. Controls are a bit simpler because the pump can be controlled using the contact provided with the chiller controller. This ensures that the pump starts and stops when the chiller wants it to. With Option B, the control of the isolation valves and pumps is by the direct digital control (DDC) system and must be coordinated with the needs of the chiller controller to avoid nuisance trips. For instance, the pumps generally must run for several minutes after the command for the chiller to stop so that the chiller can pump down the refrigerant. 3. Pump failures do not cause multiple chiller trips. With dedicated pumps, if a pump fails, only the chiller it serves will see a flow disruption and trip. With Option B, all operating chillers will see a flow reduction when a pump fails, possibly causing more than one chiller to trip due to low flow or high refrigerant head. However if there is a lag or standby pump with Option B that can be started quickly, trips can usually be avoided because it takes some time for refrigerant head to rise. The advantages of headered (manifolded) pumps (Option B) include: 1. Redundancy is improved. With Option A, if a pump fails and a chiller other than the one it serves also fails (albeit a rare event), then two chillers will be inoperative. With Option B, any pump can serve any chiller and under many conditions one pump can provide enough flow for two chillers to operate near full capacity.
September 2011

2. Including a standby pump is much simpler. Adding a standby pump to Option A is cumbersome and expensive because it requires extensive piping and manual or automatic isolation valves. If standby pumps are desired, Option B is the best option. 3. Isolation valves can double as head pressure control valves. See discussion on head pressure control later. For Option A, head pressure control would require the addition of variable speed drives on condenser water pumps or tower bypass valves. 4. It is easier to integrate a water-side economizer. See discussion on waterside economizers below. Since waterside economizers are only operational in cold weather when loads are generally low, the condenser water side can use one (or more) of the condenser water pumps serving chillers rather than providing a dedicated pump. This reduces first costs. Headered pumps with manual isolation valves (Option C) can have the advantages of Option A (although it works best with identical chillers) and it overcomes the redundancy disadvantage of Option A but accommodating a pump failure requires manual manipulation of valves vs. the automatic response in Option B. Including a standby pump is possible with Option C but it only works (depending on which pump fails) with the header isolation valves open and chillers must be staged by manually opening and closing their isolation valves. First costs are usually lowest with Option A if the chiller and pump pairs are close-coupled and the manual isolation valves between the two are eliminated (each chiller-pump pair is isolated for service as a pair). Option C is usually less expensive than Option B, but Option B is usually the best choice where head pressure control and standby pumps are required.

Refrigerant Head Pressure Control
All chillers will require a minimum refrigerant head (lift) between the evaporator and condenser. This can be quite high
ASHRAE Journal 27

ASHRAE Journal - September 2011

Table of Contents for the Digital Edition of ASHRAE Journal - September 2011