Central Plant Versus Local Plant in a Building
There is no rule about when a central plant is the right answer or when distributed packages or systems should be used. Circumstances differ from project to project, and location to location. The good designer will assess each project on the merits of that situation and involve the client in making the most suitable choice for the project.
In this section we are going to consider, in a general way, some of the technical issues that can influence the choice. We are not going to consider the internal politics that can have major influences and costs in time and money. In addition to politics, the availability of money for installation versus operating costs can have a major impact on system choices. For minimum installation-cost, the pack- age approach usually wins.
Here are some true statements in favor of central plants. Read them. Can you think of a reason why each one of them might, in some circumstances, be wrong, or irrelevant? Write down your suggested reason.
“It is easy to have someone watching the plant if it is all in one place.”
“The large central plant equipment is always much more efficient than small local plant.”
“The endless cost of local plant replacement makes it uneconomic compared to a main central plant.”
It is alright if you did not think of reasons, but do be aware that technology has changed over the last half century and you should think about whether categorical statements or “rules-of-thumb” are correct or relevant in your particular situation. You cannot go against the laws of physics, but there are many more ways of doing things than there were.
Let us consider each of the above statements in turn.
“It is easy to have someone watching the plant if it is all in one place.”
This statement is true if visual inspection of the plant is useful. A hundred years ago, the look and sound of the plant were the best, and only, indicators of performance. Operators “knew their plant” and almost intuitively knew when things needed attention. Now, in the 21st century, plant is much more complex and we have excellent monitoring equipment available at a reasonable price. The information from those monitors can be instantly, and remotely, available. So instead of paying someone to physically watch the central plant, the building owner can pay someone to monitor the performance of, not just the central plant, but all the plant, regardless of where it is located in the buildings. Now, using the internet, many buildings can be monitored from anywhere in the world with fast and reliable internet service.
The second statement, “The large central plant equipment is always more efficient than small local plant,” is generally true but not always relevant. For example, an apartment building might have a large central boiler that provides both hot water for heating, and domestic hot water. In winter this is an efficient system. However throughout the summer the boiler will be running sporadically at very low load. It will take a considerable amount of energy to heat up the boiler before it starts to heat the domestic water, and this heat will dissipate to atmosphere before it is called on to heat the water again—very inefficient. The unit has a high efficiency at full load but when its efficiency is averaged over the year, “seasonal efficiency,” may be surprisingly low.
In this situation, it may be beneficial to install a series of small hot-water heaters for the domestic hot water, although they are not as efficient as the main boiler at full load. Their advantage is that they only run when needed and have low standby losses.
The last statement “The endless cost of local plant replacement makes it uneconomic compared to a main central plant.” is also true in some cases, but definitely not in other cases. In many organizations, replacement of smaller pieces of equipment are paid for as part of the maintenance operations’ budget. On the other hand, major plant replacements are paid for out of a separate ‘capital’ fund. From the point-of-view of the maintenance managers, small, local plant is an endless expense to their maintenance budget, while other budgets fund large, central- plant replacements from the capital account. When it comes to new facilities, the maintenance managers in this situation are likely to be biased against small, packaged-plant equipment, because its replacement costs will all fall on their maintenance budget.
Let us go back to the reasons you wrote down as to why these three statements about central plant might be wrong. Are you still comfortable with them and can you think of others?
This section has deliberately been encouraging you to think about the some of the pros and cons of central plants. Now let us consider three other advantages.
1. “It is so much easier to maintain a high standard of operation and maintenance of a few large units in a single place, instead of lots of little packages all over the site.”
Plant operators know that having complete information about the plant, all the tools in one place, space to work, and protection from the weather, all make central plant maintenance very attractive.
2. “Trying to optimize many package units is really difficult compared to the two identical chillers and boilers in our central plant.”
A few central pieces of equipment can be monitored relatively easily and adjusted by the maintenance staff. When there are many units all over the building, it becomes difficult to remember which one is which and their individual quirks and characteristics.
3. “Heat recovery from central plant chillers and boilers is financially worth while.”
Heat recovery is the recovery of heat that would otherwise have gone to waste. For example, the chiller absorbs heat from the chilled water and rejects it through the condenser to atmosphere. In a hospital with substantial hot water loads, some of this waste heat could be used to preheat the domestic hot water and perhaps to heat the air-conditioning reheat coils.
In a similar way, additional heat can be recovered from boiler flue gases by means of a recuperator. This is a device consisting of water sprays in a corrosion resistant section of flue. The water heats to around 120°F and is pumped through a water-to-water heat exchanger to provide water at about 115°F. This water can be used in an oversized coil for preheating outdoor air.
Both the heat-recovery from the chillers and recuperator heat from the boilers are examples of the improved energy efficiency that is often not economically feasible on the smaller distributed-packaged equipment.