Designing energy plants the smart way
Low-load control is essential for efficient operation of heating and cooling plants and providing comfort during low-load and changeover conditions between heating and cooling. Complaints from occupants to maintenance personnel often peak during times of low load. Generally, low-load and changeover conditions occur for many hours during a year, many more times than high and low design loads. Low-load and changeover occurs when the outside temperature is between 45 and 75 F.
Philadelphia, for example, has approximately 1800 hours annually of low-load and changeover operation during office hours compared with 150 hours of high-load operation. This is equivalent annually to 160 working days of low loads compared with 13 days of high loads. That means one third to one half of the working time is in low-load or changeover condition, making this a very important period.
Steam-to-water heat exchanger control
In multi-building facilities served with central steam, or urban buildings served by district steam from utility companies, during low heating load or, in many cases, low and medium loads, there is very poor control of the heating system and the steam-to-water heat exchanger for the building.
This causes overheating in thousands of buildings that maintenance personnel are called to respond to, wastes thousands of dollars in energy, and causes occupant discomfort—a triple whammy that costs millions of dollars every year.
The heating system design usually has not been fully analyzed and documented. Detailed selection and sizing, and control of the heat exchanger and pressure reduction station are not explained or documented.
There are many textbooks and engineering articles on how to control steam-to-water heat exchangers, steam pressure reducing stations, and heating systems, but none I have read detail the various loads imposed on the exchanger. Common incorrect assumptions are that the heat exchanger and the steam pressure reducing station size are close to the operating size of the heating load and that either one valve or two valves with a 1/3, 2/3 split is considered the satisfactory or even ideal solution. There is no detailed system analysis to back up these assumptions, and no commissioning information that I have found that corrects this insidious problem.
Historical development of the system
Steam-to-water heat exchangers and steam pressure reducing stations with a one-third/two-thirds semiautomatic valve arrangement worked well for the system they were developed for almost 100 years ago: high-mass buildings with a radiator heating system that had a good load during the daytime. The size of the valves was often less than the heat exchanger size because an operator was present 24/7 to open a manual valve at high loads or during emergencies.
Unfortunately, this system has been moved to a fully automatic control system without a complete analysis. Modern buildings have a low mass and high insulation, causing lower daytime loads. The more popular air systems have a faster response time, which, together with night setback, produce both much lower daytime loads and a heat exchanger sizing much larger than the operating loads.
To resolve this, one must fully document and analyze the heating systems and heat exchanger sizing and control strategies together with the steam system control.
For example, a typical building could be part of an office, hospital, university, or school complex that is supplied with steam from a central plant or from utility company steam.
The heating design load we shall assume has been calculated as 10 million Btu/hr. The start-up load on a design day from night setback is usually 30% greater than the design load, say 13 million Btu/hr. The catastrophic breakdown load, also known as a cold building or cold start load, is an additional 20% greater than the design start-up load, say 15 million Btu/hr for the final size of the heat exchanger. The actual operating design day for an occupied running load is usually between 50% and 80% of the heating design load, depending on many factors; let us assume 67% for our example, or 7 million Btu/hr. We now have an operating scenario of a system installed that is more than twice the operating design load. This is typical sizing for a steam heat exchanger system; the installed heat exchanger is often two to three times larger than the actual operating design load, and there is nothing wrong with this.
We need to estimate the lowest realistic load that will be required to be controlled. This calculation sizes the system to control at low load. The heating temperature balance point for a building is when there is little to no heating needed during occupied hours, and is often around 50 to 60 F outside temperature, depending on many factors.
A building heating balance point is when the temperature outside causes a heat load balanced by the internal heat load, often found to be around 55 F. When the design outside temperature is 15 F, we will have an outside temperature swing of 40 F that causes the operating load to move from high to minimum, or none. Indoor temperature control is often quoted as +/-2 F, and a 2 F swing is 5% of a 40 F temperature swing. Terminal units, reheat coils, or VAV boxes might be several controls away from the steam valve on the heat exchanger. However, including the total system heat losses, the heat from the steam pipe entering the building needs to match the heat required, and with a heat exchanger two times the operating design load, we estimate that a 2 F building temperature rise causes a 2.5% heat exchanger load, plus losses.
A cross-check for a minimum operating load sizing is to assess the load for a cool day when the heating is on in the morning for warm-up and then the internal loads start kicking in to make some areas warm while a quarter of the areas require 4 F of heating to maintain comfort. The load will be a quarter of the 4 F heating load. A 4 F heating load is 10% of the 40 F temperature swing, approximately 2.5% of the design running load, or 1.25% of the exchanger load, plus losses.
A steam control valve that controls down to 2%, 3%, or 4% of heat exchanger size is required. Most steam valves control down to 40% of their full load consistently, and up to 80% of their full load. To control down to 2% would require a valve 5% or 1/20 of the steam exchanger size. A 1/3 valve is over six times larger than needed; it’s little wonder we have low-load problems.
If we adopt the 2/3, 1/3 control valve sizing sequence, as in Figure 1, we need to add a 1/6, a 1/12, and a 1/24 valve to the series to control low loads. A better scenario could be to install two small valves for low load. If we are installing a new control system with valves that have a 4:1 turndown capacity, we might want to install two 1/20, one 1/4, and one 2/3 automatic valve sequence, as in Figure 2. There will be manual bypass valves for emergency operation. These valves and control systems are not expensive to install and will save many times their cost over their lifecycle. As important as the energy savings are the improved comfort and reduced maintenance calls.
To lower the hot water distribution losses and develop a more controllable heating system at low loads, we could lower the hot water temperature during low and medium loads. This will reduce the losses during low and medium loads to less than 1% of the full running load.
Figure 3 shows a heat exchanger and no three-way mixing valve on the hot water side. This can provide more stable control with only one control point that can be controlled accurately. Another control strategy is to control the return water temperature to a constant 170 F, to be determined on-site, which would provide the equivalent of outside temperature compensation.
It costs about $1,000 to install a 3/8-in. control valve, but this valve can save much more than $1,000 every year. The steam pressure reducing station may also require a smaller valve for low loads.
Buildings already in operation can be modified easily; new buildings can be designed to allow for low-load control. The payback on this fix should be within one season.
Most boiler plants serving buildings have poor low-load characteristics.
In a similar fashion to the above steam heat exchanger analysis, we need to estimate the installed load, the actual working load, and an actual low-load situation. Modular boilers with modulating burners, as shown in Figure 4, will often allow moving to a low-load condition efficiently, but whether that operation at the estimated low load is possible must be checked. An eight-boiler modular plant with two of the boilers able to modulate down to 20% load will provide a real low load of 7% if six of the boilers represent the working full load.
There can often be a payback of less than three years for converting a boiler plant when all the operating costs are analyzed and some low-load installations can have an even better payback.
Chillers cost more to install and operate than boilers in most facilities, so an investment opportunity could be to install a small chiller to handle low loads efficiently. The long-term aim should be to eliminate electric chiller plants for comfort conditioning wherever possible.
One of the most annoying things I come across in projects being designed today is chillers that will require false loading in order to operate during low load. False loading chillers is a common practice in far too many facilities. It should be a thing of the past, but I have seen this practice in new schools and university projects, even U.S. Green Building Council LEED projects currently being designed.
Obviously, the chiller is not announced as being oversized, but where there is one chiller installed for a building, it’s a safe bet that it is at least 50% larger than the full operating load, and often 100% larger. Most centrifugal water chillers work down to 40% load, and variable speed drives work down to 10% of their full load, but they are expensive. In order for a chiller to operate at low building loads, it has to be false loaded, wasting both heating and cooling. With total chiller capacity designed at under about 300 tons, it is often a better choice to install two or three small reciprocating air-cooled chillers, hopefully 1/2 or 2/3 the original chiller size, with provision for expansion, that will work down to low load without false loading and cost less to install and maintain.
Modifying existing chiller plants typically has a three-year payback due to the high installation costs. Many large central chiller plants I have examined would benefit greatly from a total overview where the lifecycle performance together with the total mechanical systems concepts were reviewed as well as low-load profile.
For example, if we have a new chiller selection of 250 tons by standard design load methods, often the operating full load will be about 125 to 175 tons. If we use an accurate building simulation program, we can often negotiate with the owner and architect to reduce the installed size of the chiller by 50%, say to 125 tons. If we install three 60-ton air-cooled reciprocating machines that will each work down to 7.5 tons, with a space for a fourth or for even twice the tonnage, for expansion if and when necessary, it will be less expensive to install and cost much less to operate than a 250-ton centrifugal, and provide good low-load control as well as have a built in standby or backup. The 7.5-ton low load will represent around 6% of the operating full load, significantly different from the 100-ton low load of a 250-ton centrifugal chiller.
Grahame E. Maisey is chief engineer at Building Services Consultants and has more than 45 years of experience in the United Kingdom and United States planning and developing leading-edge building energy systems. In that time, he has also created energy master plans that move buildings and facilities to energy independence and financial stability and success, and has evolved commissioning to Total Quality Commissioning that provides whole lifecycle performance assurance.