Improving distribution system efficiency
March 1, 2015 | By Amin Delagah
Part II in a series dealing with restaurant hot water systems.
The distribution system is often overlooked as a component of the commercial kitchen hot water system that affects both energy and water use. There are four main types of distribution systems applicable to the foodservice environment (manifold distribution systems are generally not applicable to commercial food service).
1. Simple distribution (supply piping with no return loop)
2. Continuous recirculation (supply piping with return loop and pump)
3. Demand circulation (demand pump with or without a return loop)
4. Distributed generation (primary and point-of-use heating)
The majority of systems installed in restaurants are based either on simple distribution or continuous recirculation. Regardless of which type, many of these systems operate inefficiently and lack the hot water delivery performance desired in the foodservice environment. Demand circulation and distributed generation provide hot water on demand at the point of use and are more compatible with tankless heaters, unlike continuous recirculation systems where there may be issues with tankless heaters.
A simple distribution system uses a trunk, branch and twig configuration to deliver water from the heater to the points of use (see Figure 1). The benefit of this system is that it is simple and reliable and is compatible with all water heaters. The drawback is a potentially long wait time for hot water, especially at first use or after periods when water cools down in the pipes. Increasing the length or diameter of the distribution line increases wait times at the farthest fixtures because a larger volume of room-temperature water must be purged before hot water arrives. Simple distribution systems are often used for small quick-service restaurants and specialty shops where distribution lines are less than 60 ft.
The two most popular configurations include a single-line distribution system that feeds all sinks and equipment and a two-line distribution system that provides hot water (typically at 140F) to the sanitation sinks and dishwasher, while a second line provides a mix of hot and cold water (typically 100F to 110F) to handwashing sinks.
Continuously circulating hot water through the main distribution line and back to the heater ensures that there is hot water in the trunk line at all times, in essence moving the water heater closer to points of use as shown in Figure 2. However, depending on the branch and twig pipe size (i.e., volume of water in pipes between the trunk line and point of use) and fixture flow-rates, this configuration does not always ensure immediate delivery of hot water to the faucet. This is particularly the case when low-flow aerators have been specified. But regardless of how well the strategy works, water is being circulated at 140F (or more), continuously losing heat to the surroundings. The hotter the water is in the lines, and the poorer the insulation, the greater the heat loss and energy consumed by the water heater.
Demand circulation, as the name implies, incorporates a control strategy that operates the recirc pump only when there is a need for hot water. Pump operation can be initiated by an occupancy sensor in the vicinity of a hot water fixture. One system on the market features an integrated pump and temperature sensor placed under the hand sink (Figure 3). When the occupancy sensor triggers the pump, it purges room temperature (or luke-warm) water from the hot water supply line and transfers it into the adjacent cold water supply line or dedicated return line to the water heater.
Remember that the cold water line coming into the building is also connected to the inlet of the water heater. When the built-in temperature sensor measures an increase in water temperature, it assumes that hot water is just about to arrive at the fixture and it shuts off the pump, preventing hot water from being transferred into the cold water line. Now, when the hot water tap is turned on, the delivery of hot water happens within seconds. Every time the occupancy sensor calls for the pump to run, the unit checks the water temperature. If it senses that the water in the pipe has not cooled down, it does not activate the pump.
If the demand control pump is installed at the furthest end-use sink from the water heater, it will keep the trunk line hot between the heater and all points of use. Therefore, as illustrated in Figure 3, hot water will be delivered quickly to the other fixtures and appliances in the restaurant.
Distributed generation can be a 100 per cent distributed system utilizing point-of-use water heaters or a hybrid hot water system that combines a central water heater (storage type or tankless) with point-of-use electric heaters. In the hybrid configuration a simple distribution system delivers hot water to sanitation equipment and kitchen sinks, which are clustered near the primary water heater. On-demand or mini-tank electric water heaters are placed strategically below the hand sinks in remote lavatories. The point-of-use heaters that are sized appropriately for flow rate and temperature rise can be plumbed to the cold water line, thus eliminating the need for a dedicated hot water line. Using distributed electric heaters for hand-washing sinks is a cost-effective option, especially when specifying the “best-in-class” 0.375 gpm aerator for the faucets. This approach minimizes water and energy use while enhancing the customer experience by reducing the wait for hot water. Research is recommended when specifying a unit, as the reliability of some point-of-use electric heaters in heavy use applications has been called into question.
Pipe insulation is by far the most effective solution (if not the most overlooked) to improving the effectiveness of the distribution system to deliver hot water on demand. Typically, fibreglass or foam insulation can be used on hot water piping to prevent heat loss. This saves energy, extends the cool-down time, reduces operating cost, and improves the effectiveness of the distribution system to deliver hot water. Mandatory minimum requirements for pipe insulation thickness for service water heating systems in commercial facilities vary. Whatever the requirements are, adding insulation to the entire hot water distribution system should be a design specification. This includes pipes leading from the water heater, above the ceiling tiles, behind the walls, and leading from the wall to the appliance or sink.
RECIRCULATION PUMP TIMER
To reduce operating costs associated with the continuous operation of a circulation system, the recirc pump should be time-controlled and operated only when the restaurant needs hot water. Adding a time clock or timer (Figure 4) that turns the pump on shortly before restaurant staff begins preparation and off just before closing hours could save five to 10 hours of run time per day. The recirc pump can also be temperature-controlled with an aquastat that only runs the pump when the temperature of the return loop falls below a specified value.
HOT WATER DELIVERY ISSUES
There are two areas in food service, the dishwasher and hand sinks, where immediate delivery of hot water is critical to completing the task in a sanitary fashion. The rinse operation of the dishwasher requires inlet water temperatures typically in the 140F range to the dishwasher (for low-temp applications) or to the booster heater (for high-temp applications).
If hot water has cooled to room temperature in the pipes (between the recirc line and the dishwasher), the dishwasher may not be able to compensate for the sub-140F inlet water on its next load, thus compromising sanitation.
A more noticeable problem is the timely delivery of hot water to hand-washing sinks. With a simple distribution system, a hand sink could be located 60 ft. from the heater and the trunk must be purged of room temperature water the first time the hand sink is used. If the piping is not insulated, hot water may never reach a hand sink that is being used intermittently because the water always cools down between uses. The sad part is that the operator pays to heat the water that never makes it to the tap.
The goal is to reduce hot water wait time to 10 seconds or less, which is considered acceptable for public lavatories. A wait time of 11 to 30 seconds is considered borderline and a wait time of 30 seconds or more is unacceptable.2
EXTENDING COOL-DOWN TIME
Insulation extends the cool-down time of the hot water in pipes. It increases the time that branch and twig piping stays hot between water use events, thereby increasing the availability of hot water at the faucet and mitigating hot water delivery problems. Applied to -in. and ½-in. diameter pipes, 1-in. thick insulation will double the cool-down time compared with
uninsulated pipes; applied to ¾-in. diameter pipes, it triples the cool-down time.3
IMPROVING HOT WATER DELIVERY
A strategy to improve delivery performance is to reduce the diameter of the branch and/or twig piping leading from the trunk line to the hand sink(s). Some examples of pipe size and resulting hot water wait times are shown in Figure 5. To simplify the wait time estimation, it was assumed that the portion of twig line leading from the shut-off valve to the faucet aerator holds 0.024 gallons of water, which is equivalent to using two feet of ½-in. diameter piping and corresponds to three seconds of additional wait time.
Restaurant designs typically specify ¾-in. diameter branch piping (leading to the shut-off valve) for two lavatories or more. With 10 feet of ¾-in. diameter branch piping and a 0.5 gpm aerator installed, the wait time would be 33 seconds before the 0.28 gallons of water is purged and hot water reaches the faucet. For best design, ¾-in. branch piping should be used when serving five lavatories or more; using it to serve two lavatories is overkill. Similarly, ½-in. piping is appropriate for four lavatories that have a maximum flow rate of two gpm (Figure 6).
If ¾-in. branch piping must be used between the trunk line and the twig line, then two feet is the longest length before wait times breach the 10 second threshold for acceptable performance. In this situation, extending the trunk line vertically down the wall reduces the length of branch piping to the faucet (Figure 7). While this approach resolves the delivery performance issue, it marginally increases the length of the trunk line and system cost.
Using electrical heat trace is another way of maintaining the temperature in the hot water pipe. In this application, shown in Figure 8, a heating cable runs the length of the pipe under the insulation and maintains the temperature of the hot water by passing heat through the pipe into the water, counteracting the heat loss through the pipe and insulation. The inherent disadvantage of this method is that the water is being heated twice.
Watch for more on evolving technologies, and improving commercial kitchen hot water system performance in upcoming issues. <>
Distribution System Components
Twig: Serves one fixture
Branch: Serves two or more twigs
Trunk: Serves two or more branches; may be connected to a return line leading back to the water heater1
FINAL POINTS TO CONSIDER
• Mirror men’s and women’s lavatories so they are on opposite sides of the same wall and share a common branch pipe.
• Place higher flow rate sanitation equipment in a cluster and make sure the trunk line services these fixtures first to reduce the diameter of piping needed for the rest of the distribution line.
• Centralize the water heater in or near the kitchen to minimize pipe runs to equipment and fixtures.
• Consider point-of-use heaters for remote hand sinks, eliminating the need for hot water lines.
1 Klein, Gary. Hot Water and How Best to Get it. IAPMO Official.Winter 2009, pp. 44-45. www.allianceforwaterefficiency.org/Residential_Hot_Water_Distribution_System_Introduction.aspx
2. ASPE. Domestic Water Heating Design Manual, Second Edition. 2003. Reprint of Chapter 14: “Recirculating Domestic Hot Water Systems” www.psdmagazine.org.
3.Klein, Gary. Hot Water Distribution Research. IAPMO Official. Sep/Oct 2006, pp. 39-44. www.iapmo.org/Official Articles/2006-09 Hot Water Distribution Research.pdf.
4.Larkin, Brian. Electric Heat Trace vs. Recirculation. ACEEE Forum on Water Heating and Use. [Online] June 2, 2008. [Cited: March 4, 2010.] http://aceee.org/conf/08WHForum/#program.