Optimal Control in Three-deck Multi-Zone Air-Handling Units: A Case-Study

A multi-zone air-handling unit was popular several decades ago due to the convenience of small sized modular units, which were inexpensive to install and easily maintained in a mechanical room. The cost and convenience proved to be of little benefit as the units perform poorly from an energy usage perspective. A “three-deck” multi-zone unit is a hybrid of its kind, and it can be very efficient when controlled properly. In theory, there will not be simultaneous heating and cooling if its heating damper is controlled separately from the control of the cooling damper. When the zone load is neutral (not heating or cooling), for example, all the mixed air will be bypassed through the bypass deck. However, there are opportunities and challenges in this system. This paper presents theoretical backgrounds of advantages and challenges in the system operation of the three-deck multi-zone unit and methods to optimize temperature and economizer control to improve energy efficiency. A case-study will be presented examining a medical facility’s utilization of 35 three-deck multi-zone units serving most hospital areas, as well as illustrates a 10% savings in total gas and electric consumptions over the period of one year. INTRODUCTION A multi-zone air-handling unit was popular several decades ago due to the convenience of small size modular units. However, it performs poorly in its energy usage perspective. The lack of energy efficiency is a result of constant fan operation and simultaneous heating and cooling. The system generally serves three (3) to ten (10) zones from a centrally located air-handling unit. Since its number of serving areas is limited, the unit is typically too small for the zone and tends to operate with a constant speed fan. Each zone requires different heating or cooling load that is accommodated by mixing cold and warm air through a zone damper. [ASHRAE 2000] For each zone damper, the hot deck zone damper is interconnected with the cold deck zone damper in opposed direction. Therefore, simultaneous heating and cooling is not avoidable unless all the zones are exposed to full cooling or full heating. In fact, these units are currently installed as an option for small serving zones in many facilities, taking advantage of the centrally located system configuration leading to reduced installation costs and easy maintenance. As an alternative to the multizone air-handling unit, engineers may select a threedeck (3-deck) multi-zone unit or a Texas multi-zone unit depending on their budgets or preferences in order to improve energy efficiency, thus save energy costs. Figure 1. Schematic of a Typical Multi-Zone Unit This paper discusses a case study of optimization of three-deck (3-deck) multi-zone units in a medical facility. The 3-deck multi-zone unit is a hybrid of its kind, and it can be very efficient when controlled properly. The system has three (3) distinct decks: a heating deck on the top, a cooling deck at the bottom, and a bypass deck in the middle. The heating deck heats up mixed air by a heating coil, and the cooling deck cools down the mixed air by a cooling coil. The bypass deck has no coils. In each zone, the hot deck damper is linked with a bypass damper in an opposed direction while the cold deck damper is linked with another bypass damper in an opposed direction. [ASHRAE 2000; McDowall 2007]