1、OR-05-3-3 D i rect D i g it a I Tem pera t u re, H u m i d i ty, and Condensate Control for a Dedicated Outdoor Ai r-Ceiling Radiant Cooling Panel System Stanley A. Mumma, PhD, PE Fellow ASHRAE ABSTRACT The central thrust of this paper is to provide automation and control design guidance for enginee
2、rs considering dedi- cated outdoor air systems (DOAS) operating in parallel with ceiling radiant cooling panel systems (CRCP) in nonresiden- tial commercial and institutional applications. Thepaper iden- tijes the issues that must be addressed in the design of the control system and illustrates that
3、 the controls need not be complicated. The simplici of the controls is demonstrated via a case study of an existing DOAS-CRCP facility. Finally, the control challengespresented by low-occupancy spaces such as ofJices will be contrasted with those of high-density spaces such as schools andplaces of a
4、ssembly. INTRODUCTION DOAS-CRCP systems are applicable in low-density spaces typical of offices and for high-density spaces typical of schools and places of assembly. However, in the case of high- density spaces, reheat from recovered energy at the DOAS is necessary to avoid wasteful terminal reheat
5、 at off-design conditions. To minimize first cost and terminal reheat, high- density spaces call for a control approach uniquely different from that for low-density spaces where overcooling with the DOAS is extremely rare and terminal reheat is used sparingly. The DOAS-CRCP system design philosophy
6、is simple and straightfornard. The DOAS is used to deliver the required ventilation air to the breathing zone of each occupant without first mixing with recirculated air in a central AHU, thereby decoupling the ventilation function from the main thermal conditioning function. By adequately dehumidi%
7、ing the ventilation air, it can also serve to remove all of the space latent loads and, of course, the entire OA latent load. Depending Jae-Weon Jeong, PhD Associate Member ASHRAE upon the DOAS design supply air temperature (can be equal to the required DPT for low-density spaces or as high as room
8、neutral temperature for high-density spaces), the balance of the space sensible load is borne by the CRCP. The supply air temperature has a profound impact on both the first cost of the CRCP and the operating cost by virtue of the extent to which terminal reheat must be used. The DOAS-CRCP system ha
9、rdware and control design must consider and address the following issues: Comfort cooling Comfort heating Dehumidification during periods of elevated OA dew- point temperatures Humidification during periods of low OA dew-point temperatures Indoor air quality (IAQ) Air dimision performance index (ADP
10、I) (Mumma 2004) Condensate control, both active and passive (Mumma Spaces with movable sash, which offer additional con- densate control challenges (Mumma 2003) Internal generation and use patterns Selection of the variables to measure and the accuracy of instrumentation Transient response of instru
11、ments and system to pattern of use and weather changes Keep it simple to make the systems design, installation, maintenance, and operation easy The control hardware and software The need and/or desire for Web-based accessibility The need and desire for BACnet compatibility 2002) Stanley A. Mumma is
12、a professor and Jae-Weon Jeong is an instructor in the Department of Architectural Engineering, The Pennsylvania State University, University Park, Pa. 02005 ASHRAE. 547 Design documentation, schematics, points list, sequence of operation, and compliance with ASHRAE guideline 13-2000 (ASHRAE 2000) C
13、ommissioning Continuous monitoring and assessment (Mumma 2003) The above issues will be selectively addressed in this paper. BACNET-COMPATIBLE WEB-BASED CONTROLS FOR A SINGLE-ZONE DOAS-CRCP SYSTEM Such an operating system is illustrated in the Figure I schematic. Briefly, the OA is preconditioned as
14、 necessary with an enthalpy wheel, using the room return air, then cooled and dehumidified further with the cooling coil. The supply air temperature leaving the cooling coil is controlled with the three-way control valve V1. Finally the cold and dry 100% outdoor ventilation supply air is delivered t
15、o the space via high induction overhead diffusers. This system does not have any auxiliary preheat, terminal, or space heating. All heat comes from internal generation, and the OA is tempered with exhaust air heat recovered by the enthalpy wheel. During cold OA conditions when the OA must be tempere
16、d, the enthalpy wheels capacity to recover heat is achieved by modulating the enthalpy wheel on and off as necessary (limited to no more than four on-off cycles per hour) based upon the space conditions. The supply air temperature during cooling is modulated to satis9 the space conditions down to a
17、DPT of 52F (ll.l“C), but no lower. With thc supply air DPT at 52F (1 1.1 OC), the space DPT is maintained low enough that the CRCP will never form condensation when radiant cooling is used to meet the balance of the space sensible load. Two 5-ton (17.6 kW) air-cooled chillers working in parallel pro
18、vide chilled water. The chilled water first satisfies the needs of the cooling coil, then the CRCP. The three-way control valve V2 is modulated as necessary to meet the space DBT setpoint, limited by the space DPT. The room return air temperature and relative humidity are used to compute the space D
19、PT. The CRCP inlet water temperature is never permitted to drop below the space DPT. A passive fail-safe condensate sensor is used as a condensation prevention backup. The fail-safe condensate sensor uses a normally open switch, which opens when the first drop of condensate from the supply piping fa
20、lls on it. The sensors switch is hard wired into the three-way NC spring return control valve V2s power supply. The DOAS is constant volume, with no provision for space pressurization control. However, the space pressure is constantly monitored, and the relief fan performance period- ically adjusted
21、, to ensure long-term pressurization. It is desired to keep the space slightly pressurized (approximately 0.001 in. w.g. 0.25 Pa) to avoid the introduction of latent loads by way of infiltration. This is easily achieved, even in the very leaky early 1900s construction building where all six enclosur
22、e planes are subjected to exterior vapor and atmo- Radiant Panel I coranioneu space FM5 Figure I A single-zone DOAS-CRCP system. 548 D2 I A JOBRelaY Fresh Air Verlator I ASHRAE Transactions: Symposia spheric pressure. Introduction of infiltration at the envelope not only permits the undesirable tran
23、sport of moisture but also makes controlling the thermal climate near the exterior walls difficult during both summer and winter. As far as energy consumption is concerned, air leakage into the space is much more costly (almost seven times) than bringing that air into the building by way of a qualit
24、y total energy recovery (recovery effectiveness of 85%) DOAS. It is also very desirable to return as much of the supplied air to the enthalpy wheel for total energy recovery, so overpressurization must be avoided. ASHRAE Standard 90.1-200 1 addresses the issue of building envelope tightness, and it
25、is strongly recommended that in new construction the building envelope conform or exceed Section 5.2.3. 5.2.3 5.2.3.1 Envelope Air Leakage (ASHRAE 2001) Building Envelope Sealing. The following areas of the building envelope SHALL be sealed, caulked, gas- keted. or weather-stripped to minimize air l
26、eakage: Joints around fenestration and door frames Junctions between walls and foundations, between walls at building comers, between walls and struc- turalfloors or roof, and between walls and roof or wall panels Openings at penetrations of utility services through roof, walls, and floors Site-buil
27、t fenestration and doors Building assemblies used as ducts or plenums Joints, seams, and penetrations of vapor retarders All other openings in the building envelope. Out- side air intakes, exhaust outlets, relief outlets, stair shaft, elevator shaft smoke relief openings, and other similar elements
28、shall also be very low leak- age. 5.2.3.2 Fenestration and Doors. Air leakage for fen- estration and doors shall be determined in accordance with NFRC 400. Air leakage shall be determined by a laboratory accredited by a nationally recognized accreditation organiza- tion, such as the National Fenestr
29、ation Rating Council, and shall be labeled and certified by the manufacturer. Air leakage shall not exceed 1 .O cfm/f? for glazed swinging entrance doors and for revolving doors and 0.4 cW for all other products. This DOAS-CRCP cooling-only system was built and exists to do the following: Controls r
30、elated: Overcome/answer the negative perception of ceiling radiant cooling held by much of the industry. Demonstrate that the space sensible and latent loads can be decoupled using DOAS. Prove that condensation formation and subsequent dam- age need never occur, even in the leaky single-glazed non-i
31、deal early 1900 building space used for the project. Demonstrate the simplicity of the Web-based BACnet- compatible DDC used for the integrated systems. Provide an educational resource for university students, faculty, staff, owners, investors, and design profession- als worldwide via first-hand sit
32、e visits or via virtual tours and real-time data displays (available since Web- based) at www.doas-radiant.psu.edu. Not controls related: Provide first-hand thermal comfort experience working in a radiant cooling field. Demonstrate that the sensible load of the space can be met with radiant panels w
33、hen applied in parallel with a DOAS, using only about 20% of the ceiling area. Provide a basis for comparison of the DOAS-CRCP indoor environment with that of conventional all-air VAV systems. SPECIFIC CONTROL FUNCTIONS A control logic schematic diagram for the single-zone DOAS-CRCP system is presen
34、ted in Figure 2. The logic consists of seven parts. Each part will be briefly discussed below. 1. 2. Setpoints The setpoints consist of the 100% OA ventilation supply air temperature and the summer and winter operative temperature setpoints. Operative temperature, the arith- metic average of the spa
35、ce DBT and the space mean radi- ant temperature (MRT), is used as the space-controlled variable. The summer setpoint is used to control the cool- ing performance of the cooling coil and radiant panels. The winter setpoint is used to modulate the enthalpy wheel when the OA must be tempered in the win
36、ter. System odoffconrol This section has three functions. First, since there is no auxiliary heating in the space, the DOAS system is shut down when the space MRT drops below the setpoint minus the throttling range set in the “If ” microblock. Second, there is provision to manually set the occupancy
37、 status on or off to accommodate scheduled vacancy, such as vacations. Third, and finally, there is provision to shut the system down if, during the winter, frost could form in the enthalpy wheel (a condition that can occur when the line on the psychrometric chart connecting the two EW entering air
38、state points intersects the saturation curve). Mumma (2001) and Freund et al. (2003) addressed the effective ways to avoid frost or freeze problem on the EW surface during the winter operation. This precaution is required since there is no preheat available in the system. However, it has yet to occu
39、r since the space has no active humidification, and the room RH is low in the winter. The code written inside the freeze control microblock follows on page 552: ASH RAE Transactions: Symposia 549 I _ Note: A11 setpointe are I-P Unies I i. M E fracrs?-sapC calculatien Cleenine We tine fraction (% C3?
40、 Enthah uheel contro1 EL) 0FMd.e Full W2.0 at ion= 1.5 r-“ In t i.- - =O.O) then Il fD is equal to or higher than O, the line is the tangent or the intersect line to the saturation curve. Begin aifa=(-F+SQRT(D)/(2*E) /I Positive solution of the quadratic equation If (alfa=T 1)then ONOFF=off Else (i.
41、e. EWshould be 08 ONOFF=on Il Ifthe positive solution is less than OA DB7: the frost problem might occur on E W surface. End ind Ise NOFF=on IXITPROG Il End of the freeze control microblock 3. Enthalpy wheel control The enthalpy wheel operates in one of four modes: Full on when it reduces the enthal
42、py of the OA entering the CC, as when it is hot and humid Full off when it is warm and humid, but operation would increase the enthalpy of the OA entering the cc Modulating in the winter to temper the OA entering the space to avoid overcooling A cleaning cycle, necessary when the enthalpy wheel is o
43、ff to prevent it from becoming an unwanted filter (in this mode, the wheel rotates for a few minutes each hour, or about 40 revolutionslon cycle) The first three modes are achieved by use of the two microblocks, EW mode and EW control. The code for each of these microblocks follows: 552 ASHRAE Trans
44、actions: Symposia EW mode microblock code: TITLE EW mode II Defining the name of the EWoperation mode microblock 4iNPUT h-oa MNPUT h-ra 4INPUT DPTo 4INPUT DPTs AOUTPUT Opmd Il OA enthalpy BtuAbm : Analog input II EA enthalpy Btu/lbm : Analog input Il OA Dewpoint temperature F : Analog input Il SA DP
45、T set point F : Analog input II Enthalpy wheel operation mode : Analog output (O=O I = Modulation) if (h-oah-ra) and (DPToDPTs) then Dpmd=2.0 on the psychrometric chart il Defining the hot and humid region If (h-oaDPTs) then Il Defining the warm and humid region Opmd=O.O on the psychrometric chart t
46、f (h-oa=h-ra) and (DPTo=DPTs) then II Defining the regionfor the EW Opmd=l .O EXITPROG modulation on the psychrometric chart Il End of the EWoperation mode microblock EW control microblock code: TITLE EW control DINPUT ONOF DINPUT EWof DINPUT EWmd DINPUT EWfl DOUTPUT Full Il Defining the name of the
47、 EW control microblock Il Input from the occupancy mode : Digital input li EW off mode : Digital input Il EW modulation mode : Digital input Il EWfull speed mode : Digital input II Output for activating E W full speed operation Il Output for activating EW modulation operation : Digital output : Digi
48、tal output DOUTPUT Modu IF (ONOF=on) and (EWof=on) and (EWmd=o and (EWfl=o THEN BEGIN Il Input signals to define EW off mode Modu=off Full=off END IF (ONOF=on) and (EWof=o and (EWmd=on) and (EWfl=o THEN BEGIN II Input signals to define EW modulation mode Modu=on Full=off END IF (ONOF=on) and (EWof=o
49、f) and (EWmd=o and (EWfl=on) THEN BEGIN II Input signals to dejne EW full speed mode Modu=off Full=on END IF (ONOF=oQ THEN BEGIN Modu=off Full=off END Il Under the unoccupied mode EXITPROG Il End of the EWcontrol microblock ASHRAE Transactions: Symposia 553 4. 5. 6. 7. Chiller control This is a quite simple control, which sends a signal to the air-cooled chiller and chilled water pump PI to operate whenever the OA temperature rises above a setpoint. Since there are two 5-ton (17.6 kW) units, the second unit is only enabled when the space operative temperatur
